Re: xfs/ima: Regression caching i_version

[Frederick Lawler <fred@cloudflare.com>]

On Fri, Dec 12, 2025 at 06:41:00AM +0900, Jeff Layton wrote:
> On Thu, 2025-12-11 at 15:12 -0600, Frederick Lawler wrote:
> > On Fri, Dec 12, 2025 at 05:55:45AM +0900, Jeff Layton wrote:
> > > On Thu, 2025-12-11 at 14:29 -0600, Frederick Lawler wrote:
> > > > Hi Jeff,
> > > > 
> > > > While testing 6.18, I think I found a regression with
> > > > commit 1cf7e834a6fb ("xfs: switch to multigrain timestamps") since 6.13
> > > > where IMA is no longer able to properly cache i_version when we overlay
> > > > tmpfs on top of XFS. Each measurement diff check in function
> > > > process_measurement() reports that the i_version is
> > > > always set to zero for iint->real_inode.version.
> > > > 
> > > > The function ima_collect_measurement() is looking to extract the version
> > > > from the cookie on next measurement to cache i_version.
> > > > 
> > > > I'm unclear from the commit description what the right approach here is:
> > > > update in IMA land by checking for time changes, or do
> > > > something else such as adding the cookie back.
> > > > 
> > > > 
> > > 
> > > What we probably want to do is switch to using the ctime to manufacture
> > > a change attribute when STATX_CHANGE_ATTRIBUTE is not set in the statx
> > > reply.
> > > 
> > > IIRC, IMA doesn't need to persist these values across reboot, so
> > > something like this (completely untested) might work, but it may be
> > > better to lift nfsd4_change_attribute() into a common header and use
> > > the same mechanism for both:
> > 
> > I agree lifting nfsd4_change_attribute(), if anything else, a consistent
> > place to fetch the i_version from. Am I correct in my understanding that
> > the XOR on the times will cancel out and result in just the i_version?
> 
> No. I was just using the XOR to mix the tv_sec and tv_nsec fields
> together in a way that (hopefully) wouldn't generate collisions. It's
> quite not as robust as what nfsd4_change_attribute() does, but might be
> sane enough for IMA.
> 
> > IMA is calling into inode_eq_iversion() to perform the comparison
> > between the cached value and inode.i_version.
> 
> That just looks at the i_version field directly without going through -
> >getattr, so that would need to be switched over as well. Could
> integrity_inode_attrs_changed() use vfs_getattr_nosec() and compare the
> result?

That makes sense to me. I'll look through it a bit more, roll a patch, and
see what the IMA folks have to say (unless they comment here first).

Thanks Jeff

> 
> 
> > > 
> > > diff --git a/security/integrity/ima/ima_api.c b/security/integrity/ima/ima_api.c
> > > index c35ea613c9f8..5a71845f579e 100644
> > > --- a/security/integrity/ima/ima_api.c
> > > +++ b/security/integrity/ima/ima_api.c
> > > @@ -272,10 +272,14 @@ int ima_collect_measurement(struct ima_iint_cache *iint, struct file *file,
> > >          * to an initial measurement/appraisal/audit, but was modified to
> > >          * assume the file changed.
> > >          */
> > > -       result = vfs_getattr_nosec(&file->f_path, &stat, STATX_CHANGE_COOKIE,
> > > +       result = vfs_getattr_nosec(&file->f_path, &stat, STATX_CHANGE_COOKIE | STATX_CTIME,
> > >                                    AT_STATX_SYNC_AS_STAT);
> > > -       if (!result && (stat.result_mask & STATX_CHANGE_COOKIE))
> > > -               i_version = stat.change_cookie;
> > > +       if (!result) {
> > > +               if (stat.result_mask & STATX_CHANGE_COOKIE)
> > > +                       i_version = stat.change_cookie;
> > > +               else if (stat.result_mask & STATX_CTIME)
> > > +                       i_version = stat.ctime.tv_sec ^ stat.ctime.tv_nsec;
> > > +       }
> > >         hash.hdr.algo = algo;
> > >         hash.hdr.length = hash_digest_size[algo];
> > >  
> 
> -- 
> Jeff Layton <jlayton@kernel.org>