Re: [PATCH 0/2] kernfs, autofs: fix pipe lock cycles through sysfs stores

[Krystian Kaniewski]

Hi Ian,

On Tue, 2026-10-27 at 07:11 +0800, Ian Kent wrote:
> Commit 1e8c813b083c ("PM: hibernate: don't use early_lookup_bdev
> in resume_store") looks like it's been around for a while so I
> assume the reproducer should work on 7.2.12?

Yes. That commit entered v6.5-rc1, so 7.2.12 contains the relevant
code.

I used the unmodified C reproducer linked from the syzbot report:

https://syzkaller.appspot.com/text?tag=ReproC&x=11c0f4c9580000

I also used the configuration provided with the report:

https://syzkaller.appspot.com/text?tag=KernelConfig&x=7d012d9c67977ee4

> What should I expect, a hang, it doesn't.

A permanent hang is not expected. The result to look for is this
lockdep warning in dmesg:

        WARNING: possible circular locking dependency detected
        ...
        &pipe->mutex --> &of->mutex --> &sbi->pipe_mutex

The reproducer records the first dependency by splicing into the
cgroup v1 devices.deny file. It records the opposite dependencies
through a normal write to /sys/power/resume. These operations use
different lock instances, so the exact reproducer reports a possible
cycle rather than deadlocking.

The write waits for an autofs daemon that never replies, but the C
wrapper kills the blocked child after five seconds and starts another
iteration. In my test the warning appeared in the first iteration.

The test needs lockdep and CONFIG_PROVE_LOCKING, the cgroup v1 devices
controller, autofs, and working hibernation support. It must also run
as root. Without lockdep, the program can appear to do nothing useful.

> Also it looks to me like the reproducer is trying exploit an
> invalid request similar to what I described earlier

Whether the string identifies a usable resume device is separate from
the autofs request. resume_store() first passes it to lookup_bdev(),
which performs a pathname lookup before the numeric fallback.

The reproducer changes its current directory to the root of a detached
indirect autofs mount. It then opens /sys/power/resume using an
absolute path and writes the relative name
"0x0000000004000921".

The resulting lookup is therefore:

        <autofs root>/0x0000000004000921

For autofs, this is an ordinary lookup of a new immediate child of the
indirect mount root, so it is a normal mount trigger. There is no
autofs mount under /sys/power.

In parallel, I am preparing a revised patch following Tejun's
suggestion. It disables splice for writable opens of
/sys/power/resume and gives its of->mutex a separate lockdep class.

If you send your autofs change, I can run the same reproducer against
it and check both the request and the lockdep warning.

Thanks,
Krystian