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Date:   Sat, 9 Dec 2017 19:23:43 +0800
From:   Xin Long <lucien.xin@...il.com>
To:     syzbot 
        <bot+ed0838d0fa4c4f2b528e20286e6dc63effc7c14d@...kaller.appspotmail.com>
Cc:     davem <davem@...emloft.net>, kuznet <kuznet@....inr.ac.ru>,
        LKML <linux-kernel@...r.kernel.org>, linux-sctp@...r.kernel.org,
        network dev <netdev@...r.kernel.org>,
        Neil Horman <nhorman@...driver.com>,
        syzkaller-bugs@...glegroups.com,
        Vlad Yasevich <vyasevich@...il.com>,
        yoshfuji <yoshfuji@...ux-ipv6.org>
Subject: Re: kernel BUG at net/core/skbuff.c:LINE! (2)

On Fri, Dec 8, 2017 at 4:45 PM, Xin Long <lucien.xin@...il.com> wrote:
> On Fri, Dec 8, 2017 at 4:16 PM, syzbot
> <bot+ed0838d0fa4c4f2b528e20286e6dc63effc7c14d@...kaller.appspotmail.com>
> wrote:
>> syzkaller has found reproducer for the following crash on
>> 82bcf1def3b5f1251177ad47c44f7e17af039b4b
>> git://git.cmpxchg.org/linux-mmots.git/master
>> compiler: gcc (GCC) 7.1.1 20170620
>> .config is attached
>> Raw console output is attached.
>>
>> syzkaller reproducer is attached. See https://goo.gl/kgGztJ
>> for information about syzkaller reproducers
>>
>>
>> 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
>>  </IRQ>
>> RIP: 0010:native_safe_halt+0x6/0x10 arch/x86/include/asm/irqflags.h:54
>> RSP: 0018:ffff8801d9f97da8 EFLAGS: 00000282 ORIG_RAX: ffffffffffffff11
>> RAX: dffffc0000000000 RBX: 1ffff1003b3f2fb8 RCX: 0000000000000000
>> RDX: 1ffffffff0c59734 RSI: 0000000000000001 RDI: ffffffff862cb9a0
>> RBP: ffff8801d9f97da8 R08: 0000000000000000 R09: 0000000000000000
>> R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000001
>> R13: ffff8801d9f97e60 R14: ffffffff869eb920 R15: 0000000000000000
>>  arch_safe_halt arch/x86/include/asm/paravirt.h:93 [inline]
>>  default_idle+0xbf/0x430 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+0x18/0x20 kernel/sched/idle.c:351
>>  start_secondary+0x330/0x460 arch/x86/kernel/smpboot.c:277
>>  secondary_startup_64+0xa5/0xb0 arch/x86/kernel/head_64.S:237
>> Code: 03 0f b6 04 01 84 c0 74 04 3c 03 7e 20 8b 4b 78 41 57 48 c7 c7 a0 38
>> bd 85 52 56 4c 89 ea 41 50 4c 89 e6 45 89 f0 e8 0c b6 3d fd <0f> 0b 4c 89 4d
>> b8 4c 89 45 c0 48 89 75 c8 48 89 55 d0 e8 7d 93
>> RIP: skb_panic+0x15c/0x1f0 net/core/skbuff.c:100 RSP: ffff8801db307508
>> ---[ end trace 941a8a0f633e271f ]---
>>
> This isn't a sctp problem, but mld's, seems when lo's mtu became 0,
> it allocs a skb without enough space in add_grec():
>               if (AVAILABLE(skb) < sizeof(*psrc) +
>                     first*sizeof(struct mld2_grec)) {
>                         if (truncate && !first)
>                                 break;   /* truncate these */
>                         if (pgr)
>                                 pgr->grec_nsrcs = htons(scount);
>                         if (skb)
>                                 mld_sendpack(skb);
>                         skb = mld_newpack(idev, dev->mtu); <---
>
> I will check this for sure later on both igmp and mld.

Fix:
--- a/net/ipv6/mcast.c
+++ b/net/ipv6/mcast.c
@@ -1766,8 +1766,8 @@ static struct sk_buff *add_grec(struct sk_buff
*skb, struct ifmcaddr6 *pmc,
                if (isquery)
                        psf->sf_gsresp = 0;

-               if (AVAILABLE(skb) < sizeof(*psrc) +
-                   first*sizeof(struct mld2_grec)) {
+               if (AVAILABLE(skb) < (int)(sizeof(*psrc) +
+                                          first * sizeof(*pgr))) {
                        if (truncate && !first)
                                break;   /* truncate these */
                        if (pgr)
@@ -1810,7 +1810,7 @@ static struct sk_buff *add_grec(struct sk_buff
*skb, struct ifmcaddr6 *pmc,
                        return skb;
                if (pmc->mca_crcount || isquery || crsend) {
                        /* make sure we have room for group header */
-                       if (skb && AVAILABLE(skb) < sizeof(struct mld2_grec)) {
+                       if (skb && AVAILABLE(skb) < (int)sizeof(*pgr)) {
                                mld_sendpack(skb);
                                skb = NULL; /* add_grhead will get a new one */
                        }

do the same on igmp.

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