Message-ID: <CACT4Y+ZSyZbDTQ9VOj999eDMiowEQdQmGrNMrQ04SPHe_882pA@mail.gmail.com>
Date:   Wed, 4 Mar 2020 11:43:15 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Hillf Danton <hdanton@...a.com>
Cc:     syzbot <syzbot+0c5c2dbf76930df91489@...kaller.appspotmail.com>,
        David Miller <davem@...emloft.net>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "open list:HARDWARE RANDOM NUMBER GENERATOR CORE" 
        <linux-crypto@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>
Subject: Re: INFO: rcu detected stall in sys_keyctl

On Wed, Mar 4, 2020 at 11:29 AM Hillf Danton <hdanton@...a.com> wrote:
> On Wed, 04 Mar 2020 00:08:11 -0800
> > syzbot found the following crash on:
> >
> > HEAD commit:    63623fd4 Merge tag 'for-linus' of git://git.kernel.org/pub..
> > git tree:       upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=15257ba1e00000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=9833e26bab355358
> > dashboard link: https://syzkaller.appspot.com/bug?extid=0c5c2dbf76930df91489
> > compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
> >
> > Unfortunately, I don't have any reproducer for this crash yet.
> >
> > IMPORTANT: if you fix the bug, please add the following tag to the commit:
> > Reported-by: syzbot+0c5c2dbf76930df91489@...kaller.appspotmail.com
> >
> > rcu: INFO: rcu_preempt self-detected stall on CPU
> > rcu:  0-....: (1 GPs behind) idle=576/1/0x4000000000000002 softirq=55718/56054 fqs=5235
> >       (t=10500 jiffies g=63445 q=1523)
> > NMI backtrace for cpu 0
> > CPU: 0 PID: 18804 Comm: syz-executor.4 Not tainted 5.6.0-rc3-syzkaller #0
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> > Call Trace:
> >  <IRQ>
> >  __dump_stack lib/dump_stack.c:77 [inline]
> >  dump_stack+0x197/0x210 lib/dump_stack.c:118
> >  nmi_cpu_backtrace.cold+0x70/0xb2 lib/nmi_backtrace.c:101
> >  nmi_trigger_cpumask_backtrace+0x23b/0x28b lib/nmi_backtrace.c:62
> >  arch_trigger_cpumask_backtrace+0x14/0x20 arch/x86/kernel/apic/hw_nmi.c:38
> >  trigger_single_cpu_backtrace include/linux/nmi.h:164 [inline]
> >  rcu_dump_cpu_stacks+0x183/0x1cf kernel/rcu/tree_stall.h:254
> >  print_cpu_stall kernel/rcu/tree_stall.h:475 [inline]
> >  check_cpu_stall kernel/rcu/tree_stall.h:549 [inline]
> >  rcu_pending kernel/rcu/tree.c:3030 [inline]
> >  rcu_sched_clock_irq.cold+0x51a/0xc37 kernel/rcu/tree.c:2276
> >  update_process_times+0x2d/0x70 kernel/time/timer.c:1726
> >  tick_sched_handle+0xa2/0x190 kernel/time/tick-sched.c:171
> >  tick_sched_timer+0x53/0x140 kernel/time/tick-sched.c:1314
> >  __run_hrtimer kernel/time/hrtimer.c:1517 [inline]
> >  __hrtimer_run_queues+0x364/0xe40 kernel/time/hrtimer.c:1579
> >  hrtimer_interrupt+0x314/0x770 kernel/time/hrtimer.c:1641
> >  local_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1119 [inline]
> >  smp_apic_timer_interrupt+0x160/0x610 arch/x86/kernel/apic/apic.c:1144
> >  apic_timer_interrupt+0xf/0x20 arch/x86/entry/entry_64.S:829
> >  </IRQ>
> > RIP: 0010:__sanitizer_cov_trace_const_cmp4+0x16/0x20 kernel/kcov.c:276
> > Code: 48 89 e5 48 8b 4d 08 e8 d8 fe ff ff 5d c3 66 0f 1f 44 00 00 55 89 f2 89 fe bf 05 00 00 00 48 89 e5 48 8b 4d 08 e8 ba fe ff ff <5d> c3 0f 1f 84 00 00 00 00 00 55 48 89 f2 48 89 fe bf 07 00 00 00
> > RSP: 0018:ffffc900053877d8 EFLAGS: 00000297 ORIG_RAX: ffffffffffffff13
> > RAX: 0000000000000002 RBX: 584279fc973b765a RCX: ffffffff83b71a81
> > RDX: 00000000ffffff75 RSI: 0000000000000000 RDI: 0000000000000005
> > RBP: ffffc900053877d8 R08: ffff888041a265c0 R09: 0000000000000092
> > R10: ffffed1015d0707b R11: ffff8880ae8383db R12: ffff88809eb56398
> > R13: 000000003ab2c4e4 R14: 1b0d4377a72d08f5 R15: 00000000ffffff75
> >  mpihelp_submul_1+0x161/0x1a0 lib/mpi/generic_mpih-mul3.c:45
> >  mpihelp_divrem+0x1ce/0x1360 lib/mpi/mpih-div.c:209
> >  mpi_powm+0xffb/0x1d20 lib/mpi/mpi-pow.c:205
> >  _compute_val crypto/dh.c:39 [inline]
> >  dh_compute_value+0x373/0x610 crypto/dh.c:178
> >  crypto_kpp_generate_public_key include/crypto/kpp.h:315 [inline]
> >  __keyctl_dh_compute+0x9ae/0x1470 security/keys/dh.c:367
> >  keyctl_dh_compute+0xcf/0x12d security/keys/dh.c:422
> >  __do_sys_keyctl security/keys/keyctl.c:1818 [inline]
> >  __se_sys_keyctl security/keys/keyctl.c:1714 [inline]
> >  __x64_sys_keyctl+0x159/0x470 security/keys/keyctl.c:1714
> >  do_syscall_64+0xfa/0x790 arch/x86/entry/common.c:294
> >  entry_SYSCALL_64_after_hwframe+0x49/0xbe
> > RIP: 0033:0x45c479
> > Code: ad b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 7b b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00
> > RSP: 002b:00007f872a51fc78 EFLAGS: 00000246 ORIG_RAX: 00000000000000fa
> > RAX: ffffffffffffffda RBX: 00007f872a5206d4 RCX: 000000000045c479
> > RDX: 0000000020002700 RSI: 0000000020000400 RDI: 0000000000000017
> > RBP: 000000000076bfc0 R08: 0000000000000000 R09: 0000000000000000
> > R10: 00000000ffffff84 R11: 0000000000000246 R12: 00000000ffffffff
> > R13: 00000000000006fa R14: 00000000004c9883 R15: 000000000076bfcc
> >
> >
> > ---
> > This bug is generated by a bot. It may contain errors.
> > See https://goo.gl/tpsmEJ for more information about syzbot.
> > syzbot engineers can be reached at syzkaller@...glegroups.com.
> >
> > syzbot will keep track of this bug report. See:
> > https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
>
> Cut the chance of being a cpu hog.
>
> --- a/lib/mpi/mpih-div.c
> +++ b/lib/mpi/mpih-div.c
> @@ -215,6 +215,8 @@ q_test:
>
>                                 qp[i] = q;
>                                 n0 = np[dsize - 1];
> +
> +                               cond_resched();

Isn't it looping infinitely? cond_resched() won't help in such case.

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