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

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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@xxxxxxxxxx>

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

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.

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

 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.




Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>

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