[syzbot] BUG: unable to handle kernel access to user memory in sock_ioctl

[Dmitry Vyukov]

On Thu, Mar 18, 2021 at 4:35 PM Ben Dooks <ben.dooks at codethink.co.uk> wrote:
>
> On 18/03/2021 15:18, Dmitry Vyukov wrote:
> > On Mon, Mar 15, 2021 at 3:41 PM Ben Dooks <ben.dooks at codethink.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 at codethink.co.uk> wrote:
> >>>>
> >>>> On 14/03/2021 11:03, Dmitry Vyukov wrote:
> >>>>> On Sun, Mar 14, 2021 at 11:01 AM Dmitry Vyukov <dvyukov at google.com> wrote:
> >>>>>>> On Wed, Mar 10, 2021 at 7:28 PM syzbot
> >>>>>>> <syzbot+c23c5421600e9b454849 at syzkaller.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 at syzkaller.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
>
> I think so. I've posted a patch that you can test, which should force
> the flags to be saved over switch_to(). I think the sanitisers are just
> making it easier to see.
>
> There is a seperate issue of passing complicated things to put_user()
> as for security, the function may be executed with the user-space
> protections turned off. I plan to raise this on the kernel list later
> once I've done some more testing.

Thanks for quick debugging and the fix. This is the top crasher on the
syzbot instance, so this will unblock real testing.
I think I will trust your testing. syzbot instance is now on
riscv/fixes branch, so it will pick it up as soon as it's in that tree
(hopefully soon) and will do as exhaustive testing as possible :)