[PATCH v4 13/30] evm: add post set acl hook

[Christian Brauner]

On Fri, Sep 30, 2022 at 07:48:31AM -0400, Mimi Zohar wrote:
> 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: 
> > > On Thu, 2022-09-29 at 17:30 +0200, Christian Brauner wrote:
> > > > 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.
> >  
> > 
> > > 
> > > > The callchain for evm_inode_post_setxattr() is:
> > > > 
> > > > -> evm_inode_post_setxattr()
> > > 
> > > Resets the EVM status flag for both EVM signatures and HMAC.
> > > 
> > > >    -> 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.

Ok.

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

Sure.