Messages in this thread Patch in this message | | | From | Dmitry Vyukov <> | Date | Fri, 12 Mar 2021 18:38:57 +0100 | Subject | Re: [syzbot] BUG: unable to handle kernel access to user memory in schedule_tail |
| |
On Fri, Mar 12, 2021 at 6:34 PM Dmitry Vyukov <dvyukov@google.com> wrote: > > 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.
The inverted check crashes during boot:
--- a/arch/riscv/mm/fault.c +++ b/arch/riscv/mm/fault.c @@ -249,7 +249,7 @@ asmlinkage void do_page_fault(struct pt_regs *regs) flags |= FAULT_FLAG_USER;
if (!user_mode(regs) && addr < TASK_SIZE && - unlikely(!(regs->status & SR_SUM))) + unlikely(regs->status & SR_SUM)) die_kernel_fault("access to user memory without uaccess routines", addr, regs);
[ 77.349329][ T1] Run /sbin/init as init process [ 77.868371][ T1] Unable to handle kernel access to user memory without uaccess routines at virtual address 00000000000e8e39 [ 77.870355][ T1] Oops [#1] [ 77.870766][ T1] Modules linked in: [ 77.871326][ T1] CPU: 0 PID: 1 Comm: init Not tainted 5.12.0-rc2-00010-g0d7588ab9ef9-dirty #42 [ 77.872057][ T1] Hardware name: riscv-virtio,qemu (DT) [ 77.872620][ T1] epc : __clear_user+0x36/0x4e [ 77.873285][ T1] ra : padzero+0x9c/0xb0 [ 77.873849][ T1] epc : ffffffe000bb7136 ra : ffffffe0004f42a0 sp : ffffffe006f8fbc0 [ 77.874438][ T1] gp : ffffffe005d25718 tp : ffffffe006f98000 t0 : 00000000000e8e40 [ 77.875031][ T1] t1 : 00000000000e9000 t2 : 000000000001c49c s0 : ffffffe006f8fbf0 [ 77.875618][ T1] s1 : 00000000000001c7 a0 : 00000000000e8e39 a1 : 00000000000001c7 [ 77.876204][ T1] a2 : 0000000000000002 a3 : 00000000000e9000 a4 : ffffffe006f99000 [ 77.876787][ T1] a5 : 0000000000000000 a6 : 0000000000f00000 a7 : ffffffe00031c088 [ 77.877367][ T1] s2 : 00000000000e8e39 s3 : 0000000000001000 s4 : 0000003ffffffe39 [ 77.877952][ T1] s5 : 00000000000e8e39 s6 : 00000000000e9570 s7 : 00000000000e8e39 [ 77.878535][ T1] s8 : 0000000000000001 s9 : 00000000000e8e39 s10: ffffffe00c65f608 [ 77.879126][ T1] s11: ffffffe00816e8d8 t3 : ea3af0fa372b8300 t4 : 0000000000000003 [ 77.879711][ T1] t5 : ffffffc401dc45d8 t6 : 0000000000040000 [ 77.880209][ T1] status: 0000000000040120 badaddr: 00000000000e8e39 cause: 000000000000000f [ 77.880846][ T1] Call Trace: [ 77.881213][ T1] [<ffffffe000bb7136>] __clear_user+0x36/0x4e [ 77.881912][ T1] [<ffffffe0004f523e>] load_elf_binary+0xf8a/0x2400 [ 77.882562][ T1] [<ffffffe0003e1802>] bprm_execve+0x5b0/0x1080 [ 77.883145][ T1] [<ffffffe0003e38bc>] kernel_execve+0x204/0x288 [ 77.883727][ T1] [<ffffffe003b70e94>] run_init_process+0x1fe/0x212 [ 77.884337][ T1] [<ffffffe003b70ec6>] try_to_run_init_process+0x1e/0x66 [ 77.884956][ T1] [<ffffffe003bc0864>] kernel_init+0x14a/0x200 [ 77.885541][ T1] [<ffffffe000005570>] ret_from_exception+0x0/0x14 [ 77.886955][ T1] ---[ end trace 1e934d07b8a4bed8 ]--- [ 77.887705][ T1] Kernel panic - not syncing: Fatal exception [ 77.888333][ T1] SMP: stopping secondary CPUs [ 77.889357][ T1] Rebooting in 86400 seconds..
| |