diff mbox series

[v4,4/5] evm: Align evm_inode_init_security() definition with LSM infrastructure

Message ID 20221110094639.3086409-5-roberto.sassu@huaweicloud.com (mailing list archive)
State Handled Elsewhere
Headers show
Series evm: Prepare for moving to the LSM infrastructure | expand

Commit Message

Roberto Sassu Nov. 10, 2022, 9:46 a.m. UTC 
From: Roberto Sassu <roberto.sassu@huawei.com>

Change the evm_inode_init_security() definition to align with the LSM
infrastructure, in preparation for moving IMA and EVM to that
infrastructure.

This requires passing only the xattr array allocated by
security_inode_init_security(), instead of the first LSM xattr and the
place where the EVM xattr should be filled.

It also requires positioning after the last filled xattr (by checking the
xattr name), since the beginning of the xattr array is given.

If EVM is moved to the LSM infrastructure, it will use the xattr
reservation mechanism too, i.e. it positions itself in the xattr array with
the offset given by the LSM infrastructure.

Finally, make evm_inode_init_security() return value compatible with the
inode_init_security hook conventions, i.e. return -EOPNOTSUPP if it is not
setting an xattr.

EVM is a bit tricky, because xattrs is both an input and an output. If it
was just output, EVM should have returned zero if xattrs is NULL. But,
since xattrs is also input, EVM is unable to do its calculations, so return
-EOPNOTSUPP and handle this error in security_inode_init_security().

Don't change the return value in the inline function
evm_inode_init_security() in include/linux/evm.h, as the function will be
removed if EVM is moved to the LSM infrastructure.

Last note, this patch does not fix a possible crash if the xattr array is
empty (due to calling evm_protected_xattr() with a NULL argument). It will
be fixed with 'evm: Support multiple LSMs providing an xattr', as it will
first ensure that the xattr name is not NULL before calling
evm_protected_xattr().

Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
---
 include/linux/evm.h               | 12 ++++++------
 security/integrity/evm/evm_main.c | 20 +++++++++++++-------
 security/security.c               |  5 ++---
 3 files changed, 21 insertions(+), 16 deletions(-)

Comments

Mimi Zohar Nov. 17, 2022, 5:07 p.m. UTC | #1 
On Thu, 2022-11-10 at 10:46 +0100, Roberto Sassu wrote:
> From: Roberto Sassu <roberto.sassu@huawei.com>
> 
> Change the evm_inode_init_security() definition to align with the LSM
> infrastructure, in preparation for moving IMA and EVM to that
> infrastructure.
> 
> This requires passing only the xattr array allocated by
> security_inode_init_security(), instead of the first LSM xattr and the
> place where the EVM xattr should be filled.
> 
> It also requires positioning after the last filled xattr (by checking the
> xattr name), since the beginning of the xattr array is given.

Perhaps combine this sentence to the previous paragraph and start the
sentence with
"In lieu of passing the EVM xattr, ..." 

> If EVM is moved to the LSM infrastructure, it will use the xattr
> reservation mechanism too, i.e. it positions itself in the xattr array with
> the offset given by the LSM infrastructure.

The LSM infrastructure will need to support EVM as the last LSM.  Is
there a reason for including this comment in this patch description.

> Finally, make evm_inode_init_security() return value compatible with the
> inode_init_security hook conventions, i.e. return -EOPNOTSUPP if it is not
> setting an xattr.

> EVM is a bit tricky, because xattrs is both an input and an output. If it
> was just output, EVM should have returned zero if xattrs is NULL. But,
> since xattrs is also input, EVM is unable to do its calculations, so return
> -EOPNOTSUPP and handle this error in security_inode_init_security().
> 
> Don't change the return value in the inline function
> evm_inode_init_security() in include/linux/evm.h, as the function will be
> removed if EVM is moved to the LSM infrastructure.
> 
> Last note, this patch does not fix a possible crash if the xattr array is
> empty (due to calling evm_protected_xattr() with a NULL argument). It will
> be fixed with 'evm: Support multiple LSMs providing an xattr', as it will
> first ensure that the xattr name is not NULL before calling
> evm_protected_xattr().

From my reading of the code, although there might be multiple LSM
xattrs, this patch only includes the first LSM xattr in the security
EVM calculation.  So it only checks the first xattr's name.  Support
for including multiple LSM xattrs in the EVM hmac calculation is added
in the subsequent patch.

thanks,

Mimi

> 
> Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Roberto Sassu Nov. 18, 2022, 9:30 a.m. UTC | #2 
On 11/17/2022 6:07 PM, Mimi Zohar wrote:
> On Thu, 2022-11-10 at 10:46 +0100, Roberto Sassu wrote:
>> From: Roberto Sassu <roberto.sassu@huawei.com>
>>
>> Change the evm_inode_init_security() definition to align with the LSM
>> infrastructure, in preparation for moving IMA and EVM to that
>> infrastructure.
>>
>> This requires passing only the xattr array allocated by
>> security_inode_init_security(), instead of the first LSM xattr and the
>> place where the EVM xattr should be filled.
>>
>> It also requires positioning after the last filled xattr (by checking the
>> xattr name), since the beginning of the xattr array is given.
> 
> Perhaps combine this sentence to the previous paragraph and start the
> sentence with
> "In lieu of passing the EVM xattr, ..."

Ok.

>> If EVM is moved to the LSM infrastructure, it will use the xattr
>> reservation mechanism too, i.e. it positions itself in the xattr array with
>> the offset given by the LSM infrastructure.
> 
> The LSM infrastructure will need to support EVM as the last LSM.  Is
> there a reason for including this comment in this patch description.

The idea is to first make EVM work like other LSMs, and then add 
limitations that are EVM-specific.

As a regular LSM, EVM could be placed anywhere in the list of LSMs. This 
would mean that whenever EVM is called, it will process xattrs that are 
set by previous LSMs, not the subsequent ones.

What we would need to do EVM-specific is that EVM is the last in the 
list of LSMs, to ensure that all xattrs are protected.

>> Finally, make evm_inode_init_security() return value compatible with the
>> inode_init_security hook conventions, i.e. return -EOPNOTSUPP if it is not
>> setting an xattr.
> 
>> EVM is a bit tricky, because xattrs is both an input and an output. If it
>> was just output, EVM should have returned zero if xattrs is NULL. But,
>> since xattrs is also input, EVM is unable to do its calculations, so return
>> -EOPNOTSUPP and handle this error in security_inode_init_security().
>>
>> Don't change the return value in the inline function
>> evm_inode_init_security() in include/linux/evm.h, as the function will be
>> removed if EVM is moved to the LSM infrastructure.
>>
>> Last note, this patch does not fix a possible crash if the xattr array is
>> empty (due to calling evm_protected_xattr() with a NULL argument). It will
>> be fixed with 'evm: Support multiple LSMs providing an xattr', as it will
>> first ensure that the xattr name is not NULL before calling
>> evm_protected_xattr().
> 
>  From my reading of the code, although there might be multiple LSM
> xattrs, this patch only includes the first LSM xattr in the security
> EVM calculation.  So it only checks the first xattr's name.  Support
> for including multiple LSM xattrs in the EVM hmac calculation is added
> in the subsequent patch.

I tried to include in this patch just the function definition change and 
keep the existing behavior.

The problem is trying to access xattr->name at the beginning of 
evm_inode_init_security().

That would disappear in patch 5, where there is a loop checking 
xattr->value first. Patch 3 disallows combination of NULL name - !NULL 
value and !NULL name - NULL value. Not sure if the latter is correct 
(empty xattr?). Will check what callers do.

Roberto

> thanks,
> 
> Mimi
> 
>>
>> Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Mimi Zohar Nov. 18, 2022, 2:45 p.m. UTC | #3 
On Fri, 2022-11-18 at 10:30 +0100, Roberto Sassu wrote:
> On 11/17/2022 6:07 PM, Mimi Zohar wrote:
> > On Thu, 2022-11-10 at 10:46 +0100, Roberto Sassu wrote:
> >> From: Roberto Sassu <roberto.sassu@huawei.com>
> >>
> >> Change the evm_inode_init_security() definition to align with the LSM
> >> infrastructure, in preparation for moving IMA and EVM to that
> >> infrastructure.
> >>
> >> This requires passing only the xattr array allocated by
> >> security_inode_init_security(), instead of the first LSM xattr and the
> >> place where the EVM xattr should be filled.
> >>
> >> It also requires positioning after the last filled xattr (by checking the
> >> xattr name), since the beginning of the xattr array is given.
> > 
> > Perhaps combine this sentence to the previous paragraph and start the
> > sentence with
> > "In lieu of passing the EVM xattr, ..."
> 
> Ok.
> 
> >> If EVM is moved to the LSM infrastructure, it will use the xattr
> >> reservation mechanism too, i.e. it positions itself in the xattr array with
> >> the offset given by the LSM infrastructure.
> > 
> > The LSM infrastructure will need to support EVM as the last LSM.  Is
> > there a reason for including this comment in this patch description.
> 
> The idea is to first make EVM work like other LSMs, and then add 
> limitations that are EVM-specific.
> 
> As a regular LSM, EVM could be placed anywhere in the list of LSMs. This 
> would mean that whenever EVM is called, it will process xattrs that are 
> set by previous LSMs, not the subsequent ones.
> 
> What we would need to do EVM-specific is that EVM is the last in the 
> list of LSMs, to ensure that all xattrs are protected.
> 
> >> Finally, make evm_inode_init_security() return value compatible with the
> >> inode_init_security hook conventions, i.e. return -EOPNOTSUPP if it is not
> >> setting an xattr.
> > 
> >> EVM is a bit tricky, because xattrs is both an input and an output. If it
> >> was just output, EVM should have returned zero if xattrs is NULL. But,
> >> since xattrs is also input, EVM is unable to do its calculations, so return
> >> -EOPNOTSUPP and handle this error in security_inode_init_security().
> >>
> >> Don't change the return value in the inline function
> >> evm_inode_init_security() in include/linux/evm.h, as the function will be
> >> removed if EVM is moved to the LSM infrastructure.
> >>
> >> Last note, this patch does not fix a possible crash if the xattr array is
> >> empty (due to calling evm_protected_xattr() with a NULL argument). It will
> >> be fixed with 'evm: Support multiple LSMs providing an xattr', as it will
> >> first ensure that the xattr name is not NULL before calling
> >> evm_protected_xattr().
> > 
> >  From my reading of the code, although there might be multiple LSM
> > xattrs, this patch only includes the first LSM xattr in the security
> > EVM calculation.  So it only checks the first xattr's name.  Support
> > for including multiple LSM xattrs in the EVM hmac calculation is added
> > in the subsequent patch.
> 
> I tried to include in this patch just the function definition change and 
> keep the existing behavior.

That's fine.
> 
> The problem is trying to access xattr->name at the beginning of 
> evm_inode_init_security().
> 
> That would disappear in patch 5, where there is a loop checking 
> xattr->value first. Patch 3 disallows combination of NULL name - !NULL 
> value and !NULL name - NULL value. Not sure if the latter is correct 
> (empty xattr?). Will check what callers do.

My comments here and above were for improving the patch description:
- Just say what this patch is doing, not what subsequent changes will
do in the future.  We'll come to that when the time comes.

- Say something only the lines that this patch includes only one LSM
security xattr in the EVM calculation, like previously.

thanks,

Mimi
Mimi Zohar Nov. 18, 2022, 3:11 p.m. UTC | #4 
On Fri, 2022-11-18 at 10:30 +0100, Roberto Sassu wrote:
> On 11/17/2022 6:07 PM, Mimi Zohar wrote:
> > On Thu, 2022-11-10 at 10:46 +0100, Roberto Sassu wrote:
> >> From: Roberto Sassu <roberto.sassu@huawei.com>
> >>
> >> Change the evm_inode_init_security() definition to align with the LSM
> >> infrastructure, in preparation for moving IMA and EVM to that
> >> infrastructure.
> >>
> >> This requires passing only the xattr array allocated by
> >> security_inode_init_security(), instead of the first LSM xattr and the
> >> place where the EVM xattr should be filled.
> >>
> >> It also requires positioning after the last filled xattr (by checking the
> >> xattr name), since the beginning of the xattr array is given.
> > 
> > Perhaps combine this sentence to the previous paragraph and start the
> > sentence with
> > "In lieu of passing the EVM xattr, ..."
> 
> Ok.
> 
> >> If EVM is moved to the LSM infrastructure, it will use the xattr
> >> reservation mechanism too, i.e. it positions itself in the xattr array with
> >> the offset given by the LSM infrastructure.
> > 
> > The LSM infrastructure will need to support EVM as the last LSM.  Is
> > there a reason for including this comment in this patch description.
> 
> The idea is to first make EVM work like other LSMs, and then add 
> limitations that are EVM-specific.
> 
> As a regular LSM, EVM could be placed anywhere in the list of LSMs. This 
> would mean that whenever EVM is called, it will process xattrs that are 
> set by previous LSMs, not the subsequent ones.
> 
> What we would need to do EVM-specific is that EVM is the last in the 
> list of LSMs, to ensure that all xattrs are protected.
> 
> >> Finally, make evm_inode_init_security() return value compatible with the
> >> inode_init_security hook conventions, i.e. return -EOPNOTSUPP if it is not
> >> setting an xattr.
> > 
> >> EVM is a bit tricky, because xattrs is both an input and an output. If it
> >> was just output, EVM should have returned zero if xattrs is NULL. But,
> >> since xattrs is also input, EVM is unable to do its calculations, so return
> >> -EOPNOTSUPP and handle this error in security_inode_init_security().
> >>
> >> Don't change the return value in the inline function
> >> evm_inode_init_security() in include/linux/evm.h, as the function will be
> >> removed if EVM is moved to the LSM infrastructure.
> >>
> >> Last note, this patch does not fix a possible crash if the xattr array is
> >> empty (due to calling evm_protected_xattr() with a NULL argument). It will
> >> be fixed with 'evm: Support multiple LSMs providing an xattr', as it will
> >> first ensure that the xattr name is not NULL before calling
> >> evm_protected_xattr().
> > 
> >  From my reading of the code, although there might be multiple LSM
> > xattrs, this patch only includes the first LSM xattr in the security
> > EVM calculation.  So it only checks the first xattr's name.  Support
> > for including multiple LSM xattrs in the EVM hmac calculation is added
> > in the subsequent patch.
> 
> I tried to include in this patch just the function definition change and 
> keep the existing behavior.

That's fine.
> 
> The problem is trying to access xattr->name at the beginning of 
> evm_inode_init_security().
> 
> That would disappear in patch 5, where there is a loop checking 
> xattr->value first. Patch 3 disallows combination of NULL name - !NULL 
> value and !NULL name - NULL value. Not sure if the latter is correct 
> (empty xattr?). Will check what callers do.

My comments here and above were for improving the patch description:
- Just say what this patch is doing, not what subsequent changes will
do in the future.  We'll come to that when the time comes.

- Say something only the lines that this patch includes only one LSM
security xattr in the EVM calculation, like previously.

thanks,

Mimi
diff mbox series

Patch

diff --git a/include/linux/evm.h b/include/linux/evm.h
index aa63e0b3c0a2..3bb2ae9fe098 100644
--- a/include/linux/evm.h
+++ b/include/linux/evm.h
@@ -35,9 +35,9 @@  extern int evm_inode_removexattr(struct user_namespace *mnt_userns,
 				 struct dentry *dentry, const char *xattr_name);
 extern void evm_inode_post_removexattr(struct dentry *dentry,
 				       const char *xattr_name);
-extern int evm_inode_init_security(struct inode *inode,
-				   const struct xattr *xattr_array,
-				   struct xattr *evm);
+extern int evm_inode_init_security(struct inode *inode, struct inode *dir,
+				   const struct qstr *qstr,
+				   struct xattr *xattrs);
 extern bool evm_revalidate_status(const char *xattr_name);
 extern int evm_protected_xattr_if_enabled(const char *req_xattr_name);
 extern int evm_read_protected_xattrs(struct dentry *dentry, u8 *buffer,
@@ -108,9 +108,9 @@  static inline void evm_inode_post_removexattr(struct dentry *dentry,
 	return;
 }
 
-static inline int evm_inode_init_security(struct inode *inode,
-					  const struct xattr *xattr_array,
-					  struct xattr *evm)
+static inline int evm_inode_init_security(struct inode *inode, struct inode *dir,
+					  const struct qstr *qstr,
+					  struct xattr *xattrs)
 {
 	return 0;
 }
diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
index 23d484e05e6f..0a312cafb7de 100644
--- a/security/integrity/evm/evm_main.c
+++ b/security/integrity/evm/evm_main.c
@@ -845,23 +845,29 @@  void evm_inode_post_setattr(struct dentry *dentry, int ia_valid)
 /*
  * evm_inode_init_security - initializes security.evm HMAC value
  */
-int evm_inode_init_security(struct inode *inode,
-				 const struct xattr *lsm_xattr,
-				 struct xattr *evm_xattr)
+int evm_inode_init_security(struct inode *inode, struct inode *dir,
+			    const struct qstr *qstr,
+			    struct xattr *xattrs)
 {
 	struct evm_xattr *xattr_data;
+	struct xattr *xattr, *evm_xattr;
 	int rc;
 
-	if (!(evm_initialized & EVM_INIT_HMAC) ||
-	    !evm_protected_xattr(lsm_xattr->name))
-		return 0;
+	if (!(evm_initialized & EVM_INIT_HMAC) || !xattrs ||
+	    !evm_protected_xattr(xattrs->name))
+		return -EOPNOTSUPP;
+
+	for (xattr = xattrs; xattr->value != NULL; xattr++)
+		;
+
+	evm_xattr = xattr;
 
 	xattr_data = kzalloc(sizeof(*xattr_data), GFP_NOFS);
 	if (!xattr_data)
 		return -ENOMEM;
 
 	xattr_data->data.type = EVM_XATTR_HMAC;
-	rc = evm_init_hmac(inode, lsm_xattr, xattr_data->digest);
+	rc = evm_init_hmac(inode, xattrs, xattr_data->digest);
 	if (rc < 0)
 		goto out;
 
diff --git a/security/security.c b/security/security.c
index b62f192de6da..999102101a75 100644
--- a/security/security.c
+++ b/security/security.c
@@ -1170,9 +1170,8 @@  int security_inode_init_security(struct inode *inode, struct inode *dir,
 		}
 	}
 
-	ret = evm_inode_init_security(inode, new_xattrs,
-				      new_xattrs + cur_xattrs);
-	if (ret)
+	ret = evm_inode_init_security(inode, dir, qstr, new_xattrs);
+	if (ret && ret != -EOPNOTSUPP)
 		goto out;
 	ret = initxattrs(inode, new_xattrs, fs_data);
 out: