Re: [PATCH net] net: bridge: mst: fix vlan use-after-free

[Simon Horman <horms@kernel.org>]

On Mon, May 13, 2024 at 02:06:27PM +0300, Nikolay Aleksandrov wrote:
> syzbot reported a suspicious rcu usage[1] in bridge's mst code. While
> fixing it I noticed that nothing prevents a vlan to be freed while
> walking the list from the same path (br forward delay timer). Fix the rcu
> usage and also make sure we are not accessing freed memory by making
> br_mst_vlan_set_state use rcu read lock.
> 
> [1]
>  WARNING: suspicious RCU usage
>  6.9.0-rc6-syzkaller #0 Not tainted
>  -----------------------------
>  net/bridge/br_private.h:1599 suspicious rcu_dereference_protected() usage!
>  ...
>  stack backtrace:
>  CPU: 1 PID: 8017 Comm: syz-executor.1 Not tainted 6.9.0-rc6-syzkaller #0
>  Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/27/2024
>  Call Trace:
>   <IRQ>
>   __dump_stack lib/dump_stack.c:88 [inline]
>   dump_stack_lvl+0x241/0x360 lib/dump_stack.c:114
>   lockdep_rcu_suspicious+0x221/0x340 kernel/locking/lockdep.c:6712
>   nbp_vlan_group net/bridge/br_private.h:1599 [inline]
>   br_mst_set_state+0x1ea/0x650 net/bridge/br_mst.c:105
>   br_set_state+0x28a/0x7b0 net/bridge/br_stp.c:47
>   br_forward_delay_timer_expired+0x176/0x440 net/bridge/br_stp_timer.c:88
>   call_timer_fn+0x18e/0x650 kernel/time/timer.c:1793
>   expire_timers kernel/time/timer.c:1844 [inline]
>   __run_timers kernel/time/timer.c:2418 [inline]
>   __run_timer_base+0x66a/0x8e0 kernel/time/timer.c:2429
>   run_timer_base kernel/time/timer.c:2438 [inline]
>   run_timer_softirq+0xb7/0x170 kernel/time/timer.c:2448
>   __do_softirq+0x2c6/0x980 kernel/softirq.c:554
>   invoke_softirq kernel/softirq.c:428 [inline]
>   __irq_exit_rcu+0xf2/0x1c0 kernel/softirq.c:633
>   irq_exit_rcu+0x9/0x30 kernel/softirq.c:645
>   instr_sysvec_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1043 [inline]
>   sysvec_apic_timer_interrupt+0xa6/0xc0 arch/x86/kernel/apic/apic.c:1043
>   </IRQ>
>   <TASK>
>  asm_sysvec_apic_timer_interrupt+0x1a/0x20 arch/x86/include/asm/idtentry.h:702
>  RIP: 0010:lock_acquire+0x264/0x550 kernel/locking/lockdep.c:5758
>  Code: 2b 00 74 08 4c 89 f7 e8 ba d1 84 00 f6 44 24 61 02 0f 85 85 01 00 00 41 f7 c7 00 02 00 00 74 01 fb 48 c7 44 24 40 0e 36 e0 45 <4b> c7 44 25 00 00 00 00 00 43 c7 44 25 09 00 00 00 00 43 c7 44 25
>  RSP: 0018:ffffc90013657100 EFLAGS: 00000206
>  RAX: 0000000000000001 RBX: 1ffff920026cae2c RCX: 0000000000000001
>  RDX: dffffc0000000000 RSI: ffffffff8bcaca00 RDI: ffffffff8c1eaa60
>  RBP: ffffc90013657260 R08: ffffffff92efe507 R09: 1ffffffff25dfca0
>  R10: dffffc0000000000 R11: fffffbfff25dfca1 R12: 1ffff920026cae28
>  R13: dffffc0000000000 R14: ffffc90013657160 R15: 0000000000000246
> 
> Fixes: ec7328b59176 ("net: bridge: mst: Multiple Spanning Tree (MST) mode")
> Reported-by: syzbot+fa04eb8a56fd923fc5d8@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=fa04eb8a56fd923fc5d8
> Signed-off-by: Nikolay Aleksandrov <razor@blackwall.org>

Reviewed-by: Simon Horman <horms@kernel.org>