Messages in this thread | | | Subject | Re: [syzbot] BUG: unable to handle kernel access to user memory in sock_ioctl | From | Ben Dooks <> | Date | Thu, 18 Mar 2021 15:34:55 +0000 |
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On 18/03/2021 15:18, Dmitry Vyukov wrote: > On Mon, Mar 15, 2021 at 3:41 PM Ben Dooks <ben.dooks@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@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@google.com> wrote: >>>>>>> On Wed, Mar 10, 2021 at 7:28 PM syzbot >>>>>>> <syzbot+c23c5421600e9b454849@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@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.
-- Ben Dooks http://www.codethink.co.uk/ Senior Engineer Codethink - Providing Genius
https://www.codethink.co.uk/privacy.html
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