netdev - Re: [PATCH net] ipv6: mcast: better catch silly mtu values

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Date:   Tue, 12 Dec 2017 20:55:34 +0800
From:   Xin Long <lucien.xin@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     David Miller <davem@...emloft.net>,
        network dev <netdev@...r.kernel.org>
Subject: Re: [PATCH net] ipv6: mcast: better catch silly mtu values

On Mon, Dec 11, 2017 at 11:03 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> From: Eric Dumazet <edumazet@...gle.com>
>
> syzkaller reported crashes in IPv6 stack [1]
>
> Xin Long found that lo MTU was set to silly values.
>
> IPv6 stack reacts to changes to small MTU, by disabling itself under
> RTNL.
>
> But there is a window where threads not using RTNL can see a wrong
> device mtu. This can lead to surprises, in mld code where it is assumed
> the mtu is suitable.
>
> Fix this by reading device mtu once and checking IPv6 minimal MTU.
>
> [1]
>  skbuff: skb_over_panic: text:0000000010b86b8d len:196 put:20
>  head:000000003b477e60 data:000000000e85441e tail:0xd4 end:0xc0 dev:lo
>  ------------[ cut here ]------------
>  kernel BUG at net/core/skbuff.c:104!
>  invalid opcode: 0000 [#1] SMP KASAN
>  Dumping ftrace buffer:
>     (ftrace buffer empty)
>  Modules linked in:
>  CPU: 1 PID: 0 Comm: swapper/1 Not tainted 4.15.0-rc2-mm1+ #39
>  Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
>  Google 01/01/2011
>  RIP: 0010:skb_panic+0x15c/0x1f0 net/core/skbuff.c:100
>  RSP: 0018:ffff8801db307508 EFLAGS: 00010286
>  RAX: 0000000000000082 RBX: ffff8801c517e840 RCX: 0000000000000000
>  RDX: 0000000000000082 RSI: 1ffff1003b660e61 RDI: ffffed003b660e95
>  RBP: ffff8801db307570 R08: 1ffff1003b660e23 R09: 0000000000000000
>  R10: 0000000000000000 R11: 0000000000000000 R12: ffffffff85bd4020
>  R13: ffffffff84754ed2 R14: 0000000000000014 R15: ffff8801c4e26540
>  FS:  0000000000000000(0000) GS:ffff8801db300000(0000) knlGS:0000000000000000
>  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>  CR2: 0000000000463610 CR3: 00000001c6698000 CR4: 00000000001406e0
>  DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>  DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>  Call Trace:
>   <IRQ>
>   skb_over_panic net/core/skbuff.c:109 [inline]
>   skb_put+0x181/0x1c0 net/core/skbuff.c:1694
>   add_grhead.isra.24+0x42/0x3b0 net/ipv6/mcast.c:1695
>   add_grec+0xa55/0x1060 net/ipv6/mcast.c:1817
>   mld_send_cr net/ipv6/mcast.c:1903 [inline]
>   mld_ifc_timer_expire+0x4d2/0x770 net/ipv6/mcast.c:2448
>   call_timer_fn+0x23b/0x840 kernel/time/timer.c:1320
>   expire_timers kernel/time/timer.c:1357 [inline]
>   __run_timers+0x7e1/0xb60 kernel/time/timer.c:1660
>   run_timer_softirq+0x4c/0xb0 kernel/time/timer.c:1686
>   __do_softirq+0x29d/0xbb2 kernel/softirq.c:285
>   invoke_softirq kernel/softirq.c:365 [inline]
>   irq_exit+0x1d3/0x210 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:920
>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Reported-by: syzbot <syzkaller@...glegroups.com>
Tested-by: Xin Long <lucien.xin@...il.com>