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

From: Eric Dumazet <[email-masked]>
Date: Sat, 09 Dec 2017 08:59:30 -0800

On Sat, 2017-12-09 at 19:23 +0800, Xin Long wrote:
> On Fri, Dec 8, 2017 at 4:45 PM, Xin Long <[email-masked]>
> wrote:
> > On Fri, Dec 8, 2017 at 4:16 PM, syzbot
> > <[email-masked]
> > .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)