diff mbox series

[v5,09/12] evm: Allow setxattr() and setattr() for unmodified metadata

Message ID 20210407105252.30721-10-roberto.sassu@huawei.com (mailing list archive)
State New
Headers show
Series evm: Improve usability of portable signatures | expand

Commit Message

Roberto Sassu April 7, 2021, 10:52 a.m. UTC
With the patch to allow xattr/attr operations if a portable signature
verification fails, cp and tar can copy all xattrs/attrs so that at the
end of the process verification succeeds.

However, it might happen that the xattrs/attrs are already set to the
correct value (taken at signing time) and signature verification succeeds
before the copy has completed. For example, an archive might contains files
owned by root and the archive is extracted by root.

Then, since portable signatures are immutable, all subsequent operations
fail (e.g. fchown()), even if the operation is legitimate (does not alter
the current value).

This patch avoids this problem by reporting successful operation to user
space when that operation does not alter the current value of xattrs/attrs.

Cc: Christian Brauner <christian.brauner@ubuntu.com>
Cc: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
---
 security/integrity/evm/evm_main.c | 107 ++++++++++++++++++++++++++++++
 1 file changed, 107 insertions(+)

Comments

Christian Brauner April 7, 2021, 12:05 p.m. UTC | #1
On Wed, Apr 07, 2021 at 12:52:49PM +0200, Roberto Sassu wrote:
> With the patch to allow xattr/attr operations if a portable signature
> verification fails, cp and tar can copy all xattrs/attrs so that at the
> end of the process verification succeeds.
> 
> However, it might happen that the xattrs/attrs are already set to the
> correct value (taken at signing time) and signature verification succeeds
> before the copy has completed. For example, an archive might contains files
> owned by root and the archive is extracted by root.
> 
> Then, since portable signatures are immutable, all subsequent operations
> fail (e.g. fchown()), even if the operation is legitimate (does not alter
> the current value).
> 
> This patch avoids this problem by reporting successful operation to user
> space when that operation does not alter the current value of xattrs/attrs.
> 
> Cc: Christian Brauner <christian.brauner@ubuntu.com>
> Cc: Andreas Gruenbacher <agruenba@redhat.com>
> Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
> ---
>  security/integrity/evm/evm_main.c | 107 ++++++++++++++++++++++++++++++
>  1 file changed, 107 insertions(+)
> 
> diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
> index 74f9f3a2ae53..2a8fcba67d47 100644
> --- a/security/integrity/evm/evm_main.c
> +++ b/security/integrity/evm/evm_main.c
> @@ -18,6 +18,7 @@
>  #include <linux/integrity.h>
>  #include <linux/evm.h>
>  #include <linux/magic.h>
> +#include <linux/posix_acl_xattr.h>
>  
>  #include <crypto/hash.h>
>  #include <crypto/hash_info.h>
> @@ -328,6 +329,89 @@ static enum integrity_status evm_verify_current_integrity(struct dentry *dentry)
>  	return evm_verify_hmac(dentry, NULL, NULL, 0, NULL);
>  }
>  
> +/*
> + * evm_xattr_acl_change - check if passed ACL changes the inode mode
> + * @mnt_userns: user namespace of the idmapped mount
> + * @dentry: pointer to the affected dentry
> + * @xattr_name: requested xattr
> + * @xattr_value: requested xattr value
> + * @xattr_value_len: requested xattr value length
> + *
> + * Check if passed ACL changes the inode mode, which is protected by EVM.
> + *
> + * Returns 1 if passed ACL causes inode mode change, 0 otherwise.
> + */
> +static int evm_xattr_acl_change(struct user_namespace *mnt_userns,
> +				struct dentry *dentry, const char *xattr_name,
> +				const void *xattr_value, size_t xattr_value_len)
> +{
> +	umode_t mode;
> +	struct posix_acl *acl = NULL, *acl_res;
> +	struct inode *inode = d_backing_inode(dentry);
> +	int rc;
> +
> +	/* user_ns is not relevant here, ACL_USER/ACL_GROUP don't have impact
> +	 * on the inode mode (see posix_acl_equiv_mode()).
> +	 */
> +	acl = posix_acl_from_xattr(&init_user_ns, xattr_value, xattr_value_len);
> +	if (IS_ERR_OR_NULL(acl))
> +		return 1;
> +
> +	acl_res = acl;
> +	/* Passing mnt_userns is necessary to correctly determine the GID in
> +	 * an idmapped mount, as the GID is used to clear the setgid bit in
> +	 * the inode mode.
> +	 */
> +	rc = posix_acl_update_mode(mnt_userns, inode, &mode, &acl_res);
> +
> +	posix_acl_release(acl);
> +
> +	if (rc)
> +		return 1;
> +
> +	if (inode->i_mode != mode)
> +		return 1;
> +
> +	return 0;
> +}
> +
> +/*
> + * evm_xattr_change - check if passed xattr value differs from current value
> + * @mnt_userns: user namespace of the idmapped mount
> + * @dentry: pointer to the affected dentry
> + * @xattr_name: requested xattr
> + * @xattr_value: requested xattr value
> + * @xattr_value_len: requested xattr value length
> + *
> + * Check if passed xattr value differs from current value.
> + *
> + * Returns 1 if passed xattr value differs from current value, 0 otherwise.
> + */
> +static int evm_xattr_change(struct user_namespace *mnt_userns,
> +			    struct dentry *dentry, const char *xattr_name,
> +			    const void *xattr_value, size_t xattr_value_len)
> +{
> +	char *xattr_data = NULL;
> +	int rc = 0;
> +
> +	if (posix_xattr_acl(xattr_name))
> +		return evm_xattr_acl_change(mnt_userns, dentry, xattr_name,
> +					    xattr_value, xattr_value_len);
> +
> +	rc = vfs_getxattr_alloc(&init_user_ns, dentry, xattr_name, &xattr_data,
> +				0, GFP_NOFS);
> +	if (rc < 0)
> +		return 1;
> +
> +	if (rc == xattr_value_len)
> +		rc = memcmp(xattr_value, xattr_data, rc);

Afaik memcmp() can return values greater than 1 and less than 0 so it
might make sense to explicitly do sm like:
rc = memcmp() ? 1 : 0;
or
!!memcmp()
or alter the comment for evm_xattr_change().

other than that

Reviewed-by: Christian Brauner <christian.brauner@ubuntu.com>

> +	else
> +		rc = 1;
> +
> +	kfree(xattr_data);
> +	return rc;
> +}
> +
>  /*
>   * evm_protect_xattr - protect the EVM extended attribute
>   *
> @@ -389,6 +473,11 @@ static int evm_protect_xattr(struct user_namespace *mnt_userns,
>  	if (evm_status == INTEGRITY_FAIL_IMMUTABLE)
>  		return 0;
>  
> +	if (evm_status == INTEGRITY_PASS_IMMUTABLE &&
> +	    !evm_xattr_change(mnt_userns, dentry, xattr_name, xattr_value,
> +			      xattr_value_len))
> +		return 0;
> +
>  	if (evm_status != INTEGRITY_PASS)
>  		integrity_audit_msg(AUDIT_INTEGRITY_METADATA, d_backing_inode(dentry),
>  				    dentry->d_name.name, "appraise_metadata",
> @@ -532,6 +621,19 @@ void evm_inode_post_removexattr(struct dentry *dentry, const char *xattr_name)
>  	evm_update_evmxattr(dentry, xattr_name, NULL, 0);
>  }
>  
> +static int evm_attr_change(struct dentry *dentry, struct iattr *attr)
> +{
> +	struct inode *inode = d_backing_inode(dentry);
> +	unsigned int ia_valid = attr->ia_valid;
> +
> +	if ((!(ia_valid & ATTR_UID) || uid_eq(attr->ia_uid, inode->i_uid)) &&
> +	    (!(ia_valid & ATTR_GID) || gid_eq(attr->ia_gid, inode->i_gid)) &&
> +	    (!(ia_valid & ATTR_MODE) || attr->ia_mode == inode->i_mode))
> +		return 0;
> +
> +	return 1;
> +}
> +
>  /**
>   * evm_inode_setattr - prevent updating an invalid EVM extended attribute
>   * @dentry: pointer to the affected dentry
> @@ -562,6 +664,11 @@ int evm_inode_setattr(struct dentry *dentry, struct iattr *attr)
>  	    (evm_status == INTEGRITY_FAIL_IMMUTABLE) ||
>  	    (evm_ignore_error_safe(evm_status)))
>  		return 0;
> +
> +	if (evm_status == INTEGRITY_PASS_IMMUTABLE &&
> +	    !evm_attr_change(dentry, attr))
> +		return 0;
> +
>  	integrity_audit_msg(AUDIT_INTEGRITY_METADATA, d_backing_inode(dentry),
>  			    dentry->d_name.name, "appraise_metadata",
>  			    integrity_status_msg[evm_status], -EPERM, 0);
> -- 
> 2.26.2
>
kernel test robot April 7, 2021, 3:23 p.m. UTC | #2
Hi Roberto,

Thank you for the patch! Yet something to improve:

[auto build test ERROR on security/next-testing]
[also build test ERROR on integrity/next-integrity linus/master v5.12-rc6 next-20210407]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch]

url:    https://github.com/0day-ci/linux/commits/Roberto-Sassu/evm-Improve-usability-of-portable-signatures/20210407-185747
base:   https://git.kernel.org/pub/scm/linux/kernel/git/jmorris/linux-security.git next-testing
config: s390-randconfig-r034-20210407 (attached as .config)
compiler: clang version 13.0.0 (https://github.com/llvm/llvm-project c060945b23a1c54d4b2a053ff4b093a2277b303d)
reproduce (this is a W=1 build):
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # install s390 cross compiling tool for clang build
        # apt-get install binutils-s390x-linux-gnu
        # https://github.com/0day-ci/linux/commit/1bdae98f0b81260a925cf7acf785dc10bb7787fe
        git remote add linux-review https://github.com/0day-ci/linux
        git fetch --no-tags linux-review Roberto-Sassu/evm-Improve-usability-of-portable-signatures/20210407-185747
        git checkout 1bdae98f0b81260a925cf7acf785dc10bb7787fe
        # save the attached .config to linux build tree
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross ARCH=s390 

If you fix the issue, kindly add following tag as appropriate
Reported-by: kernel test robot <lkp@intel.com>

All errors (new ones prefixed by >>):

>> security/integrity/evm/evm_main.c:365:7: error: implicit declaration of function 'posix_acl_update_mode' [-Werror,-Wimplicit-function-declaration]
           rc = posix_acl_update_mode(mnt_userns, inode, &mode, &acl_res);
                ^
   1 error generated.


vim +/posix_acl_update_mode +365 security/integrity/evm/evm_main.c

   331	
   332	/*
   333	 * evm_xattr_acl_change - check if passed ACL changes the inode mode
   334	 * @mnt_userns: user namespace of the idmapped mount
   335	 * @dentry: pointer to the affected dentry
   336	 * @xattr_name: requested xattr
   337	 * @xattr_value: requested xattr value
   338	 * @xattr_value_len: requested xattr value length
   339	 *
   340	 * Check if passed ACL changes the inode mode, which is protected by EVM.
   341	 *
   342	 * Returns 1 if passed ACL causes inode mode change, 0 otherwise.
   343	 */
   344	static int evm_xattr_acl_change(struct user_namespace *mnt_userns,
   345					struct dentry *dentry, const char *xattr_name,
   346					const void *xattr_value, size_t xattr_value_len)
   347	{
   348		umode_t mode;
   349		struct posix_acl *acl = NULL, *acl_res;
   350		struct inode *inode = d_backing_inode(dentry);
   351		int rc;
   352	
   353		/* user_ns is not relevant here, ACL_USER/ACL_GROUP don't have impact
   354		 * on the inode mode (see posix_acl_equiv_mode()).
   355		 */
   356		acl = posix_acl_from_xattr(&init_user_ns, xattr_value, xattr_value_len);
   357		if (IS_ERR_OR_NULL(acl))
   358			return 1;
   359	
   360		acl_res = acl;
   361		/* Passing mnt_userns is necessary to correctly determine the GID in
   362		 * an idmapped mount, as the GID is used to clear the setgid bit in
   363		 * the inode mode.
   364		 */
 > 365		rc = posix_acl_update_mode(mnt_userns, inode, &mode, &acl_res);
   366	
   367		posix_acl_release(acl);
   368	
   369		if (rc)
   370			return 1;
   371	
   372		if (inode->i_mode != mode)
   373			return 1;
   374	
   375		return 0;
   376	}
   377	

---
0-DAY CI Kernel Test Service, Intel Corporation
https://lists.01.org/hyperkitty/list/kbuild-all@lists.01.org
kernel test robot April 7, 2021, 6:14 p.m. UTC | #3
Hi Roberto,

Thank you for the patch! Yet something to improve:

[auto build test ERROR on security/next-testing]
[also build test ERROR on integrity/next-integrity linus/master v5.12-rc6 next-20210407]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch]

url:    https://github.com/0day-ci/linux/commits/Roberto-Sassu/evm-Improve-usability-of-portable-signatures/20210407-185747
base:   https://git.kernel.org/pub/scm/linux/kernel/git/jmorris/linux-security.git next-testing
config: nios2-randconfig-s031-20210407 (attached as .config)
compiler: nios2-linux-gcc (GCC) 9.3.0
reproduce:
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # apt-get install sparse
        # sparse version: v0.6.3-279-g6d5d9b42-dirty
        # https://github.com/0day-ci/linux/commit/1bdae98f0b81260a925cf7acf785dc10bb7787fe
        git remote add linux-review https://github.com/0day-ci/linux
        git fetch --no-tags linux-review Roberto-Sassu/evm-Improve-usability-of-portable-signatures/20210407-185747
        git checkout 1bdae98f0b81260a925cf7acf785dc10bb7787fe
        # save the attached .config to linux build tree
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-9.3.0 make.cross C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__' ARCH=nios2 

If you fix the issue, kindly add following tag as appropriate
Reported-by: kernel test robot <lkp@intel.com>

All errors (new ones prefixed by >>):

   security/integrity/evm/evm_main.c: In function 'evm_xattr_acl_change':
>> security/integrity/evm/evm_main.c:365:7: error: implicit declaration of function 'posix_acl_update_mode'; did you mean 'posix_acl_equiv_mode'? [-Werror=implicit-function-declaration]
     365 |  rc = posix_acl_update_mode(mnt_userns, inode, &mode, &acl_res);
         |       ^~~~~~~~~~~~~~~~~~~~~
         |       posix_acl_equiv_mode
   cc1: some warnings being treated as errors


vim +365 security/integrity/evm/evm_main.c

   331	
   332	/*
   333	 * evm_xattr_acl_change - check if passed ACL changes the inode mode
   334	 * @mnt_userns: user namespace of the idmapped mount
   335	 * @dentry: pointer to the affected dentry
   336	 * @xattr_name: requested xattr
   337	 * @xattr_value: requested xattr value
   338	 * @xattr_value_len: requested xattr value length
   339	 *
   340	 * Check if passed ACL changes the inode mode, which is protected by EVM.
   341	 *
   342	 * Returns 1 if passed ACL causes inode mode change, 0 otherwise.
   343	 */
   344	static int evm_xattr_acl_change(struct user_namespace *mnt_userns,
   345					struct dentry *dentry, const char *xattr_name,
   346					const void *xattr_value, size_t xattr_value_len)
   347	{
   348		umode_t mode;
   349		struct posix_acl *acl = NULL, *acl_res;
   350		struct inode *inode = d_backing_inode(dentry);
   351		int rc;
   352	
   353		/* user_ns is not relevant here, ACL_USER/ACL_GROUP don't have impact
   354		 * on the inode mode (see posix_acl_equiv_mode()).
   355		 */
   356		acl = posix_acl_from_xattr(&init_user_ns, xattr_value, xattr_value_len);
   357		if (IS_ERR_OR_NULL(acl))
   358			return 1;
   359	
   360		acl_res = acl;
   361		/* Passing mnt_userns is necessary to correctly determine the GID in
   362		 * an idmapped mount, as the GID is used to clear the setgid bit in
   363		 * the inode mode.
   364		 */
 > 365		rc = posix_acl_update_mode(mnt_userns, inode, &mode, &acl_res);
   366	
   367		posix_acl_release(acl);
   368	
   369		if (rc)
   370			return 1;
   371	
   372		if (inode->i_mode != mode)
   373			return 1;
   374	
   375		return 0;
   376	}
   377	

---
0-DAY CI Kernel Test Service, Intel Corporation
https://lists.01.org/hyperkitty/list/kbuild-all@lists.01.org
Mimi Zohar May 3, 2021, 1 p.m. UTC | #4
On Wed, 2021-04-07 at 12:52 +0200, Roberto Sassu wrote:

> diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
> @@ -389,6 +473,11 @@ static int evm_protect_xattr(struct user_namespace *mnt_userns,
>  	if (evm_status == INTEGRITY_FAIL_IMMUTABLE)
>  		return 0;
>  
> +	if (evm_status == INTEGRITY_PASS_IMMUTABLE &&
> +	    !evm_xattr_change(mnt_userns, dentry, xattr_name, xattr_value,
> +			      xattr_value_len))
> +		return 0;
> +

If the purpose of evm_protect_xattr() is to prevent allowing an invalid
security.evm xattr from being re-calculated and updated, making it
valid, INTEGRITY_PASS_IMMUTABLE shouldn't need to be conditional.  Any
time there is an attr or xattr change, including setting it to the
existing value, the status flag should be reset.

I'm wondering if making INTEGRITY_PASS_IMMUTABLE conditional would
prevent the file from being resigned.

>  	if (evm_status != INTEGRITY_PASS)
>  		integrity_audit_msg(AUDIT_INTEGRITY_METADATA, d_backing_inode(dentry),
>  				    dentry->d_name.name, "appraise_metadata",

This would then be updated to if not INTEGRITY_PASS or
INTEGRITY_PASS_IMMUTABLE.  The subsequent "return" would need to be
updated as well.

thanks,

Mimi
Roberto Sassu May 3, 2021, 2:48 p.m. UTC | #5
> From: Mimi Zohar [mailto:zohar@linux.ibm.com]
> Sent: Monday, May 3, 2021 3:00 PM
> On Wed, 2021-04-07 at 12:52 +0200, Roberto Sassu wrote:
> 
> > diff --git a/security/integrity/evm/evm_main.c
> b/security/integrity/evm/evm_main.c
> > @@ -389,6 +473,11 @@ static int evm_protect_xattr(struct
> user_namespace *mnt_userns,
> >  	if (evm_status == INTEGRITY_FAIL_IMMUTABLE)
> >  		return 0;
> >
> > +	if (evm_status == INTEGRITY_PASS_IMMUTABLE &&
> > +	    !evm_xattr_change(mnt_userns, dentry, xattr_name, xattr_value,
> > +			      xattr_value_len))
> > +		return 0;
> > +
> 
> If the purpose of evm_protect_xattr() is to prevent allowing an invalid
> security.evm xattr from being re-calculated and updated, making it
> valid, INTEGRITY_PASS_IMMUTABLE shouldn't need to be conditional.  Any
> time there is an attr or xattr change, including setting it to the
> existing value, the status flag should be reset.

The status is always reset if evm_protect_xattr() returns 0. This does not
change.

Not making INTEGRITY_PASS_IMMUTABLE conditional would cause issues.
Suppose that the status is INTEGRITY_FAIL. Writing the same xattr would
cause evm_protect_xattr() to return 0 and the HMAC to be updated.

> I'm wondering if making INTEGRITY_PASS_IMMUTABLE conditional would
> prevent the file from being resigned.

INTEGRITY_FAIL_IMMUTABLE should be enough to continue the
operation.

Roberto

HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063
Managing Director: Li Peng, Li Jian, Shi Yanli

> >  	if (evm_status != INTEGRITY_PASS)
> >  		integrity_audit_msg(AUDIT_INTEGRITY_METADATA,
> d_backing_inode(dentry),
> >  				    dentry->d_name.name,
> "appraise_metadata",
> 
> This would then be updated to if not INTEGRITY_PASS or
> INTEGRITY_PASS_IMMUTABLE.  The subsequent "return" would need to be
> updated as well.
> 
> thanks,
> 
> Mimi
Roberto Sassu May 3, 2021, 3:11 p.m. UTC | #6
> From: Mimi Zohar [mailto:zohar@linux.ibm.com]
> Sent: Monday, May 3, 2021 3:00 PM
> On Wed, 2021-04-07 at 12:52 +0200, Roberto Sassu wrote:
> 
> > diff --git a/security/integrity/evm/evm_main.c
> b/security/integrity/evm/evm_main.c
> > @@ -389,6 +473,11 @@ static int evm_protect_xattr(struct
> user_namespace *mnt_userns,
> >  	if (evm_status == INTEGRITY_FAIL_IMMUTABLE)
> >  		return 0;
> >
> > +	if (evm_status == INTEGRITY_PASS_IMMUTABLE &&
> > +	    !evm_xattr_change(mnt_userns, dentry, xattr_name, xattr_value,
> > +			      xattr_value_len))
> > +		return 0;
> > +
> 
> If the purpose of evm_protect_xattr() is to prevent allowing an invalid
> security.evm xattr from being re-calculated and updated, making it
> valid, INTEGRITY_PASS_IMMUTABLE shouldn't need to be conditional.  Any
> time there is an attr or xattr change, including setting it to the
> existing value, the status flag should be reset.
> 
> I'm wondering if making INTEGRITY_PASS_IMMUTABLE conditional would
> prevent the file from being resigned.
> 
> >  	if (evm_status != INTEGRITY_PASS)
> >  		integrity_audit_msg(AUDIT_INTEGRITY_METADATA,
> d_backing_inode(dentry),
> >  				    dentry->d_name.name,
> "appraise_metadata",
> 
> This would then be updated to if not INTEGRITY_PASS or
> INTEGRITY_PASS_IMMUTABLE.  The subsequent "return" would need to be
> updated as well.

I agree on the first suggestion, to reduce the number of log messages.
For the second, if you meant that we should return 0 if the status is
INTEGRITY_PASS_IMMUTABLE, I thought we wanted to deny xattr
changes when there is an EVM portable signature.

Roberto

HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063
Managing Director: Li Peng, Li Jian, Shi Yanli

> thanks,
> 
> Mimi
Mimi Zohar May 3, 2021, 3:13 p.m. UTC | #7
On Mon, 2021-05-03 at 14:48 +0000, Roberto Sassu wrote:
> > From: Mimi Zohar [mailto:zohar@linux.ibm.com]
> > Sent: Monday, May 3, 2021 3:00 PM
> > On Wed, 2021-04-07 at 12:52 +0200, Roberto Sassu wrote:
> > 
> > > diff --git a/security/integrity/evm/evm_main.c
> > b/security/integrity/evm/evm_main.c
> > > @@ -389,6 +473,11 @@ static int evm_protect_xattr(struct
> > user_namespace *mnt_userns,
> > >  	if (evm_status == INTEGRITY_FAIL_IMMUTABLE)
> > >  		return 0;
> > >
> > > +	if (evm_status == INTEGRITY_PASS_IMMUTABLE &&
> > > +	    !evm_xattr_change(mnt_userns, dentry, xattr_name, xattr_value,
> > > +			      xattr_value_len))
> > > +		return 0;
> > > +
> > 
> > If the purpose of evm_protect_xattr() is to prevent allowing an invalid
> > security.evm xattr from being re-calculated and updated, making it
> > valid, INTEGRITY_PASS_IMMUTABLE shouldn't need to be conditional.  Any
> > time there is an attr or xattr change, including setting it to the
> > existing value, the status flag should be reset.
> 
> The status is always reset if evm_protect_xattr() returns 0. This does not
> change.
> 
> Not making INTEGRITY_PASS_IMMUTABLE conditional would cause issues.
> Suppose that the status is INTEGRITY_FAIL. Writing the same xattr would
> cause evm_protect_xattr() to return 0 and the HMAC to be updated.

This example is mixing security.evm types.  Please clarify.

> > I'm wondering if making INTEGRITY_PASS_IMMUTABLE conditional would
> > prevent the file from being resigned.
> 
> INTEGRITY_FAIL_IMMUTABLE should be enough to continue the
> operation.

Agreed.

Mimi
Mimi Zohar May 3, 2021, 3:26 p.m. UTC | #8
On Mon, 2021-05-03 at 15:11 +0000, Roberto Sassu wrote:
> > From: Mimi Zohar [mailto:zohar@linux.ibm.com]
> > Sent: Monday, May 3, 2021 3:00 PM
> > On Wed, 2021-04-07 at 12:52 +0200, Roberto Sassu wrote:
> > 
> > > diff --git a/security/integrity/evm/evm_main.c
> > b/security/integrity/evm/evm_main.c
> > > @@ -389,6 +473,11 @@ static int evm_protect_xattr(struct
> > user_namespace *mnt_userns,
> > >  	if (evm_status == INTEGRITY_FAIL_IMMUTABLE)
> > >  		return 0;
> > >
> > > +	if (evm_status == INTEGRITY_PASS_IMMUTABLE &&
> > > +	    !evm_xattr_change(mnt_userns, dentry, xattr_name, xattr_value,
> > > +			      xattr_value_len))
> > > +		return 0;
> > > +
> > 
> > If the purpose of evm_protect_xattr() is to prevent allowing an invalid
> > security.evm xattr from being re-calculated and updated, making it
> > valid, INTEGRITY_PASS_IMMUTABLE shouldn't need to be conditional.  Any
> > time there is an attr or xattr change, including setting it to the
> > existing value, the status flag should be reset.
> > 
> > I'm wondering if making INTEGRITY_PASS_IMMUTABLE conditional would
> > prevent the file from being resigned.
> > 
> > >  	if (evm_status != INTEGRITY_PASS)
> > >  		integrity_audit_msg(AUDIT_INTEGRITY_METADATA,
> > d_backing_inode(dentry),
> > >  				    dentry->d_name.name,
> > "appraise_metadata",
> > 
> > This would then be updated to if not INTEGRITY_PASS or
> > INTEGRITY_PASS_IMMUTABLE.  The subsequent "return" would need to be
> > updated as well.
> 
> I agree on the first suggestion, to reduce the number of log messages.
> For the second, if you meant that we should return 0 if the status is
> INTEGRITY_PASS_IMMUTABLE, I thought we wanted to deny xattr
> changes when there is an EVM portable signature.

Why?  I must be missing something.  As long as we're not relying on the
cached status, allowing the file metadata to be updated shouldn't be an
issue.

Mimi
Roberto Sassu May 3, 2021, 3:30 p.m. UTC | #9
> From: Mimi Zohar [mailto:zohar@linux.ibm.com]
> Sent: Monday, May 3, 2021 5:13 PM
> On Mon, 2021-05-03 at 14:48 +0000, Roberto Sassu wrote:
> > > From: Mimi Zohar [mailto:zohar@linux.ibm.com]
> > > Sent: Monday, May 3, 2021 3:00 PM
> > > On Wed, 2021-04-07 at 12:52 +0200, Roberto Sassu wrote:
> > >
> > > > diff --git a/security/integrity/evm/evm_main.c
> > > b/security/integrity/evm/evm_main.c
> > > > @@ -389,6 +473,11 @@ static int evm_protect_xattr(struct
> > > user_namespace *mnt_userns,
> > > >  	if (evm_status == INTEGRITY_FAIL_IMMUTABLE)
> > > >  		return 0;
> > > >
> > > > +	if (evm_status == INTEGRITY_PASS_IMMUTABLE &&
> > > > +	    !evm_xattr_change(mnt_userns, dentry, xattr_name, xattr_value,
> > > > +			      xattr_value_len))
> > > > +		return 0;
> > > > +
> > >
> > > If the purpose of evm_protect_xattr() is to prevent allowing an invalid
> > > security.evm xattr from being re-calculated and updated, making it
> > > valid, INTEGRITY_PASS_IMMUTABLE shouldn't need to be conditional.
> Any
> > > time there is an attr or xattr change, including setting it to the
> > > existing value, the status flag should be reset.
> >
> > The status is always reset if evm_protect_xattr() returns 0. This does not
> > change.
> >
> > Not making INTEGRITY_PASS_IMMUTABLE conditional would cause issues.
> > Suppose that the status is INTEGRITY_FAIL. Writing the same xattr would
> > cause evm_protect_xattr() to return 0 and the HMAC to be updated.
> 
> This example is mixing security.evm types.  Please clarify.

What I meant is that returning 0 when the xattr does not change should
be done only in the positive cases: for INTEGRITY_PASS it is not needed,
for INTEGRITY_PASS_IMMUTABLE it is needed as otherwise
evm_protect_xattr() would return -EPERM.

If your proposal was to return 0 only when the xattr does not change,
without checking the current status, we risk that someone does an
offline attack to corrupt xattrs and when the system is online, he simply
rewrites the same corrupted xattrs to obtain a valid HMAC.

Roberto

HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063
Managing Director: Li Peng, Li Jian, Shi Yanli

> > > I'm wondering if making INTEGRITY_PASS_IMMUTABLE conditional would
> > > prevent the file from being resigned.
> >
> > INTEGRITY_FAIL_IMMUTABLE should be enough to continue the
> > operation.
> 
> Agreed.
> 
> Mimi
Roberto Sassu May 3, 2021, 3:32 p.m. UTC | #10
> From: Mimi Zohar [mailto:zohar@linux.ibm.com]
> Sent: Monday, May 3, 2021 5:26 PM
> On Mon, 2021-05-03 at 15:11 +0000, Roberto Sassu wrote:
> > > From: Mimi Zohar [mailto:zohar@linux.ibm.com]
> > > Sent: Monday, May 3, 2021 3:00 PM
> > > On Wed, 2021-04-07 at 12:52 +0200, Roberto Sassu wrote:
> > >
> > > > diff --git a/security/integrity/evm/evm_main.c
> > > b/security/integrity/evm/evm_main.c
> > > > @@ -389,6 +473,11 @@ static int evm_protect_xattr(struct
> > > user_namespace *mnt_userns,
> > > >  	if (evm_status == INTEGRITY_FAIL_IMMUTABLE)
> > > >  		return 0;
> > > >
> > > > +	if (evm_status == INTEGRITY_PASS_IMMUTABLE &&
> > > > +	    !evm_xattr_change(mnt_userns, dentry, xattr_name, xattr_value,
> > > > +			      xattr_value_len))
> > > > +		return 0;
> > > > +
> > >
> > > If the purpose of evm_protect_xattr() is to prevent allowing an invalid
> > > security.evm xattr from being re-calculated and updated, making it
> > > valid, INTEGRITY_PASS_IMMUTABLE shouldn't need to be conditional.
> Any
> > > time there is an attr or xattr change, including setting it to the
> > > existing value, the status flag should be reset.
> > >
> > > I'm wondering if making INTEGRITY_PASS_IMMUTABLE conditional would
> > > prevent the file from being resigned.
> > >
> > > >  	if (evm_status != INTEGRITY_PASS)
> > > >  		integrity_audit_msg(AUDIT_INTEGRITY_METADATA,
> > > d_backing_inode(dentry),
> > > >  				    dentry->d_name.name,
> > > "appraise_metadata",
> > >
> > > This would then be updated to if not INTEGRITY_PASS or
> > > INTEGRITY_PASS_IMMUTABLE.  The subsequent "return" would need to
> be
> > > updated as well.
> >
> > I agree on the first suggestion, to reduce the number of log messages.
> > For the second, if you meant that we should return 0 if the status is
> > INTEGRITY_PASS_IMMUTABLE, I thought we wanted to deny xattr
> > changes when there is an EVM portable signature.
> 
> Why?  I must be missing something.  As long as we're not relying on the
> cached status, allowing the file metadata to be updated shouldn't be an
> issue.

We may want to prevent accidental changes, for example.

Roberto

HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063
Managing Director: Li Peng, Li Jian, Shi Yanli
Mimi Zohar May 3, 2021, 3:48 p.m. UTC | #11
On Mon, 2021-05-03 at 15:32 +0000, Roberto Sassu wrote:
> > From: Mimi Zohar [mailto:zohar@linux.ibm.com]
> > Sent: Monday, May 3, 2021 5:26 PM
> > On Mon, 2021-05-03 at 15:11 +0000, Roberto Sassu wrote:
> > > > From: Mimi Zohar [mailto:zohar@linux.ibm.com]
> > > > Sent: Monday, May 3, 2021 3:00 PM
> > > > On Wed, 2021-04-07 at 12:52 +0200, Roberto Sassu wrote:
> > > >
> > > > > diff --git a/security/integrity/evm/evm_main.c
> > > > b/security/integrity/evm/evm_main.c
> > > > > @@ -389,6 +473,11 @@ static int evm_protect_xattr(struct
> > > > user_namespace *mnt_userns,
> > > > >  	if (evm_status == INTEGRITY_FAIL_IMMUTABLE)
> > > > >  		return 0;
> > > > >
> > > > > +	if (evm_status == INTEGRITY_PASS_IMMUTABLE &&
> > > > > +	    !evm_xattr_change(mnt_userns, dentry, xattr_name, xattr_value,
> > > > > +			      xattr_value_len))
> > > > > +		return 0;
> > > > > +
> > > >
> > > > If the purpose of evm_protect_xattr() is to prevent allowing an invalid
> > > > security.evm xattr from being re-calculated and updated, making it
> > > > valid, INTEGRITY_PASS_IMMUTABLE shouldn't need to be conditional.
> > Any
> > > > time there is an attr or xattr change, including setting it to the
> > > > existing value, the status flag should be reset.
> > > >
> > > > I'm wondering if making INTEGRITY_PASS_IMMUTABLE conditional would
> > > > prevent the file from being resigned.
> > > >
> > > > >  	if (evm_status != INTEGRITY_PASS)
> > > > >  		integrity_audit_msg(AUDIT_INTEGRITY_METADATA,
> > > > d_backing_inode(dentry),
> > > > >  				    dentry->d_name.name,
> > > > "appraise_metadata",
> > > >
> > > > This would then be updated to if not INTEGRITY_PASS or
> > > > INTEGRITY_PASS_IMMUTABLE.  The subsequent "return" would need to
> > be
> > > > updated as well.
> > >
> > > I agree on the first suggestion, to reduce the number of log messages.
> > > For the second, if you meant that we should return 0 if the status is
> > > INTEGRITY_PASS_IMMUTABLE, I thought we wanted to deny xattr
> > > changes when there is an EVM portable signature.
> > 
> > Why?  I must be missing something.  As long as we're not relying on the
> > cached status, allowing the file metadata to be updated shouldn't be an
> > issue.
> 
> We may want to prevent accidental changes, for example.

Let's keep it simple, getting the basics working properly first.  Then
we can decide if this is something that we really want/need to defend
against.

thanks,

Mimi
diff mbox series

Patch

diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
index 74f9f3a2ae53..2a8fcba67d47 100644
--- a/security/integrity/evm/evm_main.c
+++ b/security/integrity/evm/evm_main.c
@@ -18,6 +18,7 @@ 
 #include <linux/integrity.h>
 #include <linux/evm.h>
 #include <linux/magic.h>
+#include <linux/posix_acl_xattr.h>
 
 #include <crypto/hash.h>
 #include <crypto/hash_info.h>
@@ -328,6 +329,89 @@  static enum integrity_status evm_verify_current_integrity(struct dentry *dentry)
 	return evm_verify_hmac(dentry, NULL, NULL, 0, NULL);
 }
 
+/*
+ * evm_xattr_acl_change - check if passed ACL changes the inode mode
+ * @mnt_userns: user namespace of the idmapped mount
+ * @dentry: pointer to the affected dentry
+ * @xattr_name: requested xattr
+ * @xattr_value: requested xattr value
+ * @xattr_value_len: requested xattr value length
+ *
+ * Check if passed ACL changes the inode mode, which is protected by EVM.
+ *
+ * Returns 1 if passed ACL causes inode mode change, 0 otherwise.
+ */
+static int evm_xattr_acl_change(struct user_namespace *mnt_userns,
+				struct dentry *dentry, const char *xattr_name,
+				const void *xattr_value, size_t xattr_value_len)
+{
+	umode_t mode;
+	struct posix_acl *acl = NULL, *acl_res;
+	struct inode *inode = d_backing_inode(dentry);
+	int rc;
+
+	/* user_ns is not relevant here, ACL_USER/ACL_GROUP don't have impact
+	 * on the inode mode (see posix_acl_equiv_mode()).
+	 */
+	acl = posix_acl_from_xattr(&init_user_ns, xattr_value, xattr_value_len);
+	if (IS_ERR_OR_NULL(acl))
+		return 1;
+
+	acl_res = acl;
+	/* Passing mnt_userns is necessary to correctly determine the GID in
+	 * an idmapped mount, as the GID is used to clear the setgid bit in
+	 * the inode mode.
+	 */
+	rc = posix_acl_update_mode(mnt_userns, inode, &mode, &acl_res);
+
+	posix_acl_release(acl);
+
+	if (rc)
+		return 1;
+
+	if (inode->i_mode != mode)
+		return 1;
+
+	return 0;
+}
+
+/*
+ * evm_xattr_change - check if passed xattr value differs from current value
+ * @mnt_userns: user namespace of the idmapped mount
+ * @dentry: pointer to the affected dentry
+ * @xattr_name: requested xattr
+ * @xattr_value: requested xattr value
+ * @xattr_value_len: requested xattr value length
+ *
+ * Check if passed xattr value differs from current value.
+ *
+ * Returns 1 if passed xattr value differs from current value, 0 otherwise.
+ */
+static int evm_xattr_change(struct user_namespace *mnt_userns,
+			    struct dentry *dentry, const char *xattr_name,
+			    const void *xattr_value, size_t xattr_value_len)
+{
+	char *xattr_data = NULL;
+	int rc = 0;
+
+	if (posix_xattr_acl(xattr_name))
+		return evm_xattr_acl_change(mnt_userns, dentry, xattr_name,
+					    xattr_value, xattr_value_len);
+
+	rc = vfs_getxattr_alloc(&init_user_ns, dentry, xattr_name, &xattr_data,
+				0, GFP_NOFS);
+	if (rc < 0)
+		return 1;
+
+	if (rc == xattr_value_len)
+		rc = memcmp(xattr_value, xattr_data, rc);
+	else
+		rc = 1;
+
+	kfree(xattr_data);
+	return rc;
+}
+
 /*
  * evm_protect_xattr - protect the EVM extended attribute
  *
@@ -389,6 +473,11 @@  static int evm_protect_xattr(struct user_namespace *mnt_userns,
 	if (evm_status == INTEGRITY_FAIL_IMMUTABLE)
 		return 0;
 
+	if (evm_status == INTEGRITY_PASS_IMMUTABLE &&
+	    !evm_xattr_change(mnt_userns, dentry, xattr_name, xattr_value,
+			      xattr_value_len))
+		return 0;
+
 	if (evm_status != INTEGRITY_PASS)
 		integrity_audit_msg(AUDIT_INTEGRITY_METADATA, d_backing_inode(dentry),
 				    dentry->d_name.name, "appraise_metadata",
@@ -532,6 +621,19 @@  void evm_inode_post_removexattr(struct dentry *dentry, const char *xattr_name)
 	evm_update_evmxattr(dentry, xattr_name, NULL, 0);
 }
 
+static int evm_attr_change(struct dentry *dentry, struct iattr *attr)
+{
+	struct inode *inode = d_backing_inode(dentry);
+	unsigned int ia_valid = attr->ia_valid;
+
+	if ((!(ia_valid & ATTR_UID) || uid_eq(attr->ia_uid, inode->i_uid)) &&
+	    (!(ia_valid & ATTR_GID) || gid_eq(attr->ia_gid, inode->i_gid)) &&
+	    (!(ia_valid & ATTR_MODE) || attr->ia_mode == inode->i_mode))
+		return 0;
+
+	return 1;
+}
+
 /**
  * evm_inode_setattr - prevent updating an invalid EVM extended attribute
  * @dentry: pointer to the affected dentry
@@ -562,6 +664,11 @@  int evm_inode_setattr(struct dentry *dentry, struct iattr *attr)
 	    (evm_status == INTEGRITY_FAIL_IMMUTABLE) ||
 	    (evm_ignore_error_safe(evm_status)))
 		return 0;
+
+	if (evm_status == INTEGRITY_PASS_IMMUTABLE &&
+	    !evm_attr_change(dentry, attr))
+		return 0;
+
 	integrity_audit_msg(AUDIT_INTEGRITY_METADATA, d_backing_inode(dentry),
 			    dentry->d_name.name, "appraise_metadata",
 			    integrity_status_msg[evm_status], -EPERM, 0);