Re: [RFC PATCH v14 15/19] fsverity: consume builtin signature via LSM hook

[Fan Wu]

On 3/12/2024 11:51 AM, Casey Schaufler wrote:
> On 3/12/2024 11:14 AM, Fan Wu wrote:
> > On 3/11/2024 8:07 PM, Eric Biggers wrote:
> > > On Mon, Mar 11, 2024 at 07:57:12PM -0700, Eric Biggers wrote:
> > > > As I've said before, this commit message needs some work.  It
> > > > currently doesn't
> > > > say anything about what the patch actually does.
> > > >
> > > > BTW, please make sure you're Cc'ing the fsverity mailing list
> > > > (fsverity@xxxxxxxxxxxxxxx), not fscrypt
> > > > (linux-fscrypt@xxxxxxxxxxxxxxx).
> > >
> > > Also, I thought this patch was using a new LSM hook, but I now see
> > > that you're
> > > actually abusing the existing security_inode_setsecurity() LSM hook.
> > > Currently
> > > that hook is called when an xattr is set.  I don't see any precedent for
> > > overloading it for other purposes.  This seems problematic, as it
> > > means that a
> > > request to set an xattr with the name you chose
> > > ("fsverity.builtin-sig") will be
> > > interpreted by LSMs as the fsverity builtin signature.  A dedicated
> > > LSM hook may
> > > be necessary to avoid issues with overloading the existing xattr hook
> > > like this.
> > >
> > > - Eric
> >
> > Thanks for the suggestion. I found that using
> > security_inode_setsecurity() causes issues with SMACK's
> > inode_setsecurity() hook. I will crate a dedicated new hook like
> > security_inode_setsig() in the next version.
>
> What is the issue you encountered with the Smack hook?

The current hook call for saving file signature will fail at 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/security/smack/smack_lsm.c#n2855 
.

The issue is file signature size is larger than SMK_LONGLABEL so if 
SMACK is enabled the SMACK's hook will return -EINVAL, which prevents us 
to use security_inode_setsecurity() to save file signature into security 
blob.

We could also make security_inode_setsecurity() more generic instead of 
for xattr only, any suggestions?

-Fan