Messages in this thread | | | From | Dmitry Vyukov <> | Date | Fri, 12 Mar 2021 18:34:48 +0100 | Subject | Re: [syzbot] BUG: unable to handle kernel access to user memory in schedule_tail |
| |
On Fri, Mar 12, 2021 at 5:36 PM Ben Dooks <ben.dooks@codethink.co.uk> wrote: > > 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?
I don't have access to hardware, I don't have other qemu versions ready to use. But I can teach you how to run syzkaller locally :) I am not sure anybody run it on real riscv hardware at all. When Tobias ported syzkaller, Tobias also used qemu I think.
I am now building with an inverted check to test locally.
I don't fully understand but this code, but does handle_exception reset SR_SUM around do_page_fault? If so, then looking at SR_SUM in do_page_fault won't work with positive nor negative check.
| |