netdev - Re: [syzbot] BUG: unable to handle kernel access to user memory in sock

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Date:   Thu, 18 Mar 2021 16:18:03 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Ben Dooks <ben.dooks@...ethink.co.uk>
Cc:     syzbot <syzbot+c23c5421600e9b454849@...kaller.appspotmail.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        linux-riscv <linux-riscv@...ts.infradead.org>, andrii@...nel.org,
        Alexei Starovoitov <ast@...nel.org>, bpf <bpf@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        David Miller <davem@...emloft.net>,
        John Fastabend <john.fastabend@...il.com>,
        Martin KaFai Lau <kafai@...com>, kpsingh@...nel.org,
        Jakub Kicinski <kuba@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        Song Liu <songliubraving@...com>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Yonghong Song <yhs@...com>
Subject: Re: [syzbot] BUG: unable to handle kernel access to user memory in sock_ioctl

On Mon, Mar 15, 2021 at 3:41 PM Ben Dooks <ben.dooks@...ethink.co.uk> wrote:
>
> On 15/03/2021 11:52, Dmitry Vyukov wrote:
> > On Mon, Mar 15, 2021 at 12:30 PM Ben Dooks <ben.dooks@...ethink.co.uk> wrote:
> >>
> >> On 14/03/2021 11:03, Dmitry Vyukov wrote:
> >>> On Sun, Mar 14, 2021 at 11:01 AM Dmitry Vyukov <dvyukov@...gle.com> wrote:
> >>>>> On Wed, Mar 10, 2021 at 7:28 PM syzbot
> >>>>> <syzbot+c23c5421600e9b454849@...kaller.appspotmail.com> wrote:
> >>>>>>
> >>>>>> Hello,
> >>>>>>
> >>>>>> syzbot found the following issue on:
> >>>>>>
> >>>>>> HEAD commit:    0d7588ab riscv: process: Fix no prototype for arch_dup_tas..
> >>>>>> git tree:       git://git.kernel.org/pub/scm/linux/kernel/git/riscv/linux.git fixes
> >>>>>> console output: https://syzkaller.appspot.com/x/log.txt?x=122c343ad00000
> >>>>>> kernel config:  https://syzkaller.appspot.com/x/.config?x=e3c595255fb2d136
> >>>>>> dashboard link: https://syzkaller.appspot.com/bug?extid=c23c5421600e9b454849
> >>>>>> userspace arch: riscv64
> >>>>>>
> >>>>>> Unfortunately, I don't have any reproducer for this issue yet.
> >>>>>>
> >>>>>> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> >>>>>> Reported-by: syzbot+c23c5421600e9b454849@...kaller.appspotmail.com
> >>>>>
> >>>>> +riscv maintainers
> >>>>>
> >>>>> Another case of put_user crashing.
> >>>>
> >>>> There are 58 crashes in sock_ioctl already. Somehow there is a very
> >>>> significant skew towards crashing with this "user memory without
> >>>> uaccess routines" in schedule_tail and sock_ioctl of all places in the
> >>>> kernel that use put_user... This looks very strange... Any ideas
> >>>> what's special about these 2 locations?
> >>>
> >>> I could imagine if such a crash happens after a previous stack
> >>> overflow and now task data structures are corrupted. But f_getown does
> >>> not look like a function that consumes way more than other kernel
> >>> syscalls...
> >>
> >> The last crash I looked at suggested somehow put_user got re-entered
> >> with the user protection turned back on. Either there is a path through
> >> one of the kernel handlers where this happens or there's something
> >> weird going on with qemu.
> >
> > Is there any kind of tracking/reporting that would help to localize
> > it? I could re-reproduce with that code.
>
> I'm not sure. I will have a go at debugging on qemu today just to make
> sure I can reproduce here before I have to go into the office and fix
> my Icicle board for real hardware tests.
>
> I think my first plan post reproduction is to stuff some trace points
> into the fault handlers to see if we can get a idea of faults being
> processed, etc.
>
> Maybe also add a check in the fault handler to see if the fault was
> in a fixable region and post an error if that happens / maybe retry
> the instruction with the relevant SR_SUM flag set.
>
> Hopefully tomorrow I can get a run on real hardware to confirm.
> Would have been better if the Unmatched board I ordered last year
> would turn up.

In retrospect it's obvious what's common between these 2 locations:
they both call a function inside of put_user.

#syz dup:
BUG: unable to handle kernel access to user memory in schedule_tail