Re: kernel BUG at net/core/skbuff.c:LINE! (2)

Xin Long
2017-12-10T04:37:01Z

On Sun, Dec 10, 2017 at 12:59 AM, Eric Dumazet <eric.dumazet@gmail.com> wrote:
> On Sat, 2017-12-09 at 19:23 +0800, Xin Long wrote:
>> On Fri, Dec 8, 2017 at 4:45 PM, Xin Long <lucien.xin@gmail.com>
>> wrote:
>> > On Fri, Dec 8, 2017 at 4:16 PM, syzbot
>> > <bot+ed0838d0fa4c4f2b528e20286e6dc63effc7c14d@syzkaller.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.
>
> Thanks for the tentative patch.
>
> Quite a hack if you ask me.
>
> I would rather :
>
> 1) Read dev->mtu once to avoid bad assumptions/surprises.
>
> 2) Give up if this mtu is too small for IPV6 to be functional.
>
> Something like :
>
>
> net/ipv6/mcast.c | 25 +++++++++++++++----------
> 1 file changed, 15 insertions(+), 10 deletions(-)
>
> diff --git a/net/ipv6/mcast.c b/net/ipv6/mcast.c
> index fc6d7d143f2c29aab9a3f56eae02e5337e65a97b..844642682b8363c4c32d329ed92474f834a59618 100644
> --- a/net/ipv6/mcast.c
> +++ b/net/ipv6/mcast.c
> @@ -1682,16 +1682,16 @@ static int grec_size(struct ifmcaddr6 *pmc, int type, int gdel, int sdel)
> }
>
> static struct sk_buff *add_grhead(struct sk_buff *skb, struct ifmcaddr6 *pmc,
> - int type, struct mld2_grec **ppgr)
> + int type, struct mld2_grec **ppgr, unsigned int mtu)
> {
> - struct net_device *dev = pmc->idev->dev;
> struct mld2_report *pmr;
> struct mld2_grec *pgr;
>
> - if (!skb)
> - skb = mld_newpack(pmc->idev, dev->mtu);
> - if (!skb)
> - return NULL;
> + if (!skb) {
> + skb = mld_newpack(pmc->idev, mtu);
> + if (!skb)
> + return NULL;
> + }
> pgr = skb_put(skb, sizeof(struct mld2_grec));
> pgr->grec_type = type;
> pgr->grec_auxwords = 0;
> @@ -1714,10 +1714,15 @@ static struct sk_buff *add_grec(struct sk_buff *skb, struct ifmcaddr6 *pmc,
> struct mld2_grec *pgr = NULL;
> struct ip6_sf_list *psf, *psf_next, *psf_prev, **psf_list;
> int scount, stotal, first, isquery, truncate;
> + unsigned int mtu;
>
> if (pmc->mca_flags & MAF_NOREPORT)
> return skb;
>
> + mtu = READ_ONCE(dev->mtu);
> + if (mtu < IPV6_MIN_MTU)
> + return skb;
> +
> isquery = type == MLD2_MODE_IS_INCLUDE ||
> type == MLD2_MODE_IS_EXCLUDE;
> truncate = type == MLD2_MODE_IS_EXCLUDE ||
> @@ -1738,7 +1743,7 @@ static struct sk_buff *add_grec(struct sk_buff *skb, struct ifmcaddr6 *pmc,
> AVAILABLE(skb) < grec_size(pmc, type, gdeleted, sdeleted)) {
> if (skb)
> mld_sendpack(skb);
> - skb = mld_newpack(idev, dev->mtu);
> + skb = mld_newpack(idev, mtu);
> }
> }
> first = 1;
> @@ -1774,12 +1779,12 @@ static struct sk_buff *add_grec(struct sk_buff *skb, struct ifmcaddr6 *pmc,
> pgr->grec_nsrcs = htons(scount);
> if (skb)
> mld_sendpack(skb);
> - skb = mld_newpack(idev, dev->mtu);
> + skb = mld_newpack(idev, mtu);
> first = 1;
> scount = 0;
> }
> if (first) {
> - skb = add_grhead(skb, pmc, type, &pgr);
> + skb = add_grhead(skb, pmc, type, &pgr, mtu);
> first = 0;
> }
> if (!skb)
> @@ -1814,7 +1819,7 @@ static struct sk_buff *add_grec(struct sk_buff *skb, struct ifmcaddr6 *pmc,
> mld_sendpack(skb);
> skb = NULL; /* add_grhead will get a new one */
> }
> - skb = add_grhead(skb, pmc, type, &pgr);
> + skb = add_grhead(skb, pmc, type, &pgr, mtu);
> }
> }
> if (pgr)
>
The new patch works to me, just two questions:
1. should it use "idev->cnf.mtu6" here for mld ?

2. 'if (int < unsigned int)' is still not nice, though in 'if
(AVAILABLE(skb) < sizeof())'
AVAILABLE(skb) seems always to return >= 0 after your patch.