Re: [PATCH] io_uring: fix sq array offset calculation

[Dmitry Vyukov <dvyukov@google.com>]

On Fri, Jul 17, 2020 at 4:05 PM Jens Axboe <axboe@kernel.dk> wrote:
>
> On 7/17/20 7:48 AM, Dmitry Vyukov wrote:
> > On Sat, Jul 11, 2020 at 6:16 PM Dmitry Vyukov <dvyukov@google.com> wrote:
> >>
> >> On Sat, Jul 11, 2020 at 5:52 PM Hristo Venev <hristo@venev.name> wrote:
> >>>
> >>> On Sat, 2020-07-11 at 17:31 +0200, Dmitry Vyukov wrote:
> >>>> Looking at the code more, I am not sure how it may not corrupt
> >>>> memory.
> >>>> There definitely should be some combinations where accessing
> >>>> sq_entries*sizeof(u32) more memory won't be OK.
> >>>> May be worth adding a test that allocates all possible sizes for
> >>>> sq/cq
> >>>> and fills both rings.
> >>>
> >>> The layout (after the fix) is roughly as follows:
> >>>
> >>> 1. struct io_rings - ~192 bytes, maybe 256
> >>> 2. cqes - (32 << n) bytes
> >>> 3. sq_array - (4 << n) bytes
> >>>
> >>> The bug was that the sq_array was offset by (4 << n) bytes. I think
> >>> issues can only occur when
> >>>
> >>>     PAGE_ALIGN(192 + (32 << n) + (4 << n) + (4 << n))
> >>>     !=
> >>>     PAGE_ALIGN(192 + (32 << n) + (4 << n))
> >>>
> >>> It looks like this never happens. We got lucky.
> >>
> >> Interesting. CQ entries are larger and we have at least that many of
> >> them as SQ entries. I guess this + power-of-2-pages can make it never
> >> overflow.
> >
> > Hi Jens,
> >
> > I see this patch is in block/for-5.9/io_uring
> > Is this tree merged into linux-next? I don't see it in linux-next yet.
> > Or is it possible to get it into 5.8?
>
> Yes, that tree is in linux-next, and I'm surprised you don't see it there
> as it's been queued up for almost a week. Are you sure?
>
> I'm not going to apply it to both 5.9 and 5.8 trees. The bug has
> been there for a while, but doesn't really impact functionality.
> Hence I just queued it up for 5.9. If this had been a 5.8 commit
> that introduced it, I would have queued it up for 5.8.
>
> > The reason I am asking is that we have an intern (Necip in CC) working
> > on significantly extending io_uring coverage in syzkaller:
> > https://github.com/google/syzkaller/pull/1926
> > Unfortunately we had to hardcode offset computation logic b/c the
> > intended way of using io_uring for normal programs represents an
> > additional obstacle for the fuzzer and the relations between syscalls
> > and writes to shared memory are even hard to express for the fuzzer.
> > We want to hardcode this new updated way of computing offsets, but
> > this means we probably won't get good coverage until the intern term
> > ends (+ may be good to discover some io_uring bugs before the
> > release).
>
> Sounds good
>
> > If it won't get into linux-next/mainline until 5.9, it's not a big
> > deal, but I wanted to ask.
>
> That's the plan, it'll go in as part of the 5.9 merge window.

Thanks.
linux-next is good enough, we test it. And the commit is actually
already there, now that I looked closer.