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Date:   Thu, 1 Feb 2018 14:56:46 -0800
From:   Eric Biggers <ebiggers3@gmail.com>
To:     weiwan@google.com
Cc:     davem@davemloft.net, kuznet@ms2.inr.ac.ru,
        linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
        syzkaller-bugs@googlegroups.com, yoshfuji@linux-ipv6.org,
        syzbot <syzbot+bca7109dba5d86cb0209@syzkaller.appspotmail.com>
Subject: Re: suspicious RCU usage at net/ipv6/ip6_fib.c:LINE
Message-ID: <20180201225646.dbug5oh3vupjrpy4@gmail.com>
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+weiwan@google.com

On Tue, Jan 02, 2018 at 03:58:02PM -0800, syzbot wrote:
> Hello,
> 
> syzkaller hit the following crash on
> 6bb8824732f69de0f233ae6b1a8158e149627b38
> git://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next.git/master
> compiler: gcc (GCC) 7.1.1 20170620
> .config is attached
> Raw console output is attached.
> C reproducer is attached
> syzkaller reproducer is attached. See https://goo.gl/kgGztJ
> for information about syzkaller reproducers
> 
> 
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+bca7109dba5d86cb0209@syzkaller.appspotmail.com
> It will help syzbot understand when the bug is fixed. See footer for
> details.
> If you forward the report, please keep this part and the footer.
> 
> 
> =============================
> WARNING: suspicious RCU usage
> 4.15.0-rc5+ #171 Not tainted
> -----------------------------
> net/ipv6/ip6_fib.c:1702 suspicious rcu_dereference_protected() usage!
> 
> other info that might help us debug this:
> 
> 
> rcu_scheduler_active = 2, debug_locks = 1
> 4 locks held by swapper/0/0:
>  #0:  ((&net->ipv6.ip6_fib_timer)){+.-.}, at: [<00000000d43f631b>]
> lockdep_copy_map include/linux/lockdep.h:178 [inline]
>  #0:  ((&net->ipv6.ip6_fib_timer)){+.-.}, at: [<00000000d43f631b>]
> call_timer_fn+0x1c6/0x820 kernel/time/timer.c:1310
>  #1:  (&(&net->ipv6.fib6_gc_lock)->rlock){+.-.}, at: [<000000002ff9d65c>]
> spin_lock_bh include/linux/spinlock.h:315 [inline]
>  #1:  (&(&net->ipv6.fib6_gc_lock)->rlock){+.-.}, at: [<000000002ff9d65c>]
> fib6_run_gc+0x9d/0x3c0 net/ipv6/ip6_fib.c:2007
>  #2:  (rcu_read_lock){....}, at: [<0000000091db762d>]
> __fib6_clean_all+0x0/0x3a0 net/ipv6/ip6_fib.c:1560
>  #3:  (&(&tb->tb6_lock)->rlock){+.-.}, at: [<000000009e503581>] spin_lock_bh
> include/linux/spinlock.h:315 [inline]
>  #3:  (&(&tb->tb6_lock)->rlock){+.-.}, at: [<000000009e503581>]
> __fib6_clean_all+0x1d0/0x3a0 net/ipv6/ip6_fib.c:1948
> 
> stack backtrace:
> CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.15.0-rc5+ #171
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> Google 01/01/2011
> Call Trace:
>  <IRQ>
>  __dump_stack lib/dump_stack.c:17 [inline]
>  dump_stack+0x194/0x257 lib/dump_stack.c:53
>  lockdep_rcu_suspicious+0x123/0x170 kernel/locking/lockdep.c:4585
>  fib6_del+0xcaa/0x11b0 net/ipv6/ip6_fib.c:1701
>  fib6_clean_node+0x3aa/0x4f0 net/ipv6/ip6_fib.c:1892
>  fib6_walk_continue+0x46c/0x8a0 net/ipv6/ip6_fib.c:1815
>  fib6_walk+0x91/0xf0 net/ipv6/ip6_fib.c:1863
>  fib6_clean_tree+0x1e6/0x340 net/ipv6/ip6_fib.c:1933
>  __fib6_clean_all+0x1f4/0x3a0 net/ipv6/ip6_fib.c:1949
>  fib6_clean_all net/ipv6/ip6_fib.c:1960 [inline]
>  fib6_run_gc+0x16b/0x3c0 net/ipv6/ip6_fib.c:2016
>  fib6_gc_timer_cb+0x20/0x30 net/ipv6/ip6_fib.c:2033
>  call_timer_fn+0x228/0x820 kernel/time/timer.c:1320
>  expire_timers kernel/time/timer.c:1357 [inline]
>  __run_timers+0x7ee/0xb70 kernel/time/timer.c:1660
>  run_timer_softirq+0x4c/0xb0 kernel/time/timer.c:1686
>  __do_softirq+0x2d7/0xb85 kernel/softirq.c:285
>  invoke_softirq kernel/softirq.c:365 [inline]
>  irq_exit+0x1cc/0x200 kernel/softirq.c:405
>  exiting_irq arch/x86/include/asm/apic.h:540 [inline]
>  smp_apic_timer_interrupt+0x16b/0x700 arch/x86/kernel/apic/apic.c:1052
>  apic_timer_interrupt+0xa9/0xb0 arch/x86/entry/entry_64.S:904
>  </IRQ>
> RIP: 0010:native_safe_halt+0x6/0x10 arch/x86/include/asm/irqflags.h:54
> RSP: 0018:ffffffff86407c38 EFLAGS: 00000282 ORIG_RAX: ffffffffffffff11
> RAX: dffffc0000000000 RBX: 1ffffffff0c80f8a RCX: 0000000000000000
> RDX: 1ffffffff0c99048 RSI: 0000000000000001 RDI: ffffffff864c8240
> RBP: ffffffff86407c38 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
> R13: ffffffff86407cf0 R14: ffffffff86c329a0 R15: 0000000000000000
>  arch_safe_halt arch/x86/include/asm/paravirt.h:93 [inline]
>  default_idle+0xbf/0x460 arch/x86/kernel/process.c:355
>  arch_cpu_idle+0xa/0x10 arch/x86/kernel/process.c:346
>  default_idle_call+0x36/0x90 kernel/sched/idle.c:98
>  cpuidle_idle_call kernel/sched/idle.c:156 [inline]
>  do_idle+0x24a/0x3b0 kernel/sched/idle.c:246
>  cpu_startup_entry+0x104/0x120 kernel/sched/idle.c:351
>  rest_init+0xed/0xf0 init/main.c:435
>  start_kernel+0x7f1/0x819 init/main.c:713
>  x86_64_start_reservations+0x2a/0x2c arch/x86/kernel/head64.c:378
>  x86_64_start_kernel+0x77/0x7a arch/x86/kernel/head64.c:359
>  secondary_startup_64+0xa5/0xb0 arch/x86/kernel/head_64.S:237

Wei, was this meant to have been fixed by commit 4512c43eac7e00 ("ipv6: remove
null_entry before adding default route")?  This crash actually still occurred on
net-next as recently as 43df215d99e604 (Jan 25) which included your fix.  I will
go ahead and mark this bug fixed, but syzbot will send a new report if it hits
it again.

#syz fix: ipv6: remove null_entry before adding default route

- Eric