linux-kernel - Re: [PATCH v5 3/3] ima: Use kstat.ctime as a fallback change detection for stacked fs

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Message-ID: <aYZJ4Y33oRRk5Drs@CMGLRV3>
Date: Fri, 6 Feb 2026 14:06:57 -0600
From: Frederick Lawler <fred@...udflare.com>
To: Mimi Zohar <zohar@...ux.ibm.com>
Cc: Roberto Sassu <roberto.sassu@...weicloud.com>,
	Roberto Sassu <roberto.sassu@...wei.com>,
	Dmitry Kasatkin <dmitry.kasatkin@...il.com>,
	Eric Snowberg <eric.snowberg@...cle.com>,
	Paul Moore <paul@...l-moore.com>, James Morris <jmorris@...ei.org>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	"Darrick J. Wong" <djwong@...nel.org>,
	Christian Brauner <brauner@...nel.org>,
	Josef Bacik <josef@...icpanda.com>,
	Jeff Layton <jlayton@...nel.org>, linux-kernel@...r.kernel.org,
	linux-integrity@...r.kernel.org,
	linux-security-module@...r.kernel.org, kernel-team@...udflare.com
Subject: Re: [PATCH v5 3/3] ima: Use kstat.ctime as a fallback change
 detection for stacked fs

On Wed, Feb 04, 2026 at 11:26:27PM -0500, Mimi Zohar wrote:
> On Wed, 2026-02-04 at 15:22 -0600, Frederick Lawler wrote:
> > That said, I think Mimi pointed out in an email [2] where multi-grain
> > file systems are impacted regardless of stacked fs or not due to the last
> > writer check.
> > 
> > I don't recall coming across that in my tests, but perhaps I did that
> > specific test wrong? To be sure, I created the C program, and on the VM,
> > created a XFS disk, mounted it on loopback, ran the rdwr program on
> > "somefile" multiple times, and only got 1 audit log for it, until I
> > mutated it with touch, and only got 2 hits: original + after mutation
> > after running the program multiple times. 
> > 
> > I'm not sure what's going on there, so I'll look into that a bit more,
> > but so far the impact is stacked file systems & multigrain ctime AFAIK.
> 
> Make sure you're testing without your patch set applied or at least the last
> patch.
> 

I'm still not sure what went wrong with my test suite, but spinning up a
VM similar to Mimi's, I was able to reproduce the last writer issue. I
used patch 1 as a baseline because that's noop from base 6.19, and used
that function as a convenient trace point.

And, then running the attached example test for stacked FS works as
expected on both patches 1 (multiple log messages) & 3 (just one log
message).

I left out the dump_stack() in integrity_inode_attrs_stat_change()
for readability in these results. For Mimi's test, we just see
calls into ima_check_last_writer(), and for the attached sample
in 0/0, we get the calls straight from process_measurement() for
stacked fs. Running Mimi's and then attached in that order, I get
double the logs because the FILE_CHECK is hitting the last writer,
and we get the process_measurement() for the binary run.

Based on all of this, I can drop the stacked FS wording in the patch
descriptions, and keep it all focused that the change_cookie semantics
have been changed for multigrain file systems.

Results:

PATCH 1/3

[root@...alhost ~]# ./mimi.sh
filename: /usr/bin/date-20260206140427
filename: /usr/bin/date-20260206140427
filename: /usr/bin/date-20260206140427
FAIL: Expected 1 audit event, but found 3.

[   26.033572] fred: integrity_inode_attrs_stat_changed result_mask=0 (change cookie) version=0 change_cookie=0
[   26.034372] ima: fred: ima_check_last_writer: must measure file: "/usr/bin/date-20260206140427"
[   26.037453] fred: integrity_inode_attrs_stat_changed result_mask=0 (change cookie) version=0 change_cookie=0
[   26.038425] ima: fred: ima_check_last_writer: must measure file: "/usr/bin/date-20260206140427"
[   26.039821] fred: integrity_inode_attrs_stat_changed result_mask=0 (change cookie) version=0 change_cookie=0
[   26.041383] ima: fred: ima_check_last_writer: must measure file: "/usr/bin/date-20260206140427"

[root@...alhost fred-tests]# ./simple-fedora.sh
314572800 bytes (315 MB, 300 MiB) copied, 0.132908 s, 2.4 GB/s
Fri Feb  6 14:56:18 EST 2026
Fri Feb  6 14:56:18 EST 2026
Fri Feb  6 14:56:18 EST 2026
Fri Feb  6 14:56:18 EST 2026
FAIL: Expected 1 audit event, but found 4.

Note: Does not have dmesg output because this patch didn't put the trace
function into integrity_inode_attrs_changed().

PATCH 3/3
[root@...alhost ~]# ./mimi.sh
filename: /usr/bin/date-20260206140141
filename: /usr/bin/date-20260206140141
filename: /usr/bin/date-20260206140141
PASS: Found exactly 1 audit event.

[   17.191235] fred: integrity_inode_attrs_stat_changed result_mask=0 (change cookie) version=1770404501462431821 change_cookie=0
[   17.192213] fred: integrity_inode_attrs_stat_changed result_mask=128 (ctime) version=1770404501462431821 ctime=1770404501462431821
[   17.196325] fred: integrity_inode_attrs_stat_changed result_mask=0 (change cookie) version=1770404501462431821 change_cookie=0
[   17.197380] fred: integrity_inode_attrs_stat_changed result_mask=128 (ctime) version=1770404501462431821 ctime=1770404501462431821
[   17.199750] fred: integrity_inode_attrs_stat_changed result_mask=0 (change cookie) version=1770404501462431821 change_cookie=0
[   17.200682] fred: integrity_inode_attrs_stat_changed result_mask=128 (ctime) version=1770404501462431821 ctime=1770404501462431821

[root@...alhost fred-tests]# ./simple-fedora.sh
Fri Feb  6 14:53:30 EST 2026
Fri Feb  6 14:53:30 EST 2026
Fri Feb  6 14:53:30 EST 2026
Fri Feb  6 14:53:30 EST 2026
PASS: Found exactly 1 audit event.

[   23.315358] fred: integrity_inode_attrs_stat_changed result_mask=0 (change cookie) version=1770407920086616962 change_cookie=0
[   23.328978] fred: integrity_inode_attrs_stat_changed result_mask=128 (ctime) version=1770407920086616962 ctime=1770407920086616962
[   23.332122] fred: integrity_inode_attrs_stat_changed result_mask=0 (change cookie) version=1770407920086616962 change_cookie=0
[   23.347162] fred: integrity_inode_attrs_stat_changed result_mask=128 (ctime) version=1770407920086616962 ctime=1770407920086616962
[   23.352931] fred: integrity_inode_attrs_stat_changed result_mask=0 (change cookie) version=1770407920086616962 change_cookie=0
[   23.368026] fred: integrity_inode_attrs_stat_changed result_mask=128 (ctime) version=1770407920086616962 ctime=1770407920086616962

Note that the in stacked FS case, process_measurement() skipped the check
for attrs changed, and skipped to measuring. Subsequent calls into
process_measurement() hits the integrity_inode_attrs_stat_changed().