iptunnel: NULL pointer deref for ip_md_tunnel_xmit

Naresh Kamboju noted the following oops during execution of selftest
tools/testing/selftests/bpf/test_tunnel.sh on x86_64:

[  274.120445] BUG: unable to handle kernel NULL pointer dereference
at 0000000000000000
[  274.128285] #PF error: [INSTR]
[  274.131351] PGD 8000000414a0e067 P4D 8000000414a0e067 PUD 3b6334067 PMD 0
[  274.138241] Oops: 0010 [#1] SMP PTI
[  274.141734] CPU: 1 PID: 11464 Comm: ping Not tainted
5.0.0-rc4-next-20190129 #1
[  274.149046] Hardware name: Supermicro SYS-5019S-ML/X11SSH-F, BIOS
2.0b 07/27/2017
[  274.156526] RIP: 0010:          (null)
[  274.160280] Code: Bad RIP value.
[  274.163509] RSP: 0018:ffffbc9681f83540 EFLAGS: 00010286
[  274.168726] RAX: 0000000000000000 RBX: ffffdc967fa80a18 RCX: 0000000000000000
[  274.175851] RDX: ffff9db2ee08b540 RSI: 000000000000000e RDI: ffffdc967fa809a0
[  274.182974] RBP: ffffbc9681f83580 R08: ffff9db2c4d62690 R09: 000000000000000c
[  274.190098] R10: 0000000000000000 R11: ffff9db2ee08b540 R12: ffff9db31ce7c000
[  274.197222] R13: 0000000000000001 R14: 000000000000000c R15: ffff9db3179cf400
[  274.204346] FS:  00007ff4ae7c5740(0000) GS:ffff9db31fa80000(0000)
knlGS:0000000000000000
[  274.212424] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  274.218162] CR2: ffffffffffffffd6 CR3: 00000004574da004 CR4: 00000000003606e0
[  274.225292] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  274.232416] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[  274.239541] Call Trace:
[  274.241988]  ? tnl_update_pmtu+0x296/0x3b0
[  274.246085]  ip_md_tunnel_xmit+0x1bc/0x520
[  274.250176]  gre_fb_xmit+0x330/0x390
[  274.253754]  gre_tap_xmit+0x128/0x180
[  274.257414]  dev_hard_start_xmit+0xb7/0x300
[  274.261598]  sch_direct_xmit+0xf6/0x290
[  274.265430]  __qdisc_run+0x15d/0x5e0
[  274.269007]  __dev_queue_xmit+0x2c5/0xc00
[  274.273011]  ? dev_queue_xmit+0x10/0x20
[  274.276842]  ? eth_header+0x2b/0xc0
[  274.280326]  dev_queue_xmit+0x10/0x20
[  274.283984]  ? dev_queue_xmit+0x10/0x20
[  274.287813]  arp_xmit+0x1a/0xf0
[  274.290952]  arp_send_dst.part.19+0x46/0x60
[  274.295138]  arp_solicit+0x177/0x6b0
[  274.298708]  ? mod_timer+0x18e/0x440
[  274.302281]  neigh_probe+0x57/0x70
[  274.305684]  __neigh_event_send+0x197/0x2d0
[  274.309862]  neigh_resolve_output+0x18c/0x210
[  274.314212]  ip_finish_output2+0x257/0x690
[  274.318304]  ip_finish_output+0x219/0x340
[  274.322314]  ? ip_finish_output+0x219/0x340
[  274.326493]  ip_output+0x76/0x240
[  274.329805]  ? ip_fragment.constprop.53+0x80/0x80
[  274.334510]  ip_local_out+0x3f/0x70
[  274.337992]  ip_send_skb+0x19/0x40
[  274.341391]  ip_push_pending_frames+0x33/0x40
[  274.345740]  raw_sendmsg+0xc15/0x11d0
[  274.349403]  ? __might_fault+0x85/0x90
[  274.353151]  ? _copy_from_user+0x6b/0xa0
[  274.357070]  ? rw_copy_check_uvector+0x54/0x130
[  274.361604]  inet_sendmsg+0x42/0x1c0
[  274.365179]  ? inet_sendmsg+0x42/0x1c0
[  274.368937]  sock_sendmsg+0x3e/0x50
[  274.372460]  ___sys_sendmsg+0x26f/0x2d0
[  274.376293]  ? lock_acquire+0x95/0x190
[  274.380043]  ? __handle_mm_fault+0x7ce/0xb70
[  274.384307]  ? lock_acquire+0x95/0x190
[  274.388053]  ? __audit_syscall_entry+0xdd/0x130
[  274.392586]  ? ktime_get_coarse_real_ts64+0x64/0xc0
[  274.397461]  ? __audit_syscall_entry+0xdd/0x130
[  274.401989]  ? trace_hardirqs_on+0x4c/0x100
[  274.406173]  __sys_sendmsg+0x63/0xa0
[  274.409744]  ? __sys_sendmsg+0x63/0xa0
[  274.413488]  __x64_sys_sendmsg+0x1f/0x30
[  274.417405]  do_syscall_64+0x55/0x190
[  274.421064]  entry_SYSCALL_64_after_hwframe+0x49/0xbe
[  274.426113] RIP: 0033:0x7ff4ae0e6e87
[  274.429686] Code: 64 89 02 48 c7 c0 ff ff ff ff eb b9 0f 1f 80 00
00 00 00 8b 05 ca d9 2b 00 48 63 d2 48 63 ff 85 c0 75 10 b8 2e 00 00
00 0f 05 <48> 3d 00 f0 ff ff 77 51 c3 53 48 89 f3 48 83 ec 10 48 89 7c
24 08
[  274.448422] RSP: 002b:00007ffcd9b76db8 EFLAGS: 00000246 ORIG_RAX:
000000000000002e
[  274.455978] RAX: ffffffffffffffda RBX: 0000000000000040 RCX: 00007ff4ae0e6e87
[  274.463104] RDX: 0000000000000000 RSI: 00000000006092e0 RDI: 0000000000000003
[  274.470228] RBP: 0000000000000000 R08: 00007ffcd9bc40a0 R09: 00007ffcd9bc4080
[  274.477349] R10: 000000000000060a R11: 0000000000000246 R12: 0000000000000003
[  274.484475] R13: 0000000000000016 R14: 00007ffcd9b77fa0 R15: 00007ffcd9b78da4
[  274.491602] Modules linked in: cls_bpf sch_ingress iptable_filter
ip_tables algif_hash af_alg x86_pkg_temp_thermal fuse [last unloaded:
test_bpf]
[  274.504634] CR2: 0000000000000000
[  274.507976] ---[ end trace 196d18386545eae1 ]---
[  274.512588] RIP: 0010:          (null)
[  274.516334] Code: Bad RIP value.
[  274.519557] RSP: 0018:ffffbc9681f83540 EFLAGS: 00010286
[  274.524775] RAX: 0000000000000000 RBX: ffffdc967fa80a18 RCX: 0000000000000000
[  274.531921] RDX: ffff9db2ee08b540 RSI: 000000000000000e RDI: ffffdc967fa809a0
[  274.539082] RBP: ffffbc9681f83580 R08: ffff9db2c4d62690 R09: 000000000000000c
[  274.546205] R10: 0000000000000000 R11: ffff9db2ee08b540 R12: ffff9db31ce7c000
[  274.553329] R13: 0000000000000001 R14: 000000000000000c R15: ffff9db3179cf400
[  274.560456] FS:  00007ff4ae7c5740(0000) GS:ffff9db31fa80000(0000)
knlGS:0000000000000000
[  274.568541] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  274.574277] CR2: ffffffffffffffd6 CR3: 00000004574da004 CR4: 00000000003606e0
[  274.581403] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  274.588535] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[  274.595658] Kernel panic - not syncing: Fatal exception in interrupt
[  274.602046] Kernel Offset: 0x14400000 from 0xffffffff81000000
(relocation range: 0xffffffff80000000-0xffffffffbfffffff)
[  274.612827] ---[ end Kernel panic - not syncing: Fatal exception in
interrupt ]---
[  274.620387] ------------[ cut here ]------------

I'm also seeing the same failure on x86_64, and it reproduces
consistently.

>From poking around it looks like the skb's dst entry is being used
to calculate the mtu in:

mtu = skb_dst(skb) ? dst_mtu(skb_dst(skb)) : dev->mtu;

...but because that dst_entry  has an "ops" value set to md_dst_ops,
the various ops (including mtu) are not set:

crash> struct sk_buff._skb_refdst ffff928f87447700 -x
      _skb_refdst = 0xffffcd6fbf5ea590
crash> struct dst_entry.ops 0xffffcd6fbf5ea590
  ops = 0xffffffffa0193800
crash> struct dst_ops.mtu 0xffffffffa0193800
  mtu = 0x0
crash>

I confirmed that the dst entry also has dst->input set to
dst_md_discard, so it looks like it's an entry that's been
initialized via __metadata_dst_init alright.

I think the fix here is to use skb_valid_dst(skb) - it checks
for  DST_METADATA also, and with that fix in place, the
problem - which was previously 100% reproducible - disappears.

The below patch resolves the panic and all bpf tunnel tests pass
without incident.

Fixes: c8b34e68 ("ip_tunnel: Add tnl_update_pmtu in ip_md_tunnel_xmit")
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Tested-by: Anders Roxell <anders.roxell@linaro.org>
Reported-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Tested-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ip_tunnel.c

@@ -515,9 +515,10 @@ static int tnl_update_pmtu(struct net_device *dev, struct sk_buff *skb,
 		mtu = dst_mtu(&rt->dst) - dev->hard_header_len
 					- sizeof(struct iphdr) - tunnel_hlen;
 	else
-		mtu = skb_dst(skb) ? dst_mtu(skb_dst(skb)) : dev->mtu;
+		mtu = skb_valid_dst(skb) ? dst_mtu(skb_dst(skb)) : dev->mtu;
 
-	skb_dst_update_pmtu(skb, mtu);
+	if (skb_valid_dst(skb))
+		skb_dst_update_pmtu(skb, mtu);
 
 	if (skb->protocol == htons(ETH_P_IP)) {
 		if (!skb_is_gso(skb) &&
@@ -530,9 +531,11 @@ static int tnl_update_pmtu(struct net_device *dev, struct sk_buff *skb,
 	}
 #if IS_ENABLED(CONFIG_IPV6)
 	else if (skb->protocol == htons(ETH_P_IPV6)) {
-		struct rt6_info *rt6 = (struct rt6_info *)skb_dst(skb);
+		struct rt6_info *rt6;
 		__be32 daddr;
 
+		rt6 = skb_valid_dst(skb) ? (struct rt6_info *)skb_dst(skb) :
+					   NULL;
 		daddr = md ? dst : tunnel->parms.iph.daddr;
 
 		if (rt6 && mtu < dst_mtu(skb_dst(skb)) &&