Re: [REPORT] Use-after-free Read in __fdget_raw in v5.10.y

[Lee Jones <lee.jones@linaro.org>]

On Wed, 18 May 2022, Jens Axboe wrote:

> On 5/18/22 9:14 AM, Lee Jones wrote:
> > 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.
> 
> Alright, maybe it's not enough. I can't get your reproducer to crash,
> unfortunately. I'll try on a different box.

You need to have fuzzing and kasan enabled.

Here's the .config I'm using: https://termbin.com/3lvp

Pop the invocation in a while loop:

  while true; do ./repro; done

This has a 100% success rate for me.

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