diff mbox series

[v2,24/25] commoncap: use vfs fscaps interfaces

Message ID 20240221-idmap-fscap-refactor-v2-24-3039364623bd@kernel.org (mailing list archive)
State New, archived
Headers show
Series fs: use type-safe uid representation for filesystem capabilities | expand

Commit Message

Seth Forshee (DigitalOcean) Feb. 21, 2024, 9:24 p.m. UTC
Use the vfs interfaces for fetching file capabilities for killpriv
checks and from get_vfs_caps_from_disk(). While there, update the
kerneldoc for get_vfs_caps_from_disk() to explain how it is different
from vfs_get_fscaps_nosec().

Signed-off-by: Seth Forshee (DigitalOcean) <sforshee@kernel.org>
---
 security/commoncap.c | 30 +++++++++++++-----------------
 1 file changed, 13 insertions(+), 17 deletions(-)

Comments

Roberto Sassu March 4, 2024, 10:19 a.m. UTC | #1
On Wed, 2024-02-21 at 15:24 -0600, Seth Forshee (DigitalOcean) wrote:
> Use the vfs interfaces for fetching file capabilities for killpriv
> checks and from get_vfs_caps_from_disk(). While there, update the
> kerneldoc for get_vfs_caps_from_disk() to explain how it is different
> from vfs_get_fscaps_nosec().
> 
> Signed-off-by: Seth Forshee (DigitalOcean) <sforshee@kernel.org>
> ---
>  security/commoncap.c | 30 +++++++++++++-----------------
>  1 file changed, 13 insertions(+), 17 deletions(-)
> 
> diff --git a/security/commoncap.c b/security/commoncap.c
> index a0ff7e6092e0..751bb26a06a6 100644
> --- a/security/commoncap.c
> +++ b/security/commoncap.c
> @@ -296,11 +296,12 @@ int cap_capset(struct cred *new,
>   */
>  int cap_inode_need_killpriv(struct dentry *dentry)
>  {
> -	struct inode *inode = d_backing_inode(dentry);
> +	struct vfs_caps caps;
>  	int error;
>  
> -	error = __vfs_getxattr(dentry, inode, XATTR_NAME_CAPS, NULL, 0);
> -	return error > 0;
> +	/* Use nop_mnt_idmap for no mapping here as mapping is unimportant */
> +	error = vfs_get_fscaps_nosec(&nop_mnt_idmap, dentry, &caps);
> +	return error == 0;
>  }
>  
>  /**
> @@ -323,7 +324,7 @@ int cap_inode_killpriv(struct mnt_idmap *idmap, struct dentry *dentry)
>  {
>  	int error;
>  
> -	error = __vfs_removexattr(idmap, dentry, XATTR_NAME_CAPS);
> +	error = vfs_remove_fscaps_nosec(idmap, dentry);

Uhm, I see that the change is logically correct... but the original
code was not correct, since the EVM post hook is not called (thus the
HMAC is broken, or an xattr change is allowed on a portable signature
which should be not).

For completeness, the xattr change on a portable signature should not
happen in the first place, so cap_inode_killpriv() would not be called.
However, since EVM allows same value change, we are here.

Here is how I discovered this problem.

Example:

# ls -l test-file
-rw-r-Sr--. 1 3001 3001 5 Mar  4 10:11 test-file

# getfattr -m - -d -e hex test-file
# file: test-file
security.capability=0x0100000202300000023000000000000000000000
security.evm=0x05020498c82b5300663064023052a1aa6200d08b3db60a1c636b97b52658af369ee0bf521cfca6c733671ebf5764b1b122f67030cfc688a111c19a7ed3023039895966cf92217ea55c1405212ced1396c2d830ae55dbdb517c5d199c5a43638f90d430bad48191149dcc7c01f772ac
security.ima=0x0404f2ca1bb6c7e907d06dafe4687e579fce76b37e4e93b7605022da52e6ccc26fd2
security.selinux=0x756e636f6e66696e65645f753a6f626a6563745f723a756e6c6162656c65645f743a733000

# chown 3001 test-file

# ls -l test-file
-rw-r-Sr--. 1 3001 3001 5 Mar  4 10:14 test-file

# getfattr -m - -d -e hex test-file
# file: test-file
security.evm=0x05020498c82b5300673065023100cdd772fa7f9c17aa66e654c7f9c124de1ccfd36abbe5b8100b64a296164da45d0025fd2a2dec2e9580d5c82e5a32bfca02305ea3458b74e53d743408f65e748dc6ee52964e3aedac7367a43080248f4e000c655eb8e1f4338becb81797ea37f0bca6
security.ima=0x0404f2ca1bb6c7e907d06dafe4687e579fce76b37e4e93b7605022da52e6ccc26fd2
security.selinux=0x756e636f6e66696e65645f753a6f626a6563745f723a756e6c6162656c65645f743a733000


which breaks EVM verification.

Roberto

>  	if (error == -EOPNOTSUPP)
>  		error = 0;
>  	return error;
> @@ -719,6 +720,10 @@ ssize_t vfs_caps_to_user_xattr(struct mnt_idmap *idmap,
>   * @cpu_caps:	vfs capabilities
>   *
>   * Extract the on-exec-apply capability sets for an executable file.
> + * For version 3 capabilities xattrs, returns the capabilities only if
> + * they are applicable to current_user_ns() (i.e. that the rootid
> + * corresponds to an ID which maps to ID 0 in current_user_ns() or an
> + * ancestor), and returns -ENODATA otherwise.
>   *
>   * If the inode has been found through an idmapped mount the idmap of
>   * the vfsmount must be passed through @idmap. This function will then
> @@ -731,25 +736,16 @@ int get_vfs_caps_from_disk(struct mnt_idmap *idmap,
>  			   struct vfs_caps *cpu_caps)
>  {
>  	struct inode *inode = d_backing_inode(dentry);
> -	int size, ret;
> -	struct vfs_ns_cap_data data, *nscaps = &data;
> +	int ret;
>  
>  	if (!inode)
>  		return -ENODATA;
>  
> -	size = __vfs_getxattr((struct dentry *)dentry, inode,
> -			      XATTR_NAME_CAPS, &data, XATTR_CAPS_SZ);
> -	if (size == -ENODATA || size == -EOPNOTSUPP)
> +	ret = vfs_get_fscaps_nosec(idmap, (struct dentry *)dentry, cpu_caps);
> +	if (ret == -EOPNOTSUPP || ret == -EOVERFLOW)
>  		/* no data, that's ok */
> -		return -ENODATA;
> +		ret = -ENODATA;
>  
> -	if (size < 0)
> -		return size;
> -
> -	ret = vfs_caps_from_xattr(idmap, inode->i_sb->s_user_ns,
> -				  cpu_caps, nscaps, size);
> -	if (ret == -EOVERFLOW)
> -		return -ENODATA;
>  	if (ret)
>  		return ret;
>  
>
Seth Forshee (DigitalOcean) March 4, 2024, 3:31 p.m. UTC | #2
On Mon, Mar 04, 2024 at 11:19:54AM +0100, Roberto Sassu wrote:
> On Wed, 2024-02-21 at 15:24 -0600, Seth Forshee (DigitalOcean) wrote:
> > Use the vfs interfaces for fetching file capabilities for killpriv
> > checks and from get_vfs_caps_from_disk(). While there, update the
> > kerneldoc for get_vfs_caps_from_disk() to explain how it is different
> > from vfs_get_fscaps_nosec().
> > 
> > Signed-off-by: Seth Forshee (DigitalOcean) <sforshee@kernel.org>
> > ---
> >  security/commoncap.c | 30 +++++++++++++-----------------
> >  1 file changed, 13 insertions(+), 17 deletions(-)
> > 
> > diff --git a/security/commoncap.c b/security/commoncap.c
> > index a0ff7e6092e0..751bb26a06a6 100644
> > --- a/security/commoncap.c
> > +++ b/security/commoncap.c
> > @@ -296,11 +296,12 @@ int cap_capset(struct cred *new,
> >   */
> >  int cap_inode_need_killpriv(struct dentry *dentry)
> >  {
> > -	struct inode *inode = d_backing_inode(dentry);
> > +	struct vfs_caps caps;
> >  	int error;
> >  
> > -	error = __vfs_getxattr(dentry, inode, XATTR_NAME_CAPS, NULL, 0);
> > -	return error > 0;
> > +	/* Use nop_mnt_idmap for no mapping here as mapping is unimportant */
> > +	error = vfs_get_fscaps_nosec(&nop_mnt_idmap, dentry, &caps);
> > +	return error == 0;
> >  }
> >  
> >  /**
> > @@ -323,7 +324,7 @@ int cap_inode_killpriv(struct mnt_idmap *idmap, struct dentry *dentry)
> >  {
> >  	int error;
> >  
> > -	error = __vfs_removexattr(idmap, dentry, XATTR_NAME_CAPS);
> > +	error = vfs_remove_fscaps_nosec(idmap, dentry);
> 
> Uhm, I see that the change is logically correct... but the original
> code was not correct, since the EVM post hook is not called (thus the
> HMAC is broken, or an xattr change is allowed on a portable signature
> which should be not).
> 
> For completeness, the xattr change on a portable signature should not
> happen in the first place, so cap_inode_killpriv() would not be called.
> However, since EVM allows same value change, we are here.

I really don't understand EVM that well and am pretty hesitant to try an
change any of the logic around it. But I'll hazard a thought: should EVM
have a inode_need_killpriv hook which returns an error in this
situation?
Roberto Sassu March 4, 2024, 4:17 p.m. UTC | #3
On Mon, 2024-03-04 at 09:31 -0600, Seth Forshee (DigitalOcean) wrote:
> On Mon, Mar 04, 2024 at 11:19:54AM +0100, Roberto Sassu wrote:
> > On Wed, 2024-02-21 at 15:24 -0600, Seth Forshee (DigitalOcean) wrote:
> > > Use the vfs interfaces for fetching file capabilities for killpriv
> > > checks and from get_vfs_caps_from_disk(). While there, update the
> > > kerneldoc for get_vfs_caps_from_disk() to explain how it is different
> > > from vfs_get_fscaps_nosec().
> > > 
> > > Signed-off-by: Seth Forshee (DigitalOcean) <sforshee@kernel.org>
> > > ---
> > >  security/commoncap.c | 30 +++++++++++++-----------------
> > >  1 file changed, 13 insertions(+), 17 deletions(-)
> > > 
> > > diff --git a/security/commoncap.c b/security/commoncap.c
> > > index a0ff7e6092e0..751bb26a06a6 100644
> > > --- a/security/commoncap.c
> > > +++ b/security/commoncap.c
> > > @@ -296,11 +296,12 @@ int cap_capset(struct cred *new,
> > >   */
> > >  int cap_inode_need_killpriv(struct dentry *dentry)
> > >  {
> > > -	struct inode *inode = d_backing_inode(dentry);
> > > +	struct vfs_caps caps;
> > >  	int error;
> > >  
> > > -	error = __vfs_getxattr(dentry, inode, XATTR_NAME_CAPS, NULL, 0);
> > > -	return error > 0;
> > > +	/* Use nop_mnt_idmap for no mapping here as mapping is unimportant */
> > > +	error = vfs_get_fscaps_nosec(&nop_mnt_idmap, dentry, &caps);
> > > +	return error == 0;
> > >  }
> > >  
> > >  /**
> > > @@ -323,7 +324,7 @@ int cap_inode_killpriv(struct mnt_idmap *idmap, struct dentry *dentry)
> > >  {
> > >  	int error;
> > >  
> > > -	error = __vfs_removexattr(idmap, dentry, XATTR_NAME_CAPS);
> > > +	error = vfs_remove_fscaps_nosec(idmap, dentry);
> > 
> > Uhm, I see that the change is logically correct... but the original
> > code was not correct, since the EVM post hook is not called (thus the
> > HMAC is broken, or an xattr change is allowed on a portable signature
> > which should be not).
> > 
> > For completeness, the xattr change on a portable signature should not
> > happen in the first place, so cap_inode_killpriv() would not be called.
> > However, since EVM allows same value change, we are here.
> 
> I really don't understand EVM that well and am pretty hesitant to try an
> change any of the logic around it. But I'll hazard a thought: should EVM
> have a inode_need_killpriv hook which returns an error in this
> situation?

Uhm, I think it would not work without modifying
security_inode_need_killpriv() and the hook definition.

Since cap_inode_need_killpriv() returns 1, the loop stops and EVM would
not be invoked. We would need to continue the loop and let EVM know
what is the current return value. Then EVM can reject the change.

An alternative way would be to detect that actually we are setting the
same value for inode metadata, and maybe not returning 1 from
cap_inode_need_killpriv().

I would prefer the second, since EVM allows same value change and we
would have an exception if there are fscaps.

This solves only the case of portable signatures. We would need to
change cap_inode_need_killpriv() anyway to update the HMAC for mutable
files.

Roberto
Seth Forshee (DigitalOcean) March 4, 2024, 4:56 p.m. UTC | #4
On Mon, Mar 04, 2024 at 05:17:57PM +0100, Roberto Sassu wrote:
> On Mon, 2024-03-04 at 09:31 -0600, Seth Forshee (DigitalOcean) wrote:
> > On Mon, Mar 04, 2024 at 11:19:54AM +0100, Roberto Sassu wrote:
> > > On Wed, 2024-02-21 at 15:24 -0600, Seth Forshee (DigitalOcean) wrote:
> > > > Use the vfs interfaces for fetching file capabilities for killpriv
> > > > checks and from get_vfs_caps_from_disk(). While there, update the
> > > > kerneldoc for get_vfs_caps_from_disk() to explain how it is different
> > > > from vfs_get_fscaps_nosec().
> > > > 
> > > > Signed-off-by: Seth Forshee (DigitalOcean) <sforshee@kernel.org>
> > > > ---
> > > >  security/commoncap.c | 30 +++++++++++++-----------------
> > > >  1 file changed, 13 insertions(+), 17 deletions(-)
> > > > 
> > > > diff --git a/security/commoncap.c b/security/commoncap.c
> > > > index a0ff7e6092e0..751bb26a06a6 100644
> > > > --- a/security/commoncap.c
> > > > +++ b/security/commoncap.c
> > > > @@ -296,11 +296,12 @@ int cap_capset(struct cred *new,
> > > >   */
> > > >  int cap_inode_need_killpriv(struct dentry *dentry)
> > > >  {
> > > > -	struct inode *inode = d_backing_inode(dentry);
> > > > +	struct vfs_caps caps;
> > > >  	int error;
> > > >  
> > > > -	error = __vfs_getxattr(dentry, inode, XATTR_NAME_CAPS, NULL, 0);
> > > > -	return error > 0;
> > > > +	/* Use nop_mnt_idmap for no mapping here as mapping is unimportant */
> > > > +	error = vfs_get_fscaps_nosec(&nop_mnt_idmap, dentry, &caps);
> > > > +	return error == 0;
> > > >  }
> > > >  
> > > >  /**
> > > > @@ -323,7 +324,7 @@ int cap_inode_killpriv(struct mnt_idmap *idmap, struct dentry *dentry)
> > > >  {
> > > >  	int error;
> > > >  
> > > > -	error = __vfs_removexattr(idmap, dentry, XATTR_NAME_CAPS);
> > > > +	error = vfs_remove_fscaps_nosec(idmap, dentry);
> > > 
> > > Uhm, I see that the change is logically correct... but the original
> > > code was not correct, since the EVM post hook is not called (thus the
> > > HMAC is broken, or an xattr change is allowed on a portable signature
> > > which should be not).
> > > 
> > > For completeness, the xattr change on a portable signature should not
> > > happen in the first place, so cap_inode_killpriv() would not be called.
> > > However, since EVM allows same value change, we are here.
> > 
> > I really don't understand EVM that well and am pretty hesitant to try an
> > change any of the logic around it. But I'll hazard a thought: should EVM
> > have a inode_need_killpriv hook which returns an error in this
> > situation?
> 
> Uhm, I think it would not work without modifying
> security_inode_need_killpriv() and the hook definition.
> 
> Since cap_inode_need_killpriv() returns 1, the loop stops and EVM would
> not be invoked. We would need to continue the loop and let EVM know
> what is the current return value. Then EVM can reject the change.
> 
> An alternative way would be to detect that actually we are setting the
> same value for inode metadata, and maybe not returning 1 from
> cap_inode_need_killpriv().
> 
> I would prefer the second, since EVM allows same value change and we
> would have an exception if there are fscaps.
> 
> This solves only the case of portable signatures. We would need to
> change cap_inode_need_killpriv() anyway to update the HMAC for mutable
> files.

I see. In any case this sounds like a matter for a separate patch
series.
Christian Brauner March 5, 2024, 9:12 a.m. UTC | #5
On Mon, Mar 04, 2024 at 10:56:17AM -0600, Seth Forshee (DigitalOcean) wrote:
> On Mon, Mar 04, 2024 at 05:17:57PM +0100, Roberto Sassu wrote:
> > On Mon, 2024-03-04 at 09:31 -0600, Seth Forshee (DigitalOcean) wrote:
> > > On Mon, Mar 04, 2024 at 11:19:54AM +0100, Roberto Sassu wrote:
> > > > On Wed, 2024-02-21 at 15:24 -0600, Seth Forshee (DigitalOcean) wrote:
> > > > > Use the vfs interfaces for fetching file capabilities for killpriv
> > > > > checks and from get_vfs_caps_from_disk(). While there, update the
> > > > > kerneldoc for get_vfs_caps_from_disk() to explain how it is different
> > > > > from vfs_get_fscaps_nosec().
> > > > > 
> > > > > Signed-off-by: Seth Forshee (DigitalOcean) <sforshee@kernel.org>
> > > > > ---
> > > > >  security/commoncap.c | 30 +++++++++++++-----------------
> > > > >  1 file changed, 13 insertions(+), 17 deletions(-)
> > > > > 
> > > > > diff --git a/security/commoncap.c b/security/commoncap.c
> > > > > index a0ff7e6092e0..751bb26a06a6 100644
> > > > > --- a/security/commoncap.c
> > > > > +++ b/security/commoncap.c
> > > > > @@ -296,11 +296,12 @@ int cap_capset(struct cred *new,
> > > > >   */
> > > > >  int cap_inode_need_killpriv(struct dentry *dentry)
> > > > >  {
> > > > > -	struct inode *inode = d_backing_inode(dentry);
> > > > > +	struct vfs_caps caps;
> > > > >  	int error;
> > > > >  
> > > > > -	error = __vfs_getxattr(dentry, inode, XATTR_NAME_CAPS, NULL, 0);
> > > > > -	return error > 0;
> > > > > +	/* Use nop_mnt_idmap for no mapping here as mapping is unimportant */
> > > > > +	error = vfs_get_fscaps_nosec(&nop_mnt_idmap, dentry, &caps);
> > > > > +	return error == 0;
> > > > >  }
> > > > >  
> > > > >  /**
> > > > > @@ -323,7 +324,7 @@ int cap_inode_killpriv(struct mnt_idmap *idmap, struct dentry *dentry)
> > > > >  {
> > > > >  	int error;
> > > > >  
> > > > > -	error = __vfs_removexattr(idmap, dentry, XATTR_NAME_CAPS);
> > > > > +	error = vfs_remove_fscaps_nosec(idmap, dentry);
> > > > 
> > > > Uhm, I see that the change is logically correct... but the original
> > > > code was not correct, since the EVM post hook is not called (thus the
> > > > HMAC is broken, or an xattr change is allowed on a portable signature
> > > > which should be not).
> > > > 
> > > > For completeness, the xattr change on a portable signature should not
> > > > happen in the first place, so cap_inode_killpriv() would not be called.
> > > > However, since EVM allows same value change, we are here.
> > > 
> > > I really don't understand EVM that well and am pretty hesitant to try an
> > > change any of the logic around it. But I'll hazard a thought: should EVM
> > > have a inode_need_killpriv hook which returns an error in this
> > > situation?
> > 
> > Uhm, I think it would not work without modifying
> > security_inode_need_killpriv() and the hook definition.
> > 
> > Since cap_inode_need_killpriv() returns 1, the loop stops and EVM would
> > not be invoked. We would need to continue the loop and let EVM know
> > what is the current return value. Then EVM can reject the change.
> > 
> > An alternative way would be to detect that actually we are setting the
> > same value for inode metadata, and maybe not returning 1 from
> > cap_inode_need_killpriv().
> > 
> > I would prefer the second, since EVM allows same value change and we
> > would have an exception if there are fscaps.
> > 
> > This solves only the case of portable signatures. We would need to
> > change cap_inode_need_killpriv() anyway to update the HMAC for mutable
> > files.
> 
> I see. In any case this sounds like a matter for a separate patch
> series.

Agreed.
Roberto Sassu March 5, 2024, 12:46 p.m. UTC | #6
On Tue, 2024-03-05 at 10:12 +0100, Christian Brauner wrote:
> On Mon, Mar 04, 2024 at 10:56:17AM -0600, Seth Forshee (DigitalOcean) wrote:
> > On Mon, Mar 04, 2024 at 05:17:57PM +0100, Roberto Sassu wrote:
> > > On Mon, 2024-03-04 at 09:31 -0600, Seth Forshee (DigitalOcean) wrote:
> > > > On Mon, Mar 04, 2024 at 11:19:54AM +0100, Roberto Sassu wrote:
> > > > > On Wed, 2024-02-21 at 15:24 -0600, Seth Forshee (DigitalOcean) wrote:
> > > > > > Use the vfs interfaces for fetching file capabilities for killpriv
> > > > > > checks and from get_vfs_caps_from_disk(). While there, update the
> > > > > > kerneldoc for get_vfs_caps_from_disk() to explain how it is different
> > > > > > from vfs_get_fscaps_nosec().
> > > > > > 
> > > > > > Signed-off-by: Seth Forshee (DigitalOcean) <sforshee@kernel.org>
> > > > > > ---
> > > > > >  security/commoncap.c | 30 +++++++++++++-----------------
> > > > > >  1 file changed, 13 insertions(+), 17 deletions(-)
> > > > > > 
> > > > > > diff --git a/security/commoncap.c b/security/commoncap.c
> > > > > > index a0ff7e6092e0..751bb26a06a6 100644
> > > > > > --- a/security/commoncap.c
> > > > > > +++ b/security/commoncap.c
> > > > > > @@ -296,11 +296,12 @@ int cap_capset(struct cred *new,
> > > > > >   */
> > > > > >  int cap_inode_need_killpriv(struct dentry *dentry)
> > > > > >  {
> > > > > > -	struct inode *inode = d_backing_inode(dentry);
> > > > > > +	struct vfs_caps caps;
> > > > > >  	int error;
> > > > > >  
> > > > > > -	error = __vfs_getxattr(dentry, inode, XATTR_NAME_CAPS, NULL, 0);
> > > > > > -	return error > 0;
> > > > > > +	/* Use nop_mnt_idmap for no mapping here as mapping is unimportant */
> > > > > > +	error = vfs_get_fscaps_nosec(&nop_mnt_idmap, dentry, &caps);
> > > > > > +	return error == 0;
> > > > > >  }
> > > > > >  
> > > > > >  /**
> > > > > > @@ -323,7 +324,7 @@ int cap_inode_killpriv(struct mnt_idmap *idmap, struct dentry *dentry)
> > > > > >  {
> > > > > >  	int error;
> > > > > >  
> > > > > > -	error = __vfs_removexattr(idmap, dentry, XATTR_NAME_CAPS);
> > > > > > +	error = vfs_remove_fscaps_nosec(idmap, dentry);
> > > > > 
> > > > > Uhm, I see that the change is logically correct... but the original
> > > > > code was not correct, since the EVM post hook is not called (thus the
> > > > > HMAC is broken, or an xattr change is allowed on a portable signature
> > > > > which should be not).
> > > > > 
> > > > > For completeness, the xattr change on a portable signature should not
> > > > > happen in the first place, so cap_inode_killpriv() would not be called.
> > > > > However, since EVM allows same value change, we are here.
> > > > 
> > > > I really don't understand EVM that well and am pretty hesitant to try an
> > > > change any of the logic around it. But I'll hazard a thought: should EVM
> > > > have a inode_need_killpriv hook which returns an error in this
> > > > situation?
> > > 
> > > Uhm, I think it would not work without modifying
> > > security_inode_need_killpriv() and the hook definition.
> > > 
> > > Since cap_inode_need_killpriv() returns 1, the loop stops and EVM would
> > > not be invoked. We would need to continue the loop and let EVM know
> > > what is the current return value. Then EVM can reject the change.
> > > 
> > > An alternative way would be to detect that actually we are setting the
> > > same value for inode metadata, and maybe not returning 1 from
> > > cap_inode_need_killpriv().
> > > 
> > > I would prefer the second, since EVM allows same value change and we
> > > would have an exception if there are fscaps.
> > > 
> > > This solves only the case of portable signatures. We would need to
> > > change cap_inode_need_killpriv() anyway to update the HMAC for mutable
> > > files.
> > 
> > I see. In any case this sounds like a matter for a separate patch
> > series.
> 
> Agreed.

Christian, how realistic is that we don't kill priv if we are setting
the same owner?

Serge, would we be able to replace __vfs_removexattr() (or now
vfs_get_fscaps_nosec()) with a security-equivalent alternative?

Thanks

Roberto
Christian Brauner March 5, 2024, 4:26 p.m. UTC | #7
On Tue, Mar 05, 2024 at 01:46:56PM +0100, Roberto Sassu wrote:
> On Tue, 2024-03-05 at 10:12 +0100, Christian Brauner wrote:
> > On Mon, Mar 04, 2024 at 10:56:17AM -0600, Seth Forshee (DigitalOcean) wrote:
> > > On Mon, Mar 04, 2024 at 05:17:57PM +0100, Roberto Sassu wrote:
> > > > On Mon, 2024-03-04 at 09:31 -0600, Seth Forshee (DigitalOcean) wrote:
> > > > > On Mon, Mar 04, 2024 at 11:19:54AM +0100, Roberto Sassu wrote:
> > > > > > On Wed, 2024-02-21 at 15:24 -0600, Seth Forshee (DigitalOcean) wrote:
> > > > > > > Use the vfs interfaces for fetching file capabilities for killpriv
> > > > > > > checks and from get_vfs_caps_from_disk(). While there, update the
> > > > > > > kerneldoc for get_vfs_caps_from_disk() to explain how it is different
> > > > > > > from vfs_get_fscaps_nosec().
> > > > > > > 
> > > > > > > Signed-off-by: Seth Forshee (DigitalOcean) <sforshee@kernel.org>
> > > > > > > ---
> > > > > > >  security/commoncap.c | 30 +++++++++++++-----------------
> > > > > > >  1 file changed, 13 insertions(+), 17 deletions(-)
> > > > > > > 
> > > > > > > diff --git a/security/commoncap.c b/security/commoncap.c
> > > > > > > index a0ff7e6092e0..751bb26a06a6 100644
> > > > > > > --- a/security/commoncap.c
> > > > > > > +++ b/security/commoncap.c
> > > > > > > @@ -296,11 +296,12 @@ int cap_capset(struct cred *new,
> > > > > > >   */
> > > > > > >  int cap_inode_need_killpriv(struct dentry *dentry)
> > > > > > >  {
> > > > > > > -	struct inode *inode = d_backing_inode(dentry);
> > > > > > > +	struct vfs_caps caps;
> > > > > > >  	int error;
> > > > > > >  
> > > > > > > -	error = __vfs_getxattr(dentry, inode, XATTR_NAME_CAPS, NULL, 0);
> > > > > > > -	return error > 0;
> > > > > > > +	/* Use nop_mnt_idmap for no mapping here as mapping is unimportant */
> > > > > > > +	error = vfs_get_fscaps_nosec(&nop_mnt_idmap, dentry, &caps);
> > > > > > > +	return error == 0;
> > > > > > >  }
> > > > > > >  
> > > > > > >  /**
> > > > > > > @@ -323,7 +324,7 @@ int cap_inode_killpriv(struct mnt_idmap *idmap, struct dentry *dentry)
> > > > > > >  {
> > > > > > >  	int error;
> > > > > > >  
> > > > > > > -	error = __vfs_removexattr(idmap, dentry, XATTR_NAME_CAPS);
> > > > > > > +	error = vfs_remove_fscaps_nosec(idmap, dentry);
> > > > > > 
> > > > > > Uhm, I see that the change is logically correct... but the original
> > > > > > code was not correct, since the EVM post hook is not called (thus the
> > > > > > HMAC is broken, or an xattr change is allowed on a portable signature
> > > > > > which should be not).
> > > > > > 
> > > > > > For completeness, the xattr change on a portable signature should not
> > > > > > happen in the first place, so cap_inode_killpriv() would not be called.
> > > > > > However, since EVM allows same value change, we are here.
> > > > > 
> > > > > I really don't understand EVM that well and am pretty hesitant to try an
> > > > > change any of the logic around it. But I'll hazard a thought: should EVM
> > > > > have a inode_need_killpriv hook which returns an error in this
> > > > > situation?
> > > > 
> > > > Uhm, I think it would not work without modifying
> > > > security_inode_need_killpriv() and the hook definition.
> > > > 
> > > > Since cap_inode_need_killpriv() returns 1, the loop stops and EVM would
> > > > not be invoked. We would need to continue the loop and let EVM know
> > > > what is the current return value. Then EVM can reject the change.
> > > > 
> > > > An alternative way would be to detect that actually we are setting the
> > > > same value for inode metadata, and maybe not returning 1 from
> > > > cap_inode_need_killpriv().
> > > > 
> > > > I would prefer the second, since EVM allows same value change and we
> > > > would have an exception if there are fscaps.
> > > > 
> > > > This solves only the case of portable signatures. We would need to
> > > > change cap_inode_need_killpriv() anyway to update the HMAC for mutable
> > > > files.
> > > 
> > > I see. In any case this sounds like a matter for a separate patch
> > > series.
> > 
> > Agreed.
> 
> Christian, how realistic is that we don't kill priv if we are setting
> the same owner?

Uhm, I would need to see the wider context of the proposed change. But
iiuc then you would be comparing current and new fscaps and if they are
identical you don't kill privs? I think that would work. But again, I
would need to see the actual context/change to say something meaningful.
Roberto Sassu March 5, 2024, 4:35 p.m. UTC | #8
On Tue, 2024-03-05 at 17:26 +0100, Christian Brauner wrote:
> On Tue, Mar 05, 2024 at 01:46:56PM +0100, Roberto Sassu wrote:
> > On Tue, 2024-03-05 at 10:12 +0100, Christian Brauner wrote:
> > > On Mon, Mar 04, 2024 at 10:56:17AM -0600, Seth Forshee (DigitalOcean) wrote:
> > > > On Mon, Mar 04, 2024 at 05:17:57PM +0100, Roberto Sassu wrote:
> > > > > On Mon, 2024-03-04 at 09:31 -0600, Seth Forshee (DigitalOcean) wrote:
> > > > > > On Mon, Mar 04, 2024 at 11:19:54AM +0100, Roberto Sassu wrote:
> > > > > > > On Wed, 2024-02-21 at 15:24 -0600, Seth Forshee (DigitalOcean) wrote:
> > > > > > > > Use the vfs interfaces for fetching file capabilities for killpriv
> > > > > > > > checks and from get_vfs_caps_from_disk(). While there, update the
> > > > > > > > kerneldoc for get_vfs_caps_from_disk() to explain how it is different
> > > > > > > > from vfs_get_fscaps_nosec().
> > > > > > > > 
> > > > > > > > Signed-off-by: Seth Forshee (DigitalOcean) <sforshee@kernel.org>
> > > > > > > > ---
> > > > > > > >  security/commoncap.c | 30 +++++++++++++-----------------
> > > > > > > >  1 file changed, 13 insertions(+), 17 deletions(-)
> > > > > > > > 
> > > > > > > > diff --git a/security/commoncap.c b/security/commoncap.c
> > > > > > > > index a0ff7e6092e0..751bb26a06a6 100644
> > > > > > > > --- a/security/commoncap.c
> > > > > > > > +++ b/security/commoncap.c
> > > > > > > > @@ -296,11 +296,12 @@ int cap_capset(struct cred *new,
> > > > > > > >   */
> > > > > > > >  int cap_inode_need_killpriv(struct dentry *dentry)
> > > > > > > >  {
> > > > > > > > -	struct inode *inode = d_backing_inode(dentry);
> > > > > > > > +	struct vfs_caps caps;
> > > > > > > >  	int error;
> > > > > > > >  
> > > > > > > > -	error = __vfs_getxattr(dentry, inode, XATTR_NAME_CAPS, NULL, 0);
> > > > > > > > -	return error > 0;
> > > > > > > > +	/* Use nop_mnt_idmap for no mapping here as mapping is unimportant */
> > > > > > > > +	error = vfs_get_fscaps_nosec(&nop_mnt_idmap, dentry, &caps);
> > > > > > > > +	return error == 0;
> > > > > > > >  }
> > > > > > > >  
> > > > > > > >  /**
> > > > > > > > @@ -323,7 +324,7 @@ int cap_inode_killpriv(struct mnt_idmap *idmap, struct dentry *dentry)
> > > > > > > >  {
> > > > > > > >  	int error;
> > > > > > > >  
> > > > > > > > -	error = __vfs_removexattr(idmap, dentry, XATTR_NAME_CAPS);
> > > > > > > > +	error = vfs_remove_fscaps_nosec(idmap, dentry);
> > > > > > > 
> > > > > > > Uhm, I see that the change is logically correct... but the original
> > > > > > > code was not correct, since the EVM post hook is not called (thus the
> > > > > > > HMAC is broken, or an xattr change is allowed on a portable signature
> > > > > > > which should be not).
> > > > > > > 
> > > > > > > For completeness, the xattr change on a portable signature should not
> > > > > > > happen in the first place, so cap_inode_killpriv() would not be called.
> > > > > > > However, since EVM allows same value change, we are here.
> > > > > > 
> > > > > > I really don't understand EVM that well and am pretty hesitant to try an
> > > > > > change any of the logic around it. But I'll hazard a thought: should EVM
> > > > > > have a inode_need_killpriv hook which returns an error in this
> > > > > > situation?
> > > > > 
> > > > > Uhm, I think it would not work without modifying
> > > > > security_inode_need_killpriv() and the hook definition.
> > > > > 
> > > > > Since cap_inode_need_killpriv() returns 1, the loop stops and EVM would
> > > > > not be invoked. We would need to continue the loop and let EVM know
> > > > > what is the current return value. Then EVM can reject the change.
> > > > > 
> > > > > An alternative way would be to detect that actually we are setting the
> > > > > same value for inode metadata, and maybe not returning 1 from
> > > > > cap_inode_need_killpriv().
> > > > > 
> > > > > I would prefer the second, since EVM allows same value change and we
> > > > > would have an exception if there are fscaps.
> > > > > 
> > > > > This solves only the case of portable signatures. We would need to
> > > > > change cap_inode_need_killpriv() anyway to update the HMAC for mutable
> > > > > files.
> > > > 
> > > > I see. In any case this sounds like a matter for a separate patch
> > > > series.
> > > 
> > > Agreed.
> > 
> > Christian, how realistic is that we don't kill priv if we are setting
> > the same owner?
> 
> Uhm, I would need to see the wider context of the proposed change. But
> iiuc then you would be comparing current and new fscaps and if they are
> identical you don't kill privs? I think that would work. But again, I
> would need to see the actual context/change to say something meaningful.

Ok, basically a software vendor can ship binaries with a signature over
file metadata, including UID/GID, etc.

A system can verify the signature through the public key of the
software vendor.

The problem is if someone (or even tar), executes chown on that binary,
fscaps are lost. Thus, signature verification will fail from now on.

EVM locks file metadata as soon as signature verification succeeds
(i.e. metadata are the same of those signed by the software vendor).

EVM locking works if someone is trying to set different metadata. But,
if I try to chown to the same owner as the one stored in the inode, EVM
allows it but the capability LSM removes security.capability, thus
invalidating the signature.

At least, it would be desirable that security.capability is not removed
when setting the same owner. If the owner is different, EVM will handle
that.

Roberto
Seth Forshee (DigitalOcean) March 5, 2024, 5:03 p.m. UTC | #9
On Tue, Mar 05, 2024 at 05:35:11PM +0100, Roberto Sassu wrote:
> On Tue, 2024-03-05 at 17:26 +0100, Christian Brauner wrote:
> > On Tue, Mar 05, 2024 at 01:46:56PM +0100, Roberto Sassu wrote:
> > > On Tue, 2024-03-05 at 10:12 +0100, Christian Brauner wrote:
> > > > On Mon, Mar 04, 2024 at 10:56:17AM -0600, Seth Forshee (DigitalOcean) wrote:
> > > > > On Mon, Mar 04, 2024 at 05:17:57PM +0100, Roberto Sassu wrote:
> > > > > > On Mon, 2024-03-04 at 09:31 -0600, Seth Forshee (DigitalOcean) wrote:
> > > > > > > On Mon, Mar 04, 2024 at 11:19:54AM +0100, Roberto Sassu wrote:
> > > > > > > > On Wed, 2024-02-21 at 15:24 -0600, Seth Forshee (DigitalOcean) wrote:
> > > > > > > > > Use the vfs interfaces for fetching file capabilities for killpriv
> > > > > > > > > checks and from get_vfs_caps_from_disk(). While there, update the
> > > > > > > > > kerneldoc for get_vfs_caps_from_disk() to explain how it is different
> > > > > > > > > from vfs_get_fscaps_nosec().
> > > > > > > > > 
> > > > > > > > > Signed-off-by: Seth Forshee (DigitalOcean) <sforshee@kernel.org>
> > > > > > > > > ---
> > > > > > > > >  security/commoncap.c | 30 +++++++++++++-----------------
> > > > > > > > >  1 file changed, 13 insertions(+), 17 deletions(-)
> > > > > > > > > 
> > > > > > > > > diff --git a/security/commoncap.c b/security/commoncap.c
> > > > > > > > > index a0ff7e6092e0..751bb26a06a6 100644
> > > > > > > > > --- a/security/commoncap.c
> > > > > > > > > +++ b/security/commoncap.c
> > > > > > > > > @@ -296,11 +296,12 @@ int cap_capset(struct cred *new,
> > > > > > > > >   */
> > > > > > > > >  int cap_inode_need_killpriv(struct dentry *dentry)
> > > > > > > > >  {
> > > > > > > > > -	struct inode *inode = d_backing_inode(dentry);
> > > > > > > > > +	struct vfs_caps caps;
> > > > > > > > >  	int error;
> > > > > > > > >  
> > > > > > > > > -	error = __vfs_getxattr(dentry, inode, XATTR_NAME_CAPS, NULL, 0);
> > > > > > > > > -	return error > 0;
> > > > > > > > > +	/* Use nop_mnt_idmap for no mapping here as mapping is unimportant */
> > > > > > > > > +	error = vfs_get_fscaps_nosec(&nop_mnt_idmap, dentry, &caps);
> > > > > > > > > +	return error == 0;
> > > > > > > > >  }
> > > > > > > > >  
> > > > > > > > >  /**
> > > > > > > > > @@ -323,7 +324,7 @@ int cap_inode_killpriv(struct mnt_idmap *idmap, struct dentry *dentry)
> > > > > > > > >  {
> > > > > > > > >  	int error;
> > > > > > > > >  
> > > > > > > > > -	error = __vfs_removexattr(idmap, dentry, XATTR_NAME_CAPS);
> > > > > > > > > +	error = vfs_remove_fscaps_nosec(idmap, dentry);
> > > > > > > > 
> > > > > > > > Uhm, I see that the change is logically correct... but the original
> > > > > > > > code was not correct, since the EVM post hook is not called (thus the
> > > > > > > > HMAC is broken, or an xattr change is allowed on a portable signature
> > > > > > > > which should be not).
> > > > > > > > 
> > > > > > > > For completeness, the xattr change on a portable signature should not
> > > > > > > > happen in the first place, so cap_inode_killpriv() would not be called.
> > > > > > > > However, since EVM allows same value change, we are here.
> > > > > > > 
> > > > > > > I really don't understand EVM that well and am pretty hesitant to try an
> > > > > > > change any of the logic around it. But I'll hazard a thought: should EVM
> > > > > > > have a inode_need_killpriv hook which returns an error in this
> > > > > > > situation?
> > > > > > 
> > > > > > Uhm, I think it would not work without modifying
> > > > > > security_inode_need_killpriv() and the hook definition.
> > > > > > 
> > > > > > Since cap_inode_need_killpriv() returns 1, the loop stops and EVM would
> > > > > > not be invoked. We would need to continue the loop and let EVM know
> > > > > > what is the current return value. Then EVM can reject the change.
> > > > > > 
> > > > > > An alternative way would be to detect that actually we are setting the
> > > > > > same value for inode metadata, and maybe not returning 1 from
> > > > > > cap_inode_need_killpriv().
> > > > > > 
> > > > > > I would prefer the second, since EVM allows same value change and we
> > > > > > would have an exception if there are fscaps.
> > > > > > 
> > > > > > This solves only the case of portable signatures. We would need to
> > > > > > change cap_inode_need_killpriv() anyway to update the HMAC for mutable
> > > > > > files.
> > > > > 
> > > > > I see. In any case this sounds like a matter for a separate patch
> > > > > series.
> > > > 
> > > > Agreed.
> > > 
> > > Christian, how realistic is that we don't kill priv if we are setting
> > > the same owner?
> > 
> > Uhm, I would need to see the wider context of the proposed change. But
> > iiuc then you would be comparing current and new fscaps and if they are
> > identical you don't kill privs? I think that would work. But again, I
> > would need to see the actual context/change to say something meaningful.
> 
> Ok, basically a software vendor can ship binaries with a signature over
> file metadata, including UID/GID, etc.
> 
> A system can verify the signature through the public key of the
> software vendor.
> 
> The problem is if someone (or even tar), executes chown on that binary,
> fscaps are lost. Thus, signature verification will fail from now on.
> 
> EVM locks file metadata as soon as signature verification succeeds
> (i.e. metadata are the same of those signed by the software vendor).
> 
> EVM locking works if someone is trying to set different metadata. But,
> if I try to chown to the same owner as the one stored in the inode, EVM
> allows it but the capability LSM removes security.capability, thus
> invalidating the signature.
> 
> At least, it would be desirable that security.capability is not removed
> when setting the same owner. If the owner is different, EVM will handle
> that.

When you say EVM "locks" file metadata, does that mean it prevents
modification to file metadata?

What about changes to file data? This will also result in removing
fscaps xattrs. Does EVM also block changes to file data when signature
verification succeeds?
Roberto Sassu March 5, 2024, 5:08 p.m. UTC | #10
On Tue, 2024-03-05 at 11:03 -0600, Seth Forshee (DigitalOcean) wrote:
> On Tue, Mar 05, 2024 at 05:35:11PM +0100, Roberto Sassu wrote:
> > On Tue, 2024-03-05 at 17:26 +0100, Christian Brauner wrote:
> > > On Tue, Mar 05, 2024 at 01:46:56PM +0100, Roberto Sassu wrote:
> > > > On Tue, 2024-03-05 at 10:12 +0100, Christian Brauner wrote:
> > > > > On Mon, Mar 04, 2024 at 10:56:17AM -0600, Seth Forshee (DigitalOcean) wrote:
> > > > > > On Mon, Mar 04, 2024 at 05:17:57PM +0100, Roberto Sassu wrote:
> > > > > > > On Mon, 2024-03-04 at 09:31 -0600, Seth Forshee (DigitalOcean) wrote:
> > > > > > > > On Mon, Mar 04, 2024 at 11:19:54AM +0100, Roberto Sassu wrote:
> > > > > > > > > On Wed, 2024-02-21 at 15:24 -0600, Seth Forshee (DigitalOcean) wrote:
> > > > > > > > > > Use the vfs interfaces for fetching file capabilities for killpriv
> > > > > > > > > > checks and from get_vfs_caps_from_disk(). While there, update the
> > > > > > > > > > kerneldoc for get_vfs_caps_from_disk() to explain how it is different
> > > > > > > > > > from vfs_get_fscaps_nosec().
> > > > > > > > > > 
> > > > > > > > > > Signed-off-by: Seth Forshee (DigitalOcean) <sforshee@kernel.org>
> > > > > > > > > > ---
> > > > > > > > > >  security/commoncap.c | 30 +++++++++++++-----------------
> > > > > > > > > >  1 file changed, 13 insertions(+), 17 deletions(-)
> > > > > > > > > > 
> > > > > > > > > > diff --git a/security/commoncap.c b/security/commoncap.c
> > > > > > > > > > index a0ff7e6092e0..751bb26a06a6 100644
> > > > > > > > > > --- a/security/commoncap.c
> > > > > > > > > > +++ b/security/commoncap.c
> > > > > > > > > > @@ -296,11 +296,12 @@ int cap_capset(struct cred *new,
> > > > > > > > > >   */
> > > > > > > > > >  int cap_inode_need_killpriv(struct dentry *dentry)
> > > > > > > > > >  {
> > > > > > > > > > -	struct inode *inode = d_backing_inode(dentry);
> > > > > > > > > > +	struct vfs_caps caps;
> > > > > > > > > >  	int error;
> > > > > > > > > >  
> > > > > > > > > > -	error = __vfs_getxattr(dentry, inode, XATTR_NAME_CAPS, NULL, 0);
> > > > > > > > > > -	return error > 0;
> > > > > > > > > > +	/* Use nop_mnt_idmap for no mapping here as mapping is unimportant */
> > > > > > > > > > +	error = vfs_get_fscaps_nosec(&nop_mnt_idmap, dentry, &caps);
> > > > > > > > > > +	return error == 0;
> > > > > > > > > >  }
> > > > > > > > > >  
> > > > > > > > > >  /**
> > > > > > > > > > @@ -323,7 +324,7 @@ int cap_inode_killpriv(struct mnt_idmap *idmap, struct dentry *dentry)
> > > > > > > > > >  {
> > > > > > > > > >  	int error;
> > > > > > > > > >  
> > > > > > > > > > -	error = __vfs_removexattr(idmap, dentry, XATTR_NAME_CAPS);
> > > > > > > > > > +	error = vfs_remove_fscaps_nosec(idmap, dentry);
> > > > > > > > > 
> > > > > > > > > Uhm, I see that the change is logically correct... but the original
> > > > > > > > > code was not correct, since the EVM post hook is not called (thus the
> > > > > > > > > HMAC is broken, or an xattr change is allowed on a portable signature
> > > > > > > > > which should be not).
> > > > > > > > > 
> > > > > > > > > For completeness, the xattr change on a portable signature should not
> > > > > > > > > happen in the first place, so cap_inode_killpriv() would not be called.
> > > > > > > > > However, since EVM allows same value change, we are here.
> > > > > > > > 
> > > > > > > > I really don't understand EVM that well and am pretty hesitant to try an
> > > > > > > > change any of the logic around it. But I'll hazard a thought: should EVM
> > > > > > > > have a inode_need_killpriv hook which returns an error in this
> > > > > > > > situation?
> > > > > > > 
> > > > > > > Uhm, I think it would not work without modifying
> > > > > > > security_inode_need_killpriv() and the hook definition.
> > > > > > > 
> > > > > > > Since cap_inode_need_killpriv() returns 1, the loop stops and EVM would
> > > > > > > not be invoked. We would need to continue the loop and let EVM know
> > > > > > > what is the current return value. Then EVM can reject the change.
> > > > > > > 
> > > > > > > An alternative way would be to detect that actually we are setting the
> > > > > > > same value for inode metadata, and maybe not returning 1 from
> > > > > > > cap_inode_need_killpriv().
> > > > > > > 
> > > > > > > I would prefer the second, since EVM allows same value change and we
> > > > > > > would have an exception if there are fscaps.
> > > > > > > 
> > > > > > > This solves only the case of portable signatures. We would need to
> > > > > > > change cap_inode_need_killpriv() anyway to update the HMAC for mutable
> > > > > > > files.
> > > > > > 
> > > > > > I see. In any case this sounds like a matter for a separate patch
> > > > > > series.
> > > > > 
> > > > > Agreed.
> > > > 
> > > > Christian, how realistic is that we don't kill priv if we are setting
> > > > the same owner?
> > > 
> > > Uhm, I would need to see the wider context of the proposed change. But
> > > iiuc then you would be comparing current and new fscaps and if they are
> > > identical you don't kill privs? I think that would work. But again, I
> > > would need to see the actual context/change to say something meaningful.
> > 
> > Ok, basically a software vendor can ship binaries with a signature over
> > file metadata, including UID/GID, etc.
> > 
> > A system can verify the signature through the public key of the
> > software vendor.
> > 
> > The problem is if someone (or even tar), executes chown on that binary,
> > fscaps are lost. Thus, signature verification will fail from now on.
> > 
> > EVM locks file metadata as soon as signature verification succeeds
> > (i.e. metadata are the same of those signed by the software vendor).
> > 
> > EVM locking works if someone is trying to set different metadata. But,
> > if I try to chown to the same owner as the one stored in the inode, EVM
> > allows it but the capability LSM removes security.capability, thus
> > invalidating the signature.
> > 
> > At least, it would be desirable that security.capability is not removed
> > when setting the same owner. If the owner is different, EVM will handle
> > that.
> 
> When you say EVM "locks" file metadata, does that mean it prevents
> modification to file metadata?

Yes, but only when metadata are in the final state (what the software
vendor signed). Otherwise, modifications are still allowed.

That was needed to let tar set everything (xattrs and attrs).

> What about changes to file data? This will also result in removing
> fscaps xattrs. Does EVM also block changes to file data when signature
> verification succeeds?

That would be the job of IMA. EVM would not be able to detect that. If
the file has an EVM immutable and portable signature, IMA denies
changes to it (assuming that an IMA appraisal policy was loaded, and
that file matches a policy rule).

Roberto
Roberto Sassu March 5, 2024, 5:11 p.m. UTC | #11
On Tue, 2024-03-05 at 13:46 +0100, Roberto Sassu wrote:
> On Tue, 2024-03-05 at 10:12 +0100, Christian Brauner wrote:
> > On Mon, Mar 04, 2024 at 10:56:17AM -0600, Seth Forshee (DigitalOcean) wrote:
> > > On Mon, Mar 04, 2024 at 05:17:57PM +0100, Roberto Sassu wrote:
> > > > On Mon, 2024-03-04 at 09:31 -0600, Seth Forshee (DigitalOcean) wrote:
> > > > > On Mon, Mar 04, 2024 at 11:19:54AM +0100, Roberto Sassu wrote:
> > > > > > On Wed, 2024-02-21 at 15:24 -0600, Seth Forshee (DigitalOcean) wrote:
> > > > > > > Use the vfs interfaces for fetching file capabilities for killpriv
> > > > > > > checks and from get_vfs_caps_from_disk(). While there, update the
> > > > > > > kerneldoc for get_vfs_caps_from_disk() to explain how it is different
> > > > > > > from vfs_get_fscaps_nosec().
> > > > > > > 
> > > > > > > Signed-off-by: Seth Forshee (DigitalOcean) <sforshee@kernel.org>
> > > > > > > ---
> > > > > > >  security/commoncap.c | 30 +++++++++++++-----------------
> > > > > > >  1 file changed, 13 insertions(+), 17 deletions(-)
> > > > > > > 
> > > > > > > diff --git a/security/commoncap.c b/security/commoncap.c
> > > > > > > index a0ff7e6092e0..751bb26a06a6 100644
> > > > > > > --- a/security/commoncap.c
> > > > > > > +++ b/security/commoncap.c
> > > > > > > @@ -296,11 +296,12 @@ int cap_capset(struct cred *new,
> > > > > > >   */
> > > > > > >  int cap_inode_need_killpriv(struct dentry *dentry)
> > > > > > >  {
> > > > > > > -	struct inode *inode = d_backing_inode(dentry);
> > > > > > > +	struct vfs_caps caps;
> > > > > > >  	int error;
> > > > > > >  
> > > > > > > -	error = __vfs_getxattr(dentry, inode, XATTR_NAME_CAPS, NULL, 0);
> > > > > > > -	return error > 0;
> > > > > > > +	/* Use nop_mnt_idmap for no mapping here as mapping is unimportant */
> > > > > > > +	error = vfs_get_fscaps_nosec(&nop_mnt_idmap, dentry, &caps);
> > > > > > > +	return error == 0;
> > > > > > >  }
> > > > > > >  
> > > > > > >  /**
> > > > > > > @@ -323,7 +324,7 @@ int cap_inode_killpriv(struct mnt_idmap *idmap, struct dentry *dentry)
> > > > > > >  {
> > > > > > >  	int error;
> > > > > > >  
> > > > > > > -	error = __vfs_removexattr(idmap, dentry, XATTR_NAME_CAPS);
> > > > > > > +	error = vfs_remove_fscaps_nosec(idmap, dentry);
> > > > > > 
> > > > > > Uhm, I see that the change is logically correct... but the original
> > > > > > code was not correct, since the EVM post hook is not called (thus the
> > > > > > HMAC is broken, or an xattr change is allowed on a portable signature
> > > > > > which should be not).
> > > > > > 
> > > > > > For completeness, the xattr change on a portable signature should not
> > > > > > happen in the first place, so cap_inode_killpriv() would not be called.
> > > > > > However, since EVM allows same value change, we are here.
> > > > > 
> > > > > I really don't understand EVM that well and am pretty hesitant to try an
> > > > > change any of the logic around it. But I'll hazard a thought: should EVM
> > > > > have a inode_need_killpriv hook which returns an error in this
> > > > > situation?
> > > > 
> > > > Uhm, I think it would not work without modifying
> > > > security_inode_need_killpriv() and the hook definition.
> > > > 
> > > > Since cap_inode_need_killpriv() returns 1, the loop stops and EVM would
> > > > not be invoked. We would need to continue the loop and let EVM know
> > > > what is the current return value. Then EVM can reject the change.
> > > > 
> > > > An alternative way would be to detect that actually we are setting the
> > > > same value for inode metadata, and maybe not returning 1 from
> > > > cap_inode_need_killpriv().
> > > > 
> > > > I would prefer the second, since EVM allows same value change and we
> > > > would have an exception if there are fscaps.
> > > > 
> > > > This solves only the case of portable signatures. We would need to
> > > > change cap_inode_need_killpriv() anyway to update the HMAC for mutable
> > > > files.
> > > 
> > > I see. In any case this sounds like a matter for a separate patch
> > > series.
> > 
> > Agreed.
> 
> Christian, how realistic is that we don't kill priv if we are setting
> the same owner?
> 
> Serge, would we be able to replace __vfs_removexattr() (or now
> vfs_get_fscaps_nosec()) with a security-equivalent alternative?

It seems it is not necessary.

security.capability removal occurs between evm_inode_setattr() and
evm_inode_post_setattr(), after the HMAC has been verified and before
the new HMAC is recalculated (without security.capability).

So, all good.

Christian, Seth, I pushed the kernel and the updated tests (all patches
are WIP):

https://github.com/robertosassu/linux/commits/evm-fscaps-v2/

https://github.com/robertosassu/ima-evm-utils/commits/evm-fscaps-v2/


The tests are passing:

https://github.com/robertosassu/ima-evm-utils/actions/runs/8159877004/job/22305521359

Roberto
Seth Forshee (DigitalOcean) March 5, 2024, 8:17 p.m. UTC | #12
On Tue, Mar 05, 2024 at 06:11:45PM +0100, Roberto Sassu wrote:
> On Tue, 2024-03-05 at 13:46 +0100, Roberto Sassu wrote:
> > On Tue, 2024-03-05 at 10:12 +0100, Christian Brauner wrote:
> > > On Mon, Mar 04, 2024 at 10:56:17AM -0600, Seth Forshee (DigitalOcean) wrote:
> > > > On Mon, Mar 04, 2024 at 05:17:57PM +0100, Roberto Sassu wrote:
> > > > > On Mon, 2024-03-04 at 09:31 -0600, Seth Forshee (DigitalOcean) wrote:
> > > > > > On Mon, Mar 04, 2024 at 11:19:54AM +0100, Roberto Sassu wrote:
> > > > > > > On Wed, 2024-02-21 at 15:24 -0600, Seth Forshee (DigitalOcean) wrote:
> > > > > > > > Use the vfs interfaces for fetching file capabilities for killpriv
> > > > > > > > checks and from get_vfs_caps_from_disk(). While there, update the
> > > > > > > > kerneldoc for get_vfs_caps_from_disk() to explain how it is different
> > > > > > > > from vfs_get_fscaps_nosec().
> > > > > > > > 
> > > > > > > > Signed-off-by: Seth Forshee (DigitalOcean) <sforshee@kernel.org>
> > > > > > > > ---
> > > > > > > >  security/commoncap.c | 30 +++++++++++++-----------------
> > > > > > > >  1 file changed, 13 insertions(+), 17 deletions(-)
> > > > > > > > 
> > > > > > > > diff --git a/security/commoncap.c b/security/commoncap.c
> > > > > > > > index a0ff7e6092e0..751bb26a06a6 100644
> > > > > > > > --- a/security/commoncap.c
> > > > > > > > +++ b/security/commoncap.c
> > > > > > > > @@ -296,11 +296,12 @@ int cap_capset(struct cred *new,
> > > > > > > >   */
> > > > > > > >  int cap_inode_need_killpriv(struct dentry *dentry)
> > > > > > > >  {
> > > > > > > > -	struct inode *inode = d_backing_inode(dentry);
> > > > > > > > +	struct vfs_caps caps;
> > > > > > > >  	int error;
> > > > > > > >  
> > > > > > > > -	error = __vfs_getxattr(dentry, inode, XATTR_NAME_CAPS, NULL, 0);
> > > > > > > > -	return error > 0;
> > > > > > > > +	/* Use nop_mnt_idmap for no mapping here as mapping is unimportant */
> > > > > > > > +	error = vfs_get_fscaps_nosec(&nop_mnt_idmap, dentry, &caps);
> > > > > > > > +	return error == 0;
> > > > > > > >  }
> > > > > > > >  
> > > > > > > >  /**
> > > > > > > > @@ -323,7 +324,7 @@ int cap_inode_killpriv(struct mnt_idmap *idmap, struct dentry *dentry)
> > > > > > > >  {
> > > > > > > >  	int error;
> > > > > > > >  
> > > > > > > > -	error = __vfs_removexattr(idmap, dentry, XATTR_NAME_CAPS);
> > > > > > > > +	error = vfs_remove_fscaps_nosec(idmap, dentry);
> > > > > > > 
> > > > > > > Uhm, I see that the change is logically correct... but the original
> > > > > > > code was not correct, since the EVM post hook is not called (thus the
> > > > > > > HMAC is broken, or an xattr change is allowed on a portable signature
> > > > > > > which should be not).
> > > > > > > 
> > > > > > > For completeness, the xattr change on a portable signature should not
> > > > > > > happen in the first place, so cap_inode_killpriv() would not be called.
> > > > > > > However, since EVM allows same value change, we are here.
> > > > > > 
> > > > > > I really don't understand EVM that well and am pretty hesitant to try an
> > > > > > change any of the logic around it. But I'll hazard a thought: should EVM
> > > > > > have a inode_need_killpriv hook which returns an error in this
> > > > > > situation?
> > > > > 
> > > > > Uhm, I think it would not work without modifying
> > > > > security_inode_need_killpriv() and the hook definition.
> > > > > 
> > > > > Since cap_inode_need_killpriv() returns 1, the loop stops and EVM would
> > > > > not be invoked. We would need to continue the loop and let EVM know
> > > > > what is the current return value. Then EVM can reject the change.
> > > > > 
> > > > > An alternative way would be to detect that actually we are setting the
> > > > > same value for inode metadata, and maybe not returning 1 from
> > > > > cap_inode_need_killpriv().
> > > > > 
> > > > > I would prefer the second, since EVM allows same value change and we
> > > > > would have an exception if there are fscaps.
> > > > > 
> > > > > This solves only the case of portable signatures. We would need to
> > > > > change cap_inode_need_killpriv() anyway to update the HMAC for mutable
> > > > > files.
> > > > 
> > > > I see. In any case this sounds like a matter for a separate patch
> > > > series.
> > > 
> > > Agreed.
> > 
> > Christian, how realistic is that we don't kill priv if we are setting
> > the same owner?
> > 
> > Serge, would we be able to replace __vfs_removexattr() (or now
> > vfs_get_fscaps_nosec()) with a security-equivalent alternative?
> 
> It seems it is not necessary.
> 
> security.capability removal occurs between evm_inode_setattr() and
> evm_inode_post_setattr(), after the HMAC has been verified and before
> the new HMAC is recalculated (without security.capability).
> 
> So, all good.
> 
> Christian, Seth, I pushed the kernel and the updated tests (all patches
> are WIP):
> 
> https://github.com/robertosassu/linux/commits/evm-fscaps-v2/
> 
> https://github.com/robertosassu/ima-evm-utils/commits/evm-fscaps-v2/
> 
> 
> The tests are passing:
> 
> https://github.com/robertosassu/ima-evm-utils/actions/runs/8159877004/job/22305521359

Thanks! I probably won't be able to take them exactly as-is due to other
changes for the next version (rebasing onto the changes to make IMA and
EVM LSMs, forbidding xattr handlers entirely for fscaps), but they will
serve as a good road map for what needs to happen.
Mimi Zohar March 6, 2024, 2:17 a.m. UTC | #13
On Tue, 2024-03-05 at 18:11 +0100, Roberto Sassu wrote:
> On Tue, 2024-03-05 at 13:46 +0100, Roberto Sassu wrote:
> > On Tue, 2024-03-05 at 10:12 +0100, Christian Brauner wrote:
> > > On Mon, Mar 04, 2024 at 10:56:17AM -0600, Seth Forshee (DigitalOcean)
> > > wrote:
> > > > On Mon, Mar 04, 2024 at 05:17:57PM +0100, Roberto Sassu wrote:
> > > > > On Mon, 2024-03-04 at 09:31 -0600, Seth Forshee (DigitalOcean) wrote:
> > > > > > On Mon, Mar 04, 2024 at 11:19:54AM +0100, Roberto Sassu wrote:
> > > > > > > On Wed, 2024-02-21 at 15:24 -0600, Seth Forshee (DigitalOcean)
> > > > > > > wrote:
> > > > > > > > Use the vfs interfaces for fetching file capabilities for
> > > > > > > > killpriv
> > > > > > > > checks and from get_vfs_caps_from_disk(). While there, update
> > > > > > > > the
> > > > > > > > kerneldoc for get_vfs_caps_from_disk() to explain how it is
> > > > > > > > different
> > > > > > > > from vfs_get_fscaps_nosec().
> > > > > > > > 
> > > > > > > > Signed-off-by: Seth Forshee (DigitalOcean) <sforshee@kernel.org>
> > > > > > > > ---
> > > > > > > >  security/commoncap.c | 30 +++++++++++++-----------------
> > > > > > > >  1 file changed, 13 insertions(+), 17 deletions(-)
> > > > > > > > 
> > > > > > > > diff --git a/security/commoncap.c b/security/commoncap.c
> > > > > > > > index a0ff7e6092e0..751bb26a06a6 100644
> > > > > > > > --- a/security/commoncap.c
> > > > > > > > +++ b/security/commoncap.c
> > > > > > > > @@ -296,11 +296,12 @@ int cap_capset(struct cred *new,
> > > > > > > >   */
> > > > > > > >  int cap_inode_need_killpriv(struct dentry *dentry)
> > > > > > > >  {
> > > > > > > > -	struct inode *inode = d_backing_inode(dentry);
> > > > > > > > +	struct vfs_caps caps;
> > > > > > > >  	int error;
> > > > > > > >  
> > > > > > > > -	error = __vfs_getxattr(dentry, inode, XATTR_NAME_CAPS,
> > > > > > > > NULL, 0);
> > > > > > > > -	return error > 0;
> > > > > > > > +	/* Use nop_mnt_idmap for no mapping here as mapping is
> > > > > > > > unimportant */
> > > > > > > > +	error = vfs_get_fscaps_nosec(&nop_mnt_idmap, dentry,
> > > > > > > > &caps);
> > > > > > > > +	return error == 0;
> > > > > > > >  }
> > > > > > > >  
> > > > > > > >  /**
> > > > > > > > @@ -323,7 +324,7 @@ int cap_inode_killpriv(struct mnt_idmap
> > > > > > > > *idmap, struct dentry *dentry)
> > > > > > > >  {
> > > > > > > >  	int error;
> > > > > > > >  
> > > > > > > > -	error = __vfs_removexattr(idmap, dentry,
> > > > > > > > XATTR_NAME_CAPS);
> > > > > > > > +	error = vfs_remove_fscaps_nosec(idmap, dentry);
> > > > > > > 
> > > > > > > Uhm, I see that the change is logically correct... but the
> > > > > > > original
> > > > > > > code was not correct, since the EVM post hook is not called (thus
> > > > > > > the
> > > > > > > HMAC is broken, or an xattr change is allowed on a portable
> > > > > > > signature
> > > > > > > which should be not).
> > > > > > > 
> > > > > > > For completeness, the xattr change on a portable signature should
> > > > > > > not
> > > > > > > happen in the first place, so cap_inode_killpriv() would not be
> > > > > > > called.
> > > > > > > However, since EVM allows same value change, we are here.
> > > > > > 
> > > > > > I really don't understand EVM that well and am pretty hesitant to
> > > > > > try an
> > > > > > change any of the logic around it. But I'll hazard a thought: should
> > > > > > EVM
> > > > > > have a inode_need_killpriv hook which returns an error in this
> > > > > > situation?
> > > > > 
> > > > > Uhm, I think it would not work without modifying
> > > > > security_inode_need_killpriv() and the hook definition.
> > > > > 
> > > > > Since cap_inode_need_killpriv() returns 1, the loop stops and EVM
> > > > > would
> > > > > not be invoked. We would need to continue the loop and let EVM know
> > > > > what is the current return value. Then EVM can reject the change.
> > > > > 
> > > > > An alternative way would be to detect that actually we are setting the
> > > > > same value for inode metadata, and maybe not returning 1 from
> > > > > cap_inode_need_killpriv().
> > > > > 
> > > > > I would prefer the second, since EVM allows same value change and we
> > > > > would have an exception if there are fscaps.
> > > > > 
> > > > > This solves only the case of portable signatures. We would need to
> > > > > change cap_inode_need_killpriv() anyway to update the HMAC for mutable
> > > > > files.
> > > > 
> > > > I see. In any case this sounds like a matter for a separate patch
> > > > series.
> > > 
> > > Agreed.
> > 
> > Christian, how realistic is that we don't kill priv if we are setting
> > the same owner?
> > 
> > Serge, would we be able to replace __vfs_removexattr() (or now
> > vfs_get_fscaps_nosec()) with a security-equivalent alternative?
> 
> It seems it is not necessary.
> 
> security.capability removal occurs between evm_inode_setattr() and
> evm_inode_post_setattr(), after the HMAC has been verified and before
> the new HMAC is recalculated (without security.capability).
> 
> So, all good.
> 
> Christian, Seth, I pushed the kernel and the updated tests (all patches
> are WIP):
> 
> https://github.com/robertosassu/linux/commits/evm-fscaps-v2/

Resetting the IMA status flag is insufficient.  The EVM status needs to be reset
as well.  Stefan's "ima: re-evaluate file integrity on file metadata change"
patch does something similar for overlay.

Mimi

https://lore.kernel.org/linux-integrity/20240223172513.4049959-8-stefanb@linux.ibm.com/

> 
> https://github.com/robertosassu/ima-evm-utils/commits/evm-fscaps-v2/
> 
> 
> The tests are passing:
> 
> https://github.com/robertosassu/ima-evm-utils/actions/runs/8159877004/job/22305521359
> 
> Roberto
> 
>
Roberto Sassu March 6, 2024, 8:25 a.m. UTC | #14
On Tue, 2024-03-05 at 21:17 -0500, Mimi Zohar wrote:
> On Tue, 2024-03-05 at 18:11 +0100, Roberto Sassu wrote:
> > On Tue, 2024-03-05 at 13:46 +0100, Roberto Sassu wrote:
> > > On Tue, 2024-03-05 at 10:12 +0100, Christian Brauner wrote:
> > > > On Mon, Mar 04, 2024 at 10:56:17AM -0600, Seth Forshee (DigitalOcean)
> > > > wrote:
> > > > > On Mon, Mar 04, 2024 at 05:17:57PM +0100, Roberto Sassu wrote:
> > > > > > On Mon, 2024-03-04 at 09:31 -0600, Seth Forshee (DigitalOcean) wrote:
> > > > > > > On Mon, Mar 04, 2024 at 11:19:54AM +0100, Roberto Sassu wrote:
> > > > > > > > On Wed, 2024-02-21 at 15:24 -0600, Seth Forshee (DigitalOcean)
> > > > > > > > wrote:
> > > > > > > > > Use the vfs interfaces for fetching file capabilities for
> > > > > > > > > killpriv
> > > > > > > > > checks and from get_vfs_caps_from_disk(). While there, update
> > > > > > > > > the
> > > > > > > > > kerneldoc for get_vfs_caps_from_disk() to explain how it is
> > > > > > > > > different
> > > > > > > > > from vfs_get_fscaps_nosec().
> > > > > > > > > 
> > > > > > > > > Signed-off-by: Seth Forshee (DigitalOcean) <sforshee@kernel.org>
> > > > > > > > > ---
> > > > > > > > >  security/commoncap.c | 30 +++++++++++++-----------------
> > > > > > > > >  1 file changed, 13 insertions(+), 17 deletions(-)
> > > > > > > > > 
> > > > > > > > > diff --git a/security/commoncap.c b/security/commoncap.c
> > > > > > > > > index a0ff7e6092e0..751bb26a06a6 100644
> > > > > > > > > --- a/security/commoncap.c
> > > > > > > > > +++ b/security/commoncap.c
> > > > > > > > > @@ -296,11 +296,12 @@ int cap_capset(struct cred *new,
> > > > > > > > >   */
> > > > > > > > >  int cap_inode_need_killpriv(struct dentry *dentry)
> > > > > > > > >  {
> > > > > > > > > -	struct inode *inode = d_backing_inode(dentry);
> > > > > > > > > +	struct vfs_caps caps;
> > > > > > > > >  	int error;
> > > > > > > > >  
> > > > > > > > > -	error = __vfs_getxattr(dentry, inode, XATTR_NAME_CAPS,
> > > > > > > > > NULL, 0);
> > > > > > > > > -	return error > 0;
> > > > > > > > > +	/* Use nop_mnt_idmap for no mapping here as mapping is
> > > > > > > > > unimportant */
> > > > > > > > > +	error = vfs_get_fscaps_nosec(&nop_mnt_idmap, dentry,
> > > > > > > > > &caps);
> > > > > > > > > +	return error == 0;
> > > > > > > > >  }
> > > > > > > > >  
> > > > > > > > >  /**
> > > > > > > > > @@ -323,7 +324,7 @@ int cap_inode_killpriv(struct mnt_idmap
> > > > > > > > > *idmap, struct dentry *dentry)
> > > > > > > > >  {
> > > > > > > > >  	int error;
> > > > > > > > >  
> > > > > > > > > -	error = __vfs_removexattr(idmap, dentry,
> > > > > > > > > XATTR_NAME_CAPS);
> > > > > > > > > +	error = vfs_remove_fscaps_nosec(idmap, dentry);
> > > > > > > > 
> > > > > > > > Uhm, I see that the change is logically correct... but the
> > > > > > > > original
> > > > > > > > code was not correct, since the EVM post hook is not called (thus
> > > > > > > > the
> > > > > > > > HMAC is broken, or an xattr change is allowed on a portable
> > > > > > > > signature
> > > > > > > > which should be not).
> > > > > > > > 
> > > > > > > > For completeness, the xattr change on a portable signature should
> > > > > > > > not
> > > > > > > > happen in the first place, so cap_inode_killpriv() would not be
> > > > > > > > called.
> > > > > > > > However, since EVM allows same value change, we are here.
> > > > > > > 
> > > > > > > I really don't understand EVM that well and am pretty hesitant to
> > > > > > > try an
> > > > > > > change any of the logic around it. But I'll hazard a thought: should
> > > > > > > EVM
> > > > > > > have a inode_need_killpriv hook which returns an error in this
> > > > > > > situation?
> > > > > > 
> > > > > > Uhm, I think it would not work without modifying
> > > > > > security_inode_need_killpriv() and the hook definition.
> > > > > > 
> > > > > > Since cap_inode_need_killpriv() returns 1, the loop stops and EVM
> > > > > > would
> > > > > > not be invoked. We would need to continue the loop and let EVM know
> > > > > > what is the current return value. Then EVM can reject the change.
> > > > > > 
> > > > > > An alternative way would be to detect that actually we are setting the
> > > > > > same value for inode metadata, and maybe not returning 1 from
> > > > > > cap_inode_need_killpriv().
> > > > > > 
> > > > > > I would prefer the second, since EVM allows same value change and we
> > > > > > would have an exception if there are fscaps.
> > > > > > 
> > > > > > This solves only the case of portable signatures. We would need to
> > > > > > change cap_inode_need_killpriv() anyway to update the HMAC for mutable
> > > > > > files.
> > > > > 
> > > > > I see. In any case this sounds like a matter for a separate patch
> > > > > series.
> > > > 
> > > > Agreed.
> > > 
> > > Christian, how realistic is that we don't kill priv if we are setting
> > > the same owner?
> > > 
> > > Serge, would we be able to replace __vfs_removexattr() (or now
> > > vfs_get_fscaps_nosec()) with a security-equivalent alternative?
> > 
> > It seems it is not necessary.
> > 
> > security.capability removal occurs between evm_inode_setattr() and
> > evm_inode_post_setattr(), after the HMAC has been verified and before
> > the new HMAC is recalculated (without security.capability).
> > 
> > So, all good.
> > 
> > Christian, Seth, I pushed the kernel and the updated tests (all patches
> > are WIP):
> > 
> > https://github.com/robertosassu/linux/commits/evm-fscaps-v2/
> 
> Resetting the IMA status flag is insufficient.  The EVM status needs to be reset
> as well.  Stefan's "ima: re-evaluate file integrity on file metadata change"
> patch does something similar for overlay.

Both the IMA and EVM status are reset. The IMA one is reset based on
the evm_revalidate_status() call, similarly to ACLs.

Roberto

> Mimi
> 
> https://lore.kernel.org/linux-integrity/20240223172513.4049959-8-stefanb@linux.ibm.com/
> 
> > 
> > https://github.com/robertosassu/ima-evm-utils/commits/evm-fscaps-v2/
> > 
> > 
> > The tests are passing:
> > 
> > https://github.com/robertosassu/ima-evm-utils/actions/runs/8159877004/job/22305521359
> > 
> > Roberto
> > 
> > 
>
Roberto Sassu March 6, 2024, 8:30 a.m. UTC | #15
On Tue, 2024-03-05 at 14:17 -0600, Seth Forshee (DigitalOcean) wrote:
> On Tue, Mar 05, 2024 at 06:11:45PM +0100, Roberto Sassu wrote:
> > On Tue, 2024-03-05 at 13:46 +0100, Roberto Sassu wrote:
> > > On Tue, 2024-03-05 at 10:12 +0100, Christian Brauner wrote:
> > > > On Mon, Mar 04, 2024 at 10:56:17AM -0600, Seth Forshee (DigitalOcean) wrote:
> > > > > On Mon, Mar 04, 2024 at 05:17:57PM +0100, Roberto Sassu wrote:
> > > > > > On Mon, 2024-03-04 at 09:31 -0600, Seth Forshee (DigitalOcean) wrote:
> > > > > > > On Mon, Mar 04, 2024 at 11:19:54AM +0100, Roberto Sassu wrote:
> > > > > > > > On Wed, 2024-02-21 at 15:24 -0600, Seth Forshee (DigitalOcean) wrote:
> > > > > > > > > Use the vfs interfaces for fetching file capabilities for killpriv
> > > > > > > > > checks and from get_vfs_caps_from_disk(). While there, update the
> > > > > > > > > kerneldoc for get_vfs_caps_from_disk() to explain how it is different
> > > > > > > > > from vfs_get_fscaps_nosec().
> > > > > > > > > 
> > > > > > > > > Signed-off-by: Seth Forshee (DigitalOcean) <sforshee@kernel.org>
> > > > > > > > > ---
> > > > > > > > >  security/commoncap.c | 30 +++++++++++++-----------------
> > > > > > > > >  1 file changed, 13 insertions(+), 17 deletions(-)
> > > > > > > > > 
> > > > > > > > > diff --git a/security/commoncap.c b/security/commoncap.c
> > > > > > > > > index a0ff7e6092e0..751bb26a06a6 100644
> > > > > > > > > --- a/security/commoncap.c
> > > > > > > > > +++ b/security/commoncap.c
> > > > > > > > > @@ -296,11 +296,12 @@ int cap_capset(struct cred *new,
> > > > > > > > >   */
> > > > > > > > >  int cap_inode_need_killpriv(struct dentry *dentry)
> > > > > > > > >  {
> > > > > > > > > -	struct inode *inode = d_backing_inode(dentry);
> > > > > > > > > +	struct vfs_caps caps;
> > > > > > > > >  	int error;
> > > > > > > > >  
> > > > > > > > > -	error = __vfs_getxattr(dentry, inode, XATTR_NAME_CAPS, NULL, 0);
> > > > > > > > > -	return error > 0;
> > > > > > > > > +	/* Use nop_mnt_idmap for no mapping here as mapping is unimportant */
> > > > > > > > > +	error = vfs_get_fscaps_nosec(&nop_mnt_idmap, dentry, &caps);
> > > > > > > > > +	return error == 0;
> > > > > > > > >  }
> > > > > > > > >  
> > > > > > > > >  /**
> > > > > > > > > @@ -323,7 +324,7 @@ int cap_inode_killpriv(struct mnt_idmap *idmap, struct dentry *dentry)
> > > > > > > > >  {
> > > > > > > > >  	int error;
> > > > > > > > >  
> > > > > > > > > -	error = __vfs_removexattr(idmap, dentry, XATTR_NAME_CAPS);
> > > > > > > > > +	error = vfs_remove_fscaps_nosec(idmap, dentry);
> > > > > > > > 
> > > > > > > > Uhm, I see that the change is logically correct... but the original
> > > > > > > > code was not correct, since the EVM post hook is not called (thus the
> > > > > > > > HMAC is broken, or an xattr change is allowed on a portable signature
> > > > > > > > which should be not).
> > > > > > > > 
> > > > > > > > For completeness, the xattr change on a portable signature should not
> > > > > > > > happen in the first place, so cap_inode_killpriv() would not be called.
> > > > > > > > However, since EVM allows same value change, we are here.
> > > > > > > 
> > > > > > > I really don't understand EVM that well and am pretty hesitant to try an
> > > > > > > change any of the logic around it. But I'll hazard a thought: should EVM
> > > > > > > have a inode_need_killpriv hook which returns an error in this
> > > > > > > situation?
> > > > > > 
> > > > > > Uhm, I think it would not work without modifying
> > > > > > security_inode_need_killpriv() and the hook definition.
> > > > > > 
> > > > > > Since cap_inode_need_killpriv() returns 1, the loop stops and EVM would
> > > > > > not be invoked. We would need to continue the loop and let EVM know
> > > > > > what is the current return value. Then EVM can reject the change.
> > > > > > 
> > > > > > An alternative way would be to detect that actually we are setting the
> > > > > > same value for inode metadata, and maybe not returning 1 from
> > > > > > cap_inode_need_killpriv().
> > > > > > 
> > > > > > I would prefer the second, since EVM allows same value change and we
> > > > > > would have an exception if there are fscaps.
> > > > > > 
> > > > > > This solves only the case of portable signatures. We would need to
> > > > > > change cap_inode_need_killpriv() anyway to update the HMAC for mutable
> > > > > > files.
> > > > > 
> > > > > I see. In any case this sounds like a matter for a separate patch
> > > > > series.
> > > > 
> > > > Agreed.
> > > 
> > > Christian, how realistic is that we don't kill priv if we are setting
> > > the same owner?
> > > 
> > > Serge, would we be able to replace __vfs_removexattr() (or now
> > > vfs_get_fscaps_nosec()) with a security-equivalent alternative?
> > 
> > It seems it is not necessary.
> > 
> > security.capability removal occurs between evm_inode_setattr() and
> > evm_inode_post_setattr(), after the HMAC has been verified and before
> > the new HMAC is recalculated (without security.capability).
> > 
> > So, all good.
> > 
> > Christian, Seth, I pushed the kernel and the updated tests (all patches
> > are WIP):
> > 
> > https://github.com/robertosassu/linux/commits/evm-fscaps-v2/
> > 
> > https://github.com/robertosassu/ima-evm-utils/commits/evm-fscaps-v2/
> > 
> > 
> > The tests are passing:
> > 
> > https://github.com/robertosassu/ima-evm-utils/actions/runs/8159877004/job/22305521359
> 
> Thanks! I probably won't be able to take them exactly as-is due to other
> changes for the next version (rebasing onto the changes to make IMA and
> EVM LSMs, forbidding xattr handlers entirely for fscaps), but they will
> serve as a good road map for what needs to happen.

Welcome. Yes, both ima_inode_set_fscaps() and ima_inode_remove_fscaps()
will be registered as LSM hooks in ima_appraise.c. It will be probably
straightforward for you to make those changes, but if you have any
question, let me know.

Roberto
Mimi Zohar March 6, 2024, 12:56 p.m. UTC | #16
On Wed, 2024-03-06 at 09:25 +0100, Roberto Sassu wrote:
> On Tue, 2024-03-05 at 21:17 -0500, Mimi Zohar wrote:
> > On Tue, 2024-03-05 at 18:11 +0100, Roberto Sassu wrote:
> > > On Tue, 2024-03-05 at 13:46 +0100, Roberto Sassu wrote:
> > > > On Tue, 2024-03-05 at 10:12 +0100, Christian Brauner wrote:
> > > > > On Mon, Mar 04, 2024 at 10:56:17AM -0600, Seth Forshee (DigitalOcean)
> > > > > wrote:
> > > > > > On Mon, Mar 04, 2024 at 05:17:57PM +0100, Roberto Sassu wrote:
> > > > > > > On Mon, 2024-03-04 at 09:31 -0600, Seth Forshee (DigitalOcean)
> > > > > > > wrote:
> > > > > > > > On Mon, Mar 04, 2024 at 11:19:54AM +0100, Roberto Sassu wrote:
> > > > > > > > > On Wed, 2024-02-21 at 15:24 -0600, Seth Forshee (DigitalOcean)
> > > > > > > > > wrote:
> > > > > > > > > > Use the vfs interfaces for fetching file capabilities for
> > > > > > > > > > killpriv
> > > > > > > > > > checks and from get_vfs_caps_from_disk(). While there,
> > > > > > > > > > update
> > > > > > > > > > the
> > > > > > > > > > kerneldoc for get_vfs_caps_from_disk() to explain how it is
> > > > > > > > > > different
> > > > > > > > > > from vfs_get_fscaps_nosec().
> > > > > > > > > > 
> > > > > > > > > > Signed-off-by: Seth Forshee (DigitalOcean) <
> > > > > > > > > > sforshee@kernel.org>
> > > > > > > > > > ---
> > > > > > > > > >  security/commoncap.c | 30 +++++++++++++-----------------
> > > > > > > > > >  1 file changed, 13 insertions(+), 17 deletions(-)
> > > > > > > > > > 
> > > > > > > > > > diff --git a/security/commoncap.c b/security/commoncap.c
> > > > > > > > > > index a0ff7e6092e0..751bb26a06a6 100644
> > > > > > > > > > --- a/security/commoncap.c
> > > > > > > > > > +++ b/security/commoncap.c
> > > > > > > > > > @@ -296,11 +296,12 @@ int cap_capset(struct cred *new,
> > > > > > > > > >   */
> > > > > > > > > >  int cap_inode_need_killpriv(struct dentry *dentry)
> > > > > > > > > >  {
> > > > > > > > > > -	struct inode *inode = d_backing_inode(dentry);
> > > > > > > > > > +	struct vfs_caps caps;
> > > > > > > > > >  	int error;
> > > > > > > > > >  
> > > > > > > > > > -	error = __vfs_getxattr(dentry, inode, XATTR_NAME_CAPS,
> > > > > > > > > > NULL, 0);
> > > > > > > > > > -	return error > 0;
> > > > > > > > > > +	/* Use nop_mnt_idmap for no mapping here as mapping is
> > > > > > > > > > unimportant */
> > > > > > > > > > +	error = vfs_get_fscaps_nosec(&nop_mnt_idmap, dentry,
> > > > > > > > > > &caps);
> > > > > > > > > > +	return error == 0;
> > > > > > > > > >  }
> > > > > > > > > >  
> > > > > > > > > >  /**
> > > > > > > > > > @@ -323,7 +324,7 @@ int cap_inode_killpriv(struct mnt_idmap
> > > > > > > > > > *idmap, struct dentry *dentry)
> > > > > > > > > >  {
> > > > > > > > > >  	int error;
> > > > > > > > > >  
> > > > > > > > > > -	error = __vfs_removexattr(idmap, dentry,
> > > > > > > > > > XATTR_NAME_CAPS);
> > > > > > > > > > +	error = vfs_remove_fscaps_nosec(idmap, dentry);
> > > > > > > > > 
> > > > > > > > > Uhm, I see that the change is logically correct... but the
> > > > > > > > > original
> > > > > > > > > code was not correct, since the EVM post hook is not called
> > > > > > > > > (thus
> > > > > > > > > the
> > > > > > > > > HMAC is broken, or an xattr change is allowed on a portable
> > > > > > > > > signature
> > > > > > > > > which should be not).
> > > > > > > > > 
> > > > > > > > > For completeness, the xattr change on a portable signature
> > > > > > > > > should
> > > > > > > > > not
> > > > > > > > > happen in the first place, so cap_inode_killpriv() would not
> > > > > > > > > be
> > > > > > > > > called.
> > > > > > > > > However, since EVM allows same value change, we are here.
> > > > > > > > 
> > > > > > > > I really don't understand EVM that well and am pretty hesitant
> > > > > > > > to
> > > > > > > > try an
> > > > > > > > change any of the logic around it. But I'll hazard a thought:
> > > > > > > > should
> > > > > > > > EVM
> > > > > > > > have a inode_need_killpriv hook which returns an error in this
> > > > > > > > situation?
> > > > > > > 
> > > > > > > Uhm, I think it would not work without modifying
> > > > > > > security_inode_need_killpriv() and the hook definition.
> > > > > > > 
> > > > > > > Since cap_inode_need_killpriv() returns 1, the loop stops and EVM
> > > > > > > would
> > > > > > > not be invoked. We would need to continue the loop and let EVM
> > > > > > > know
> > > > > > > what is the current return value. Then EVM can reject the change.
> > > > > > > 
> > > > > > > An alternative way would be to detect that actually we are setting
> > > > > > > the
> > > > > > > same value for inode metadata, and maybe not returning 1 from
> > > > > > > cap_inode_need_killpriv().
> > > > > > > 
> > > > > > > I would prefer the second, since EVM allows same value change and
> > > > > > > we
> > > > > > > would have an exception if there are fscaps.
> > > > > > > 
> > > > > > > This solves only the case of portable signatures. We would need to
> > > > > > > change cap_inode_need_killpriv() anyway to update the HMAC for
> > > > > > > mutable
> > > > > > > files.
> > > > > > 
> > > > > > I see. In any case this sounds like a matter for a separate patch
> > > > > > series.
> > > > > 
> > > > > Agreed.
> > > > 
> > > > Christian, how realistic is that we don't kill priv if we are setting
> > > > the same owner?
> > > > 
> > > > Serge, would we be able to replace __vfs_removexattr() (or now
> > > > vfs_get_fscaps_nosec()) with a security-equivalent alternative?
> > > 
> > > It seems it is not necessary.
> > > 
> > > security.capability removal occurs between evm_inode_setattr() and
> > > evm_inode_post_setattr(), after the HMAC has been verified and before
> > > the new HMAC is recalculated (without security.capability).
> > > 
> > > So, all good.
> > > 
> > > Christian, Seth, I pushed the kernel and the updated tests (all patches
> > > are WIP):
> > > 
> > > https://github.com/robertosassu/linux/commits/evm-fscaps-v2/
> > 
> > Resetting the IMA status flag is insufficient.  The EVM status needs to be
> > reset
> > as well.  Stefan's "ima: re-evaluate file integrity on file metadata change"
> > patch does something similar for overlay.
> 
> Both the IMA and EVM status are reset. The IMA one is reset based on
> the evm_revalidate_status() call, similarly to ACLs.

Agreed. Oh, evm_status is being reset in evm_inode_post_set_fscaps().

> 
> 
> > https://lore.kernel.org/linux-integrity/20240223172513.4049959-8-stefanb@linux.ibm.com/
> > 
> > > https://github.com/robertosassu/ima-evm-utils/commits/evm-fscaps-v2/
> > > 
> > > 
> > > The tests are passing:
> > > 
> > > https://github.com/robertosassu/ima-evm-utils/actions/runs/8159877004/job/22305521359
> > > 
> > > Roberto
> > > 
> > > 
> 
>
diff mbox series

Patch

diff --git a/security/commoncap.c b/security/commoncap.c
index a0ff7e6092e0..751bb26a06a6 100644
--- a/security/commoncap.c
+++ b/security/commoncap.c
@@ -296,11 +296,12 @@  int cap_capset(struct cred *new,
  */
 int cap_inode_need_killpriv(struct dentry *dentry)
 {
-	struct inode *inode = d_backing_inode(dentry);
+	struct vfs_caps caps;
 	int error;
 
-	error = __vfs_getxattr(dentry, inode, XATTR_NAME_CAPS, NULL, 0);
-	return error > 0;
+	/* Use nop_mnt_idmap for no mapping here as mapping is unimportant */
+	error = vfs_get_fscaps_nosec(&nop_mnt_idmap, dentry, &caps);
+	return error == 0;
 }
 
 /**
@@ -323,7 +324,7 @@  int cap_inode_killpriv(struct mnt_idmap *idmap, struct dentry *dentry)
 {
 	int error;
 
-	error = __vfs_removexattr(idmap, dentry, XATTR_NAME_CAPS);
+	error = vfs_remove_fscaps_nosec(idmap, dentry);
 	if (error == -EOPNOTSUPP)
 		error = 0;
 	return error;
@@ -719,6 +720,10 @@  ssize_t vfs_caps_to_user_xattr(struct mnt_idmap *idmap,
  * @cpu_caps:	vfs capabilities
  *
  * Extract the on-exec-apply capability sets for an executable file.
+ * For version 3 capabilities xattrs, returns the capabilities only if
+ * they are applicable to current_user_ns() (i.e. that the rootid
+ * corresponds to an ID which maps to ID 0 in current_user_ns() or an
+ * ancestor), and returns -ENODATA otherwise.
  *
  * If the inode has been found through an idmapped mount the idmap of
  * the vfsmount must be passed through @idmap. This function will then
@@ -731,25 +736,16 @@  int get_vfs_caps_from_disk(struct mnt_idmap *idmap,
 			   struct vfs_caps *cpu_caps)
 {
 	struct inode *inode = d_backing_inode(dentry);
-	int size, ret;
-	struct vfs_ns_cap_data data, *nscaps = &data;
+	int ret;
 
 	if (!inode)
 		return -ENODATA;
 
-	size = __vfs_getxattr((struct dentry *)dentry, inode,
-			      XATTR_NAME_CAPS, &data, XATTR_CAPS_SZ);
-	if (size == -ENODATA || size == -EOPNOTSUPP)
+	ret = vfs_get_fscaps_nosec(idmap, (struct dentry *)dentry, cpu_caps);
+	if (ret == -EOPNOTSUPP || ret == -EOVERFLOW)
 		/* no data, that's ok */
-		return -ENODATA;
+		ret = -ENODATA;
 
-	if (size < 0)
-		return size;
-
-	ret = vfs_caps_from_xattr(idmap, inode->i_sb->s_user_ns,
-				  cpu_caps, nscaps, size);
-	if (ret == -EOVERFLOW)
-		return -ENODATA;
 	if (ret)
 		return ret;