Re: WARNING: suspicious RCU usage in bpf_prog_array_copy_core

[Y Song]

On Tue, Aug 14, 2018 at 2:40 AM, syzbot
<syzbot+6e72317008eef84a216b@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
> Hello,
>
> syzbot found the following crash on:
>
> HEAD commit:    4110b42356f3 Add linux-next specific files for 20180810
> git tree:       linux-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=109bac02400000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=1d80606e3795a4f5
> dashboard link: https://syzkaller.appspot.com/bug?extid=6e72317008eef84a216b
> compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
> syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=157ef48a400000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=12e16cf8400000
>
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+6e72317008eef84a216b@xxxxxxxxxxxxxxxxxxxxxxxxx
>
> random: sshd: uninitialized urandom read (32 bytes read)
> random: sshd: uninitialized urandom read (32 bytes read)
> random: sshd: uninitialized urandom read (32 bytes read)
>
> =============================
> WARNING: suspicious RCU usage
> 4.18.0-rc8-next-20180810+ #36 Not tainted
> -----------------------------
> kernel/bpf/core.c:1582 suspicious rcu_dereference_check() usage!
>
> other info that might help us debug this:
>
>
> rcu_scheduler_active = 2, debug_locks = 1
> 2 locks held by syz-executor287/4449:
>  #0: 00000000d8131c0c (&ctx->mutex){+.+.}, at:
> perf_event_ctx_lock_nested+0x375/0x600 kernel/events/core.c:1276
>  #1: 000000006c916250 (bpf_event_mutex){+.+.}, at:
> perf_event_query_prog_array+0x1c6/0x380 kernel/trace/bpf_trace.c:1062
>
> stack backtrace:
> CPU: 1 PID: 4449 Comm: syz-executor287 Not tainted 4.18.0-rc8-next-20180810+
> #36
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> Google 01/01/2011
> Call Trace:
>  __dump_stack lib/dump_stack.c:77 [inline]
>  dump_stack+0x1c9/0x2b4 lib/dump_stack.c:113
>  lockdep_rcu_suspicious+0x14a/0x153 kernel/locking/lockdep.c:4562
>  bpf_prog_array_copy_core+0x2d9/0x360 kernel/bpf/core.c:1582
>  bpf_prog_array_copy_info+0x9a/0x110 kernel/bpf/core.c:1720
>  perf_event_query_prog_array+0x22e/0x380 kernel/trace/bpf_trace.c:1063
>  _perf_ioctl+0x986/0x1600 kernel/events/core.c:5079
>  perf_ioctl+0x59/0x80 kernel/events/core.c:5110
>  vfs_ioctl fs/ioctl.c:46 [inline]
>  file_ioctl fs/ioctl.c:501 [inline]
>  do_vfs_ioctl+0x1de/0x1720 fs/ioctl.c:685
>  ksys_ioctl+0xa9/0xd0 fs/ioctl.c:702
>  __do_sys_ioctl fs/ioctl.c:709 [inline]
>  __se_sys_ioctl fs/ioctl.c:707 [inline]
>  __x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:707
>  do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
>  entry_SYSCALL_64_after_hwframe+0x49/0xbe
> RIP: 0033:0x440409
> Code: 18 89 d0 c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 00 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 fb 13 fc ff c3 66 2e 0f 1f 84 00 00 00 00
> RSP: 002b:00007fffc5fc1488 EFLAGS: 00000217 ORIG_RAX: 0000000000000010
> RAX: ffffffffffffffda RBX: 00000000004002c8 RCX: 0000000000440409
> RDX: 0000000020000180 RSI: 00000000c008240a RDI: 0000000000000003
> RBP: 00000000006ca018 R08: 00000000004002c8 R09: 00000000004002c8
> R10: 00000000004002c8 R11: 0000000000000217 R12: 0000000000401c90
> R13: 0000000000401d20 R14: 0000000000000000
>

I will take a look at this one.
We have a mutex lock at kernel/trace/bpf_trace.c,

1062         mutex_lock(&bpf_event_mutex);
1063         ret = bpf_prog_array_copy_info(event->tp_event->prog_array,
1064                                        ids,
1065                                        ids_len,
1066                                        &prog_cnt);
1067         mutex_unlock(&bpf_event_mutex);

We may need a variant of rcu_dereference, e.g., rcu_dereference_protected.

>
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