Re: [PATCH v2] kernfs: recheck of->released after acquiring the active reference

[Tejun Heo]

On Fri, Aug 21, 2026 at 05:07:20AM +0000, Fan Wu wrote:
> kernfs_get_active_of(), added by commit 3c9ba2777d6c ("kernfs: Fix UAF
> in polling when open file is released"), tests @of->released before
> acquiring the active reference on @of->kn.  A hide/drain/show cycle can
> run between those steps: the drain path releases the open file, and the
> reactivation lets kernfs_get_active() succeed again.  Any entry guarded
> by kernfs_get_active_of() can consequently run its file operation on an
> already released open file; on the cgroup pressure files, the poll
> callback dereferences of->priv while forming &ctx->psi.trigger and can
> hit either stale, freed memory or NULL.
> 
>   CPU 0 (kernfs_fop_poll)       CPU 1 (echo 0/1 > cgroup.pressure)
>   -------------------------     ---------------------------------
>   of->released == false         kernfs_show(kn, false)
>   ... preempted ...               kernfs_drain()
>                                    kernfs_release_file()
>                                      ->release(of)  (frees of->priv)
>                                      of->released = true
>                                 kernfs_show(kn, true)
>                                   kernfs_activate_one(kn)
>   kernfs_get_active(of->kn)
>   ops->poll(of)
> 
> The cycle needs the file operation to be delayed between the two
> steps, but kernfs_show() cycles like the one above are fully
> userspace driven.
> 
> Acquire the active reference first and re-check @of->released after
> it.  While the reference is held, @kn cannot be drained: kernfs_drain()
> waits for kn->active to reach KN_DEACTIVATED_BIAS before draining open
> files, and the only other kernfs_release_file() caller,
> kernfs_fop_release(), is serialized against in-flight file operations
> by the VFS, so the re-read settles whether @of was released for good.
> The re-check needs no lock: @of->released is only ever set to true,
> the drain which sets it precedes the reactivation under kernfs_rwsem,
> and the fully-ordered RMW on @kn->active in kernfs_get_active() then
> orders the read after that reactivation.
> 
> This issue was found by an in-house static analysis tool.
> 
> Fixes: 3c9ba2777d6c ("kernfs: Fix UAF in polling when open file is released")
> Cc: stable@xxxxxxxxxxxxxxx
> Suggested-by: Tejun Heo <tj@xxxxxxxxxx>
> Assisted-by: Codex:gpt-5.6
> Signed-off-by: Fan Wu <fanwu01@xxxxxxxxxx>

Acked-by: Tejun Heo <tj@xxxxxxxxxx>

Thanks.

-- 
tejun