Re: INFO: rcu detected stall in sys_kill

[Daniel Axtens]

Dmitry Vyukov <dvyukov@xxxxxxxxxx> writes:

> On Thu, Jan 9, 2020 at 11:05 AM Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
>> > > > > On 1/8/2020 2:25 AM, Tetsuo Handa wrote:
>> > > > > > On 2020/01/08 15:20, Dmitry Vyukov wrote:
>> > > > > >> I temporarily re-enabled smack instance and it produced another 50
>> > > > > >> stalls all over the kernel, and now keeps spewing a dozen every hour.
>> > > > >
>> > > > > Do I have to be using clang to test this? I'm setting up to work on this,
>> > > > > and don't want to waste time using my current tool chain if the problem
>> > > > > is clang specific.
>> > > >
>> > > > Humm, interesting. Initially I was going to say that most likely it's
>> > > > not clang-related. Bug smack instance is actually the only one that
>> > > > uses clang as well (except for KMSAN of course). So maybe it's indeed
>> > > > clang-related rather than smack-related. Let me try to build a kernel
>> > > > with clang.
>> > >
>> > > +clang-built-linux, glider
>> > >
>> > > [clang-built linux is severe broken since early Dec]
>> > >
>> > > Building kernel with clang I can immediately reproduce this locally:
>> > >
>> > > $ syz-manager
>> > > 2020/01/09 09:27:15 loading corpus...
>> > > 2020/01/09 09:27:17 serving http on http://0.0.0.0:50001
>> > > 2020/01/09 09:27:17 serving rpc on tcp://[::]:45851
>> > > 2020/01/09 09:27:17 booting test machines...
>> > > 2020/01/09 09:27:17 wait for the connection from test machine...
>> > > 2020/01/09 09:29:23 machine check:
>> > > 2020/01/09 09:29:23 syscalls                : 2961/3195
>> > > 2020/01/09 09:29:23 code coverage           : enabled
>> > > 2020/01/09 09:29:23 comparison tracing      : enabled
>> > > 2020/01/09 09:29:23 extra coverage          : enabled
>> > > 2020/01/09 09:29:23 setuid sandbox          : enabled
>> > > 2020/01/09 09:29:23 namespace sandbox       : enabled
>> > > 2020/01/09 09:29:23 Android sandbox         : /sys/fs/selinux/policy
>> > > does not exist
>> > > 2020/01/09 09:29:23 fault injection         : enabled
>> > > 2020/01/09 09:29:23 leak checking           : CONFIG_DEBUG_KMEMLEAK is
>> > > not enabled
>> > > 2020/01/09 09:29:23 net packet injection    : enabled
>> > > 2020/01/09 09:29:23 net device setup        : enabled
>> > > 2020/01/09 09:29:23 concurrency sanitizer   : /sys/kernel/debug/kcsan
>> > > does not exist
>> > > 2020/01/09 09:29:23 devlink PCI setup       : PCI device 0000:00:10.0
>> > > is not available
>> > > 2020/01/09 09:29:27 corpus                  : 50226 (0 deleted)
>> > > 2020/01/09 09:29:27 VMs 20, executed 0, cover 0, crashes 0, repro 0
>> > > 2020/01/09 09:29:37 VMs 20, executed 45, cover 0, crashes 0, repro 0
>> > > 2020/01/09 09:29:47 VMs 20, executed 74, cover 0, crashes 0, repro 0
>> > > 2020/01/09 09:29:57 VMs 20, executed 80, cover 0, crashes 0, repro 0
>> > > 2020/01/09 09:30:07 VMs 20, executed 80, cover 0, crashes 0, repro 0
>> > > 2020/01/09 09:30:17 VMs 20, executed 80, cover 0, crashes 0, repro 0
>> > > 2020/01/09 09:30:27 VMs 20, executed 80, cover 0, crashes 0, repro 0
>> > > 2020/01/09 09:30:37 VMs 20, executed 80, cover 0, crashes 0, repro 0
>> > > 2020/01/09 09:30:47 VMs 20, executed 80, cover 0, crashes 0, repro 0
>> > > 2020/01/09 09:30:57 VMs 20, executed 80, cover 0, crashes 0, repro 0
>> > > 2020/01/09 09:31:07 VMs 20, executed 80, cover 0, crashes 0, repro 0
>> > > 2020/01/09 09:31:17 VMs 20, executed 80, cover 0, crashes 0, repro 0
>> > > 2020/01/09 09:31:26 vm-10: crash: INFO: rcu detected stall in do_idle
>> > > 2020/01/09 09:31:27 VMs 13, executed 80, cover 0, crashes 0, repro 0
>> > > 2020/01/09 09:31:28 vm-1: crash: INFO: rcu detected stall in sys_futex
>> > > 2020/01/09 09:31:29 vm-4: crash: INFO: rcu detected stall in sys_futex
>> > > 2020/01/09 09:31:31 vm-0: crash: INFO: rcu detected stall in sys_getsockopt
>> > > 2020/01/09 09:31:33 vm-18: crash: INFO: rcu detected stall in sys_clone3
>> > > 2020/01/09 09:31:35 vm-3: crash: INFO: rcu detected stall in sys_futex
>> > > 2020/01/09 09:31:36 vm-8: crash: INFO: rcu detected stall in do_idle
>> > > 2020/01/09 09:31:37 VMs 7, executed 80, cover 0, crashes 6, repro 0
>> > > 2020/01/09 09:31:38 vm-19: crash: INFO: rcu detected stall in schedule_tail
>> > > 2020/01/09 09:31:40 vm-6: crash: INFO: rcu detected stall in schedule_tail
>> > > 2020/01/09 09:31:42 vm-2: crash: INFO: rcu detected stall in schedule_tail
>> > > 2020/01/09 09:31:44 vm-12: crash: INFO: rcu detected stall in sys_futex
>> > > 2020/01/09 09:31:46 vm-15: crash: INFO: rcu detected stall in sys_nanosleep
>> > > 2020/01/09 09:31:47 VMs 1, executed 80, cover 0, crashes 11, repro 0
>> > > 2020/01/09 09:31:48 vm-16: crash: INFO: rcu detected stall in sys_futex
>> > > 2020/01/09 09:31:50 vm-9: crash: INFO: rcu detected stall in schedule
>> > > 2020/01/09 09:31:52 vm-13: crash: INFO: rcu detected stall in schedule_tail
>> > > 2020/01/09 09:31:54 vm-11: crash: INFO: rcu detected stall in schedule_tail
>> > > 2020/01/09 09:31:56 vm-17: crash: INFO: rcu detected stall in sys_futex
>> > > 2020/01/09 09:31:57 VMs 0, executed 80, cover 0, crashes 16, repro 0
>> > > 2020/01/09 09:31:58 vm-7: crash: INFO: rcu detected stall in sys_futex
>> > > 2020/01/09 09:32:00 vm-5: crash: INFO: rcu detected stall in dput
>> > > 2020/01/09 09:32:02 vm-14: crash: INFO: rcu detected stall in sys_nanosleep
>> > >
>> > >
>> > > Then I switched LSM to selinux and I _still_ can reproduce this. So,
>> > > Casey, you may relax, this is not smack-specific :)
>> > >
>> > > Then I disabled CONFIG_KASAN_VMALLOC and CONFIG_VMAP_STACK and it
>> > > started working normally.
>> > >
>> > > So this is somehow related to both clang and KASAN/VMAP_STACK.
>> > >
>> > > The clang I used is:
>> > > https://storage.googleapis.com/syzkaller/clang-kmsan-362913.tar.gz
>> > > (the one we use on syzbot).
>> >
>> >
>> > Clustering hangs, they all happen within very limited section of the code:
>> >
>> >       1  free_thread_stack+0x124/0x590 kernel/fork.c:284
>> >       5  free_thread_stack+0x12e/0x590 kernel/fork.c:280
>> >      39  free_thread_stack+0x12e/0x590 kernel/fork.c:284
>> >       6  free_thread_stack+0x133/0x590 kernel/fork.c:280
>> >       5  free_thread_stack+0x13d/0x590 kernel/fork.c:280
>> >       2  free_thread_stack+0x141/0x590 kernel/fork.c:280
>> >       6  free_thread_stack+0x14c/0x590 kernel/fork.c:280
>> >       9  free_thread_stack+0x151/0x590 kernel/fork.c:280
>> >       3  free_thread_stack+0x15b/0x590 kernel/fork.c:280
>> >      67  free_thread_stack+0x168/0x590 kernel/fork.c:280
>> >       6  free_thread_stack+0x16d/0x590 kernel/fork.c:284
>> >       2  free_thread_stack+0x177/0x590 kernel/fork.c:284
>> >       1  free_thread_stack+0x182/0x590 kernel/fork.c:284
>> >       1  free_thread_stack+0x186/0x590 kernel/fork.c:284
>> >      16  free_thread_stack+0x18b/0x590 kernel/fork.c:284
>> >       4  free_thread_stack+0x195/0x590 kernel/fork.c:284
>> >
>> > Here is disass of the function:
>> > https://gist.githubusercontent.com/dvyukov/a283d1aaf2ef7874001d56525279ccbd/raw/ac2478bff6472bc473f57f91a75f827cd72bb6bf/gistfile1.txt
>> >
>> > But if I am not mistaken, the function only ever jumps down. So how
>> > can it loop?...
>>
>>
>> This is a miscompilation related to static branches.
>>
>> objdump shows:
>>
>> ffffffff814878f8: 0f 1f 44 00 00        nopl   0x0(%rax,%rax,1)
>>  ./arch/x86/include/asm/jump_label.h:25
>> asm_volatile_goto("1:"
>>
>> However, the actual instruction in memory at the time is:
>>
>>    0xffffffff814878f8 <+408>: jmpq   0xffffffff8148787f <free_thread_stack+287>
>>
>> Which jumps to a wrong location in free_thread_stack and makes it loop.
>>
>> The static branch is this:
>>
>> static inline bool memcg_kmem_enabled(void)
>> {
>>   return static_branch_unlikely(&memcg_kmem_enabled_key);
>> }
>>
>> static inline void memcg_kmem_uncharge(struct page *page, int order)
>> {
>>   if (memcg_kmem_enabled())
>>     __memcg_kmem_uncharge(page, order);
>> }
>>
>> I suspect it may have something to do with loop unrolling. It may jump
>> to the right location, but in the wrong unrolled iteration.
>
>
> Kernel built with clang version 10.0.0
> (https://github.com/llvm/llvm-project.git
> c2443155a0fb245c8f17f2c1c72b6ea391e86e81) works fine.
>
> Alex, please update clang on syzbot machines.

Wow, what a bug. Very happy to be off the hook for causing it, and
feeling a lot better about my inability to reproduce it with a GCC-built
kernel!

Regards,
Daniel