Messages in this thread | | | Subject | Re: [syzbot] BUG: unable to handle kernel access to user memory in schedule_tail | From | Ben Dooks <> | Date | Fri, 12 Mar 2021 16:36:50 +0000 |
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On 12/03/2021 16:34, Ben Dooks wrote: > On 12/03/2021 16:30, Ben Dooks wrote: >> On 12/03/2021 15:12, Dmitry Vyukov wrote: >>> On Fri, Mar 12, 2021 at 2:50 PM Ben Dooks <ben.dooks@codethink.co.uk> >>> wrote: >>>> >>>> On 10/03/2021 17:16, Dmitry Vyukov wrote: >>>>> On Wed, Mar 10, 2021 at 5:46 PM syzbot >>>>> <syzbot+e74b94fe601ab9552d69@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=1212c6e6d00000 >>>>>> kernel config: >>>>>> https://syzkaller.appspot.com/x/.config?x=e3c595255fb2d136 >>>>>> dashboard link: >>>>>> https://syzkaller.appspot.com/bug?extid=e74b94fe601ab9552d69 >>>>>> 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+e74b94fe601ab9552d69@syzkaller.appspotmail.com >>>>> >>>>> +riscv maintainers >>>>> >>>>> This is riscv64-specific. >>>>> I've seen similar crashes in put_user in other places. It looks like >>>>> put_user crashes in the user address is not mapped/protected (?). >>>> >>>> I've been having a look, and this seems to be down to access of the >>>> tsk->set_child_tid variable. I assume the fuzzing here is to pass a >>>> bad address to clone? >>>> >>>> From looking at the code, the put_user() code should have set the >>>> relevant SR_SUM bit (the value for this, which is 1<<18 is in the >>>> s2 register in the crash report) and from looking at the compiler >>>> output from my gcc-10, the code looks to be dong the relevant csrs >>>> and then csrc around the put_user >>>> >>>> So currently I do not understand how the above could have happened >>>> over than something re-tried the code seqeunce and ended up retrying >>>> the faulting instruction without the SR_SUM bit set. >>> >>> I would maybe blame qemu for randomly resetting SR_SUM, but it's >>> strange that 99% of these crashes are in schedule_tail. If it would be >>> qemu, then they would be more evenly distributed... >>> >>> Another observation: looking at a dozen of crash logs, in none of >>> these cases fuzzer was actually trying to fuzz clone with some insane >>> arguments. So it looks like completely normal clone's (e..g coming >>> from pthread_create) result in this crash. >>> >>> I also wonder why there is ret_from_exception, is it normal? I see >>> handle_exception disables SR_SUM: >>> https://elixir.bootlin.com/linux/v5.12-rc2/source/arch/riscv/kernel/entry.S#L73 >>> >> >> So I think if SR_SUM is set, then it faults the access to user memory >> which the _user() routines clear to allow them access. >> >> I'm thinking there is at least one issue here: >> >> - the test in fault is the wrong way around for die kernel >> - the handler only catches this if the page has yet to be mapped. >> >> So I think the test should be: >> >> if (!user_mode(regs) && addr < TASK_SIZE && >> unlikely(regs->status & SR_SUM) >> >> This then should continue on and allow the rest of the handler to >> complete mapping the page if it is not there. >> >> I have been trying to create a very simple clone test, but so far it >> has yet to actually trigger anything. > > I should have added there doesn't seem to be a good way to use mmap() > to allocate memory but not insert a vm-mapping post the mmap(). > >
How difficult is it to try building a branch with the above test modified?
-- Ben Dooks http://www.codethink.co.uk/ Senior Engineer Codethink - Providing Genius
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