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Date:   Wed, 18 May 2022 16:14:10 +0100
From:   Lee Jones <lee.jones@linaro.org>
To:     Jens Axboe <axboe@kernel.dk>
Cc:     Pavel Begunkov <asml.silence@gmail.com>, io-uring@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [REPORT] Use-after-free Read in __fdget_raw in v5.10.y
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On Wed, 18 May 2022, Jens Axboe wrote:

> On 5/18/22 6:54 AM, Jens Axboe wrote:
> > On 5/18/22 6:52 AM, Jens Axboe wrote:
> >> On 5/18/22 6:50 AM, Lee Jones wrote:
> >>> On Tue, 17 May 2022, Jens Axboe wrote:
> >>>
> >>>> On 5/17/22 7:00 AM, Lee Jones wrote:
> >>>>> On Tue, 17 May 2022, Jens Axboe wrote:
> >>>>>
> >>>>>> On 5/17/22 6:36 AM, Lee Jones wrote:
> >>>>>>> On Tue, 17 May 2022, Jens Axboe wrote:
> >>>>>>>
> >>>>>>>> On 5/17/22 6:24 AM, Lee Jones wrote:
> >>>>>>>>> On Tue, 17 May 2022, Jens Axboe wrote:
> >>>>>>>>>
> >>>>>>>>>> On 5/17/22 5:41 AM, Lee Jones wrote:
> >>>>>>>>>>> Good afternoon Jens, Pavel, et al.,
> >>>>>>>>>>>
> >>>>>>>>>>> Not sure if you are presently aware, but there appears to be a
> >>>>>>>>>>> use-after-free issue affecting the io_uring worker driver (fs/io-wq.c)
> >>>>>>>>>>> in Stable v5.10.y.
> >>>>>>>>>>>
> >>>>>>>>>>> The full sysbot report can be seen below [0].
> >>>>>>>>>>>
> >>>>>>>>>>> The C-reproducer has been placed below that [1].
> >>>>>>>>>>>
> >>>>>>>>>>> I had great success running this reproducer in an infinite loop.
> >>>>>>>>>>>
> >>>>>>>>>>> My colleague reverse-bisected the fixing commit to:
> >>>>>>>>>>>
> >>>>>>>>>>>   commit fb3a1f6c745ccd896afadf6e2d6f073e871d38ba
> >>>>>>>>>>>   Author: Jens Axboe <axboe@kernel.dk>
> >>>>>>>>>>>   Date:   Fri Feb 26 09:47:20 2021 -0700
> >>>>>>>>>>>
> >>>>>>>>>>>        io-wq: have manager wait for all workers to exit
> >>>>>>>>>>>
> >>>>>>>>>>>        Instead of having to wait separately on workers and manager, just have
> >>>>>>>>>>>        the manager wait on the workers. We use an atomic_t for the reference
> >>>>>>>>>>>        here, as we need to start at 0 and allow increment from that. Since the
> >>>>>>>>>>>        number of workers is naturally capped by the allowed nr of processes,
> >>>>>>>>>>>        and that uses an int, there is no risk of overflow.
> >>>>>>>>>>>
> >>>>>>>>>>>        Signed-off-by: Jens Axboe <axboe@kernel.dk>
> >>>>>>>>>>>
> >>>>>>>>>>>     fs/io-wq.c | 30 ++++++++++++++++++++++--------
> >>>>>>>>>>>     1 file changed, 22 insertions(+), 8 deletions(-)
> >>>>>>>>>>
> >>>>>>>>>> Does this fix it:
> >>>>>>>>>>
> >>>>>>>>>> commit 886d0137f104a440d9dfa1d16efc1db06c9a2c02
> >>>>>>>>>> Author: Jens Axboe <axboe@kernel.dk>
> >>>>>>>>>> Date:   Fri Mar 5 12:59:30 2021 -0700
> >>>>>>>>>>
> >>>>>>>>>>     io-wq: fix race in freeing 'wq' and worker access
> >>>>>>>>>>
> >>>>>>>>>> Looks like it didn't make it into 5.10-stable, but we can certainly
> >>>>>>>>>> rectify that.
> >>>>>>>>>
> >>>>>>>>> Thanks for your quick response Jens.
> >>>>>>>>>
> >>>>>>>>> This patch doesn't apply cleanly to v5.10.y.
> >>>>>>>>
> >>>>>>>> This is probably why it never made it into 5.10-stable :-/
> >>>>>>>
> >>>>>>> Right.  It doesn't apply at all unfortunately.
> >>>>>>>
> >>>>>>>>> I'll have a go at back-porting it.  Please bear with me.
> >>>>>>>>
> >>>>>>>> Let me know if you into issues with that and I can help out.
> >>>>>>>
> >>>>>>> I think the dependency list is too big.
> >>>>>>>
> >>>>>>> Too much has changed that was never back-ported.
> >>>>>>>
> >>>>>>> Actually the list of patches pertaining to fs/io-wq.c alone isn't so
> >>>>>>> bad, I did start to back-port them all but some of the big ones have
> >>>>>>> fs/io_uring.c changes incorporated and that list is huge (256 patches
> >>>>>>> from v5.10 to the fixing patch mentioned above).
> >>>>>>
> >>>>>> The problem is that 5.12 went to the new worker setup, and this patch
> >>>>>> landed after that even though it also applies to the pre-native workers.
> >>>>>> Hence the dependency chain isn't really as long as it seems, probably
> >>>>>> just a few patches backporting the change references and completions.
> >>>>>>
> >>>>>> I'll take a look this afternoon.
> >>>>>
> >>>>> Thanks Jens.  I really appreciate it.
> >>>>
> >>>> Can you see if this helps? Untested...
> >>>
> >>> What base does this apply against please?
> >>>
> >>> I tried Mainline and v5.10.116 and both failed.
> >>
> >> It's against 5.10.116, so that's puzzling. Let me double check I sent
> >> the right one...
> > 
> > Looks like I sent the one from the wrong directory, sorry about that.
> > This one should be better:
> 
> Nope, both are the right one. Maybe your mailer is mangling the patch?
> I'll attach it gzip'ed here in case that helps.

Okay, that applied, thanks.

Unfortunately, I am still able to crash the kernel in the same way.

-- 
Lee Jones [李琼斯]
Principal Technical Lead - Developer Services
Linaro.org │ Open source software for Arm SoCs
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