Re: [PATCH] xenbus: Use kref to track req lifetime

["Marek Marczykowski-Górecki" <marmarek@invisiblethingslab.com>]

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On Tue, May 06, 2025 at 05:09:33PM -0400, Jason Andryuk wrote:
> Marek reported seeing a NULL pointer fault in the xenbus_thread
> callstack:
> BUG: kernel NULL pointer dereference, address: 0000000000000000
> RIP: e030:__wake_up_common+0x4c/0x180
> Call Trace:
>  <TASK>
>  __wake_up_common_lock+0x82/0xd0
>  process_msg+0x18e/0x2f0
>  xenbus_thread+0x165/0x1c0
> 
> process_msg+0x18e is req->cb(req).  req->cb is set to xs_wake_up(), a
> thin wrapper around wake_up(), or xenbus_dev_queue_reply().  It seems
> like it was xs_wake_up() in this case.
> 
> It seems like req may have woken up the xs_wait_for_reply(), which
> kfree()ed the req.  When xenbus_thread resumes, it faults on the zero-ed
> data.
> 
> Linux Device Drivers 2nd edition states:
> "Normally, a wake_up call can cause an immediate reschedule to happen,
> meaning that other processes might run before wake_up returns."
> ... which would match the behaviour observed.
> 
> Change to keeping two krefs on each request.  One for the caller, and
> one for xenbus_thread.  Each will kref_put() when finished, and the last
> will free it.
> 
> This use of kref matches the description in
> Documentation/core-api/kref.rst
> 
> Link: https://lore.kernel.org/xen-devel/ZO0WrR5J0xuwDIxW@mail-itl/
> Reported-by: "Marek Marczykowski-Górecki" <marmarek@invisiblethingslab.com>
> Fixes: fd8aa9095a95 ("xen: optimize xenbus driver for multiple concurrent xenstore accesses")
> Cc: stable@vger.kernel.org
> Signed-off-by: Jason Andryuk <jason.andryuk@amd.com>
> ---
> Kinda RFC-ish as I don't know if it fixes Marek's issue.  This does seem
> like the correct approach if we are seeing req free()ed out from under
> xenbus_thread.

Thanks for the patch! I don't have easy way to test if it definitely
fixes the issues (due to poor reproduction rate), but it looks very
likely. I did run it through our CI and at least there it didn't crash
(but again, it doesn't happen often).

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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