linux-kernel - Re: [REPORT] Use-after-free Read in __fdget

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Date:   Tue, 17 May 2022 14:00:17 +0100
From:   Lee Jones <lee.jones@...aro.org>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     Pavel Begunkov <asml.silence@...il.com>, io-uring@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [REPORT] Use-after-free Read in __fdget_raw in v5.10.y

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@...nel.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@...nel.dk>
> >>>>>
> >>>>>     fs/io-wq.c | 30 ++++++++++++++++++++++--------
> >>>>>     1 file changed, 22 insertions(+), 8 deletions(-)
> >>>>
> >>>> Does this fix it:
> >>>>
> >>>> commit 886d0137f104a440d9dfa1d16efc1db06c9a2c02
> >>>> Author: Jens Axboe <axboe@...nel.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.

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