Hi Christian,

On Fri, 2022-09-30 at 10:44 +0200, Christian Brauner wrote:

On Thu, Sep 29, 2022 at 09:44:45PM -0400, Mimi Zohar wrote: 

quoted

On Thu, 2022-09-29 at 17:30 +0200, Christian Brauner wrote:

quoted

The security_inode_post_setxattr() hook is used by security modules to
update their own security.* xattrs. Consequently none of the security
modules operate on posix acls. So we don't need an additional security
hook when post setting posix acls.

However, the integrity subsystem wants to be informed about posix acl
changes and specifically evm to update their hashes when the xattrs
change. 

^... to be informed about posix acl changes in order to reset the EVM
status flag.

Substituted.
 

quoted

quoted

The callchain for evm_inode_post_setxattr() is:

-> evm_inode_post_setxattr()

Resets the EVM status flag for both EVM signatures and HMAC.

quoted

   -> evm_update_evmxattr()

evm_update_evmxattr() is only called for "security.evm", not acls.

After re-reading the code with fresh eyes, I made a mistake here. 
Please revert these suggestions.

I've added both comments but note that I'm explaining this in the
paragraph below as well.

Agreed.

quoted

quoted

      -> evm_calc_hmac()
         -> evm_calc_hmac_or_hash()

and evm_cacl_hmac_or_hash() walks the global list of protected xattr
names evm_config_xattrnames. This global list can be modified via
/sys/security/integrity/evm/evm_xattrs. The write to "evm_xattrs" is
restricted to security.* xattrs and the default xattrs in
evm_config_xattrnames only contains security.* xattrs as well.

So the actual value for posix acls is currently completely irrelevant
for evm during evm_inode_post_setxattr() and frankly it should stay that
way in the future to not cause the vfs any more headaches. But if the
actual posix acl values matter then evm shouldn't operate on the binary
void blob and try to hack around in the uapi struct anyway. Instead it
should then in the future add a dedicated hook which takes a struct
posix_acl argument passing the posix acls in the proper vfs format.

For now it is sufficient to make evm_inode_post_set_acl() a wrapper
around evm_inode_post_setxattr() not passing any actual values down.
This will still cause the hashes to be updated as before.

^This will cause the EVM status flag to be reset.

Substituted.

My mistake.  Can you replace it with:

This will still cause the EVM status flag to be reset and EVM HMAC's to
be updated as before.

-- 
thanks,

Mimi