Patch "ipv6: fix udpv6 sendmsg crash caused by too small MTU" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

ipv6: fix udpv6 sendmsg crash caused by too small MTU

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
ipv6-fix-udpv6-sendmsg-crash-caused-by-too-small-mtu.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Mike Maloney <[email protected]>
Date: Wed, 10 Jan 2018 12:45:10 -0500
Subject: ipv6: fix udpv6 sendmsg crash caused by too small MTU

From: Mike Maloney <[email protected]>

[ Upstream commit 749439bfac6e1a2932c582e2699f91d329658196 ]

The logic in __ip6_append_data() assumes that the MTU is at least large enough for the headers. A device's MTU may be adjusted after being added while sendmsg() is processing data, resulting in __ip6_append_data() seeing any MTU. For an mtu smaller than the size of the fragmentation header, the math results in a negative 'maxfraglen', which causes problems when refragmenting any previous skb in the skb_write_queue, leaving it possibly malformed.

Instead sendmsg returns EINVAL when the mtu is calculated to be less than IPV6_MIN_MTU.

Found by syzkaller:
kernel BUG at ./include/linux/skbuff.h:2064!
invalid opcode: 0000 [#1] SMP KASAN
Dumping ftrace buffer:
(ftrace buffer empty)

Modules linked in:
CPU: 1 PID: 14216 Comm: syz-executor5 Not tainted 4.13.0-rc4+ #2
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
task: ffff8801d0b68580 task.stack: ffff8801ac6b8000
RIP: 0010:__skb_pull include/linux/skbuff.h:2064 [inline]
RIP: 0010:__ip6_make_skb+0x18cf/0x1f70 net/ipv6/ip6_output.c:1617
RSP: 0018:ffff8801ac6bf570 EFLAGS: 00010216
RAX: 0000000000010000 RBX: 0000000000000028 RCX: ffffc90003cce000
RDX: 00000000000001b8 RSI: ffffffff839df06f RDI: ffff8801d9478ca0
RBP: ffff8801ac6bf780 R08: ffff8801cc3f1dbc R09: 0000000000000000
R10: ffff8801ac6bf7a0 R11: 43cb4b7b1948a9e7 R12: ffff8801cc3f1dc8
R13: ffff8801cc3f1d40 R14: 0000000000001036 R15: dffffc0000000000
FS:  00007f43d740c700(0000) GS:ffff8801dc100000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f7834984000 CR3: 00000001d79b9000 CR4: 00000000001406e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 ip6_finish_skb include/net/ipv6.h:911 [inline]
 udp_v6_push_pending_frames+0x255/0x390 net/ipv6/udp.c:1093
 udpv6_sendmsg+0x280d/0x31a0 net/ipv6/udp.c:1363
 inet_sendmsg+0x11f/0x5e0 net/ipv4/af_inet.c:762
 sock_sendmsg_nosec net/socket.c:633 [inline]
 sock_sendmsg+0xca/0x110 net/socket.c:643
 SYSC_sendto+0x352/0x5a0 net/socket.c:1750
 SyS_sendto+0x40/0x50 net/socket.c:1718
 entry_SYSCALL_64_fastpath+0x1f/0xbe
RIP: 0033:0x4512e9
RSP: 002b:00007f43d740bc08 EFLAGS: 00000216 ORIG_RAX: 000000000000002c
RAX: ffffffffffffffda RBX: 00000000007180a8 RCX: 00000000004512e9
RDX: 000000000000002e RSI: 0000000020d08000 RDI: 0000000000000005
RBP: 0000000000000086 R08: 00000000209c1000 R09: 000000000000001c
R10: 0000000000040800 R11: 0000000000000216 R12: 00000000004b9c69
R13: 00000000ffffffff R14: 0000000000000005 R15: 00000000202c2000
Code: 9e 01 fe e9 c5 e8 ff ff e8 7f 9e 01 fe e9 4a ea ff ff 48 89 f7 e8 52 9e 01 fe e9 aa eb ff ff e8 a8 b6 cf fd 0f 0b e8 a1 b6 cf fd <0f> 0b 49 8d 45 78 4d 8d 45 7c 48 89 85 78 fe ff ff 49 8d 85 ba
RIP: __skb_pull include/linux/skbuff.h:2064 [inline] RSP: ffff8801ac6bf570
RIP: __ip6_make_skb+0x18cf/0x1f70 net/ipv6/ip6_output.c:1617 RSP: ffff8801ac6bf570

---
 net/ipv6/ip6_output.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

@@ -1246,14 +1246,16 @@ static int ip6_setup_cork(struct sock *s
 	v6_cork->tclass = tclass;
 	if (rt->dst.flags & DST_XFRM_TUNNEL)
 		mtu = np->pmtudisc >= IPV6_PMTUDISC_PROBE ?
-		      rt->dst.dev->mtu : dst_mtu(&rt->dst);
+		      READ_ONCE(rt->dst.dev->mtu) : dst_mtu(&rt->dst);
 	else
 		mtu = np->pmtudisc >= IPV6_PMTUDISC_PROBE ?
-		      rt->dst.dev->mtu : dst_mtu(rt->dst.path);
+		      READ_ONCE(rt->dst.dev->mtu) : dst_mtu(rt->dst.path);
 	if (np->frag_size < mtu) {
 		if (np->frag_size)
 			mtu = np->frag_size;
 	}
+	if (mtu < IPV6_MIN_MTU)
+		return -EINVAL;
 	cork->base.fragsize = mtu;
 	if (dst_allfrag(rt->dst.path))
 		cork->base.flags |= IPCORK_ALLFRAG;

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/ipv6-fix-udpv6-sendmsg-crash-caused-by-too-small-mtu.patch