Re: xfs/ima: Regression caching i_version

[Frederick Lawler <fred@cloudflare.com>]

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?
IMA is calling into inode_eq_iversion() to perform the comparison
between the cached value and inode.i_version.
> 
> 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];
>