xfrm6: avoid potential infinite loop in _decode_session6()

syzbot found a way to trigger an infinitie loop by overflowing
@offset variable that has been forced to use u16 for some very
obscure reason in the past.

We probably want to look at NEXTHDR_FRAGMENT handling which looks
wrong, in a separate patch.

In net-next, we shall try to use skb_header_pointer() instead of
pskb_may_pull().

watchdog: BUG: soft lockup - CPU#1 stuck for 134s! [syz-executor738:4553]
Modules linked in:
irq event stamp: 13885653
hardirqs last  enabled at (13885652): [<ffffffff878009d5>] restore_regs_and_return_to_kernel+0x0/0x2b
hardirqs last disabled at (13885653): [<ffffffff87800905>] interrupt_entry+0xb5/0xf0 arch/x86/entry/entry_64.S:625
softirqs last  enabled at (13614028): [<ffffffff84df0809>] tun_napi_alloc_frags drivers/net/tun.c:1478 [inline]
softirqs last  enabled at (13614028): [<ffffffff84df0809>] tun_get_user+0x1dd9/0x4290 drivers/net/tun.c:1825
softirqs last disabled at (13614032): [<ffffffff84df1b6f>] tun_get_user+0x313f/0x4290 drivers/net/tun.c:1942
CPU: 1 PID: 4553 Comm: syz-executor738 Not tainted 4.17.0-rc3+ #40
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:check_kcov_mode kernel/kcov.c:67 [inline]
RIP: 0010:__sanitizer_cov_trace_pc+0x20/0x50 kernel/kcov.c:101
RSP: 0018:ffff8801d8cfe250 EFLAGS: 00000246 ORIG_RAX: ffffffffffffff13
RAX: ffff8801d88a8080 RBX: ffff8801d7389e40 RCX: 0000000000000006
RDX: 0000000000000000 RSI: ffffffff868da4ad RDI: ffff8801c8a53277
RBP: ffff8801d8cfe250 R08: ffff8801d88a8080 R09: ffff8801d8cfe3e8
R10: ffffed003b19fc87 R11: ffff8801d8cfe43f R12: ffff8801c8a5327f
R13: 0000000000000000 R14: ffff8801c8a4e5fe R15: ffff8801d8cfe3e8
FS:  0000000000d88940(0000) GS:ffff8801daf00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffffffffff600400 CR3: 00000001acab3000 CR4: 00000000001406e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 _decode_session6+0xc1d/0x14f0 net/ipv6/xfrm6_policy.c:150
 __xfrm_decode_session+0x71/0x140 net/xfrm/xfrm_policy.c:2368
 xfrm_decode_session_reverse include/net/xfrm.h:1213 [inline]
 icmpv6_route_lookup+0x395/0x6e0 net/ipv6/icmp.c:372
 icmp6_send+0x1982/0x2da0 net/ipv6/icmp.c:551
 icmpv6_send+0x17a/0x300 net/ipv6/ip6_icmp.c:43
 ip6_input_finish+0x14e1/0x1a30 net/ipv6/ip6_input.c:305
 NF_HOOK include/linux/netfilter.h:288 [inline]
 ip6_input+0xe1/0x5e0 net/ipv6/ip6_input.c:327
 dst_input include/net/dst.h:450 [inline]
 ip6_rcv_finish+0x29c/0xa10 net/ipv6/ip6_input.c:71
 NF_HOOK include/linux/netfilter.h:288 [inline]
 ipv6_rcv+0xeb8/0x2040 net/ipv6/ip6_input.c:208
 __netif_receive_skb_core+0x2468/0x3650 net/core/dev.c:4646
 __netif_receive_skb+0x2c/0x1e0 net/core/dev.c:4711
 netif_receive_skb_internal+0x126/0x7b0 net/core/dev.c:4785
 napi_frags_finish net/core/dev.c:5226 [inline]
 napi_gro_frags+0x631/0xc40 net/core/dev.c:5299
 tun_get_user+0x3168/0x4290 drivers/net/tun.c:1951
 tun_chr_write_iter+0xb9/0x154 drivers/net/tun.c:1996
 call_write_iter include/linux/fs.h:1784 [inline]
 do_iter_readv_writev+0x859/0xa50 fs/read_write.c:680
 do_iter_write+0x185/0x5f0 fs/read_write.c:959
 vfs_writev+0x1c7/0x330 fs/read_write.c:1004
 do_writev+0x112/0x2f0 fs/read_write.c:1039
 __do_sys_writev fs/read_write.c:1112 [inline]
 __se_sys_writev fs/read_write.c:1109 [inline]
 __x64_sys_writev+0x75/0xb0 fs/read_write.c:1109
 do_syscall_64+0x1b1/0x800 arch/x86/entry/common.c:287
 entry_SYSCALL_64_after_hwframe+0x49/0xbe

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Steffen Klassert <steffen.klassert@secunet.com>
Cc: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Reported-by: syzbot+0053c8...@syzkaller.appspotmail.com
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/ipv6/xfrm6_policy.c

@@ -126,7 +126,7 @@ _decode_session6(struct sk_buff *skb, struct flowi *fl, int reverse)
 	struct flowi6 *fl6 = &fl->u.ip6;
 	int onlyproto = 0;
 	const struct ipv6hdr *hdr = ipv6_hdr(skb);
-	u16 offset = sizeof(*hdr);
+	u32 offset = sizeof(*hdr);
 	struct ipv6_opt_hdr *exthdr;
 	const unsigned char *nh = skb_network_header(skb);
 	u16 nhoff = IP6CB(skb)->nhoff;