ipv6: mcast: better catch silly mtu values

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 <[e-mail]>
Reported-by: syzbot <[e-mail]>
Tested-by: Xin Long <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/ipv6/mcast.c

···
 }
 
 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;
···
 	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 ||
···
 		    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;
···
 				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)
···
 				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)