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SubjectRe: bpf: test_tunnel.sh: BUG: unable to handle kernel NULL pointer dereference
On Fri, 1 Feb 2019, Naresh Kamboju wrote:

> Kernel panic while running bpf: test_tunnel.sh on linux -next
> 5.0.0-rc4-next-20190129.
>
> selftests: bpf: test_tunnel.sh
> [ 273.997647] IPv6: ADDRCONF(NETDEV_CHANGE): veth1: link becomes ready
> [ 274.054068] ip (11458) used greatest stack depth: 11448 bytes left
> [ 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 ]------------
>
> The above log is from x86_64 device.
>


I'm also seeing the same panic during test_tunnel.sh execution
on x86_64.

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.

However what we should do in terms of path MTU setting for
such cases (use ip*_update_pmtu() perhaps?), I'm not sure.

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

Fixes: c8b34e680a09 ("ip_tunnel: Add tnl_update_pmtu in ip_md_tunnel_xmit")

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
---
net/ipv4/ip_tunnel.c | 9 ++++++---
1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/ip_tunnel.c b/net/ipv4/ip_tunnel.c
index 893f013..5dcf50c 100644
--- a/net/ipv4/ip_tunnel.c
+++ b/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)) &&
--
1.8.3.1
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