[PATCH v4.14-stable] sch_netem: restore skb->dev after dequeuing from the rbtree

[Christoph Paasch <cpaasch@apple.com>]

Upstream commit bffa72cf7f9d ("net: sk_buff rbnode reorg") got
backported as commit 6b921536f170 ("net: sk_buff rbnode reorg") into the
v4.14.x-tree.

However, the backport does not include the changes in sch_netem.c

We need these, as otherwise the skb->dev pointer is not set when
dequeueing from the netem rbtree, resulting in a panic:

[   15.427748] BUG: unable to handle kernel NULL pointer dereference at 00000000000000d0
[   15.428863] IP: netif_skb_features+0x24/0x230
[   15.429402] PGD 0 P4D 0
[   15.429733] Oops: 0000 [#1] SMP PTI
[   15.430169] Modules linked in:
[   15.430614] CPU: 3 PID: 0 Comm: swapper/3 Not tainted 4.14.77.mptcp #77
[   15.431497] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 0.5.1 01/01/2011
[   15.432568] task: ffff88042db19680 task.stack: ffffc90000070000
[   15.433356] RIP: 0010:netif_skb_features+0x24/0x230
[   15.433977] RSP: 0018:ffff88043fd83e70 EFLAGS: 00010286
[   15.434665] RAX: ffff880429ad80c0 RBX: ffff88042bd0e400 RCX: ffff880429ad8000
[   15.435585] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff88042bd0e400
[   15.436551] RBP: ffff88042bd0e400 R08: ffff88042a4b6c9c R09: 0000000000000001
[   15.437485] R10: 0000000000000004 R11: 0000000000000000 R12: ffff88042c700000
[   15.438393] R13: ffff88042c700000 R14: ffff88042a4b6c00 R15: ffff88042c6bb000
[   15.439315] FS:  0000000000000000(0000) GS:ffff88043fd80000(0000) knlGS:0000000000000000
[   15.440314] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   15.441084] CR2: 00000000000000d0 CR3: 000000042c374000 CR4: 00000000000006e0
[   15.442016] Call Trace:
[   15.442333]  <IRQ>
[   15.442596]  validate_xmit_skb+0x17/0x270
[   15.443134]  validate_xmit_skb_list+0x38/0x60
[   15.443698]  sch_direct_xmit+0x102/0x190
[   15.444198]  __qdisc_run+0xe3/0x240
[   15.444671]  net_tx_action+0x121/0x140
[   15.445177]  __do_softirq+0xe2/0x224
[   15.445654]  irq_exit+0xbf/0xd0
[   15.446072]  smp_apic_timer_interrupt+0x5d/0x90
[   15.446654]  apic_timer_interrupt+0x7d/0x90
[   15.447185]  </IRQ>
[   15.447460] RIP: 0010:native_safe_halt+0x2/0x10
[   15.447992] RSP: 0018:ffffc90000073f10 EFLAGS: 00000282 ORIG_RAX: ffffffffffffff10
[   15.449008] RAX: ffffffff816667d0 RBX: ffffffff820946b0 RCX: 0000000000000000
[   15.449895] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
[   15.450768] RBP: ffffffff82026940 R08: 00000004e858e5e1 R09: ffff88042a4b6d58
[   15.451643] R10: 0000000000000000 R11: 000000d0d56879bb R12: 0000000000000000
[   15.452478] R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
[   15.453340]  ? __sched_text_end+0x2/0x2
[   15.453835]  default_idle+0xf/0x20
[   15.454259]  do_idle+0x170/0x200
[   15.454653]  cpu_startup_entry+0x14/0x20
[   15.455142]  secondary_startup_64+0xa5/0xb0
[   15.455715] Code: 1f 84 00 00 00 00 00 55 53 48 89 fd 48 83 ec 08 8b 87 bc 00 00 00 48 8b 8f c0 00 00 00 0f b6 97 81 00 00 00 48 8b 77 10 48 01 c8 <48> 8b 9
[   15.458138] RIP: netif_skb_features+0x24/0x230 RSP: ffff88043fd83e70
[   15.458933] CR2: 00000000000000d0
[   15.459352] ---[ end trace 083925903ae60570 ]---

Fixes: 6b921536f170 ("net: sk_buff rbnode reorg")
Cc: Stephen Hemminger <stephen@networkplumber.org>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Soheil Hassas Yeganeh <soheil@google.com>
Cc: Wei Wang <weiwan@google.com>
Cc: Willem de Bruijn <willemb@google.com>
Signed-off-by: Christoph Paasch <cpaasch@apple.com>
---

Notes:
    This patch should only make it into v4.14-stable as that's the only branch where
    the offending commit has been backported to.

 net/sched/sch_netem.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
index 2a2ab6bfe5d8..3d325b840802 100644
--- a/net/sched/sch_netem.c
+++ b/net/sched/sch_netem.c
@@ -624,6 +624,10 @@ static struct sk_buff *netem_dequeue(struct Qdisc *sch)
 			skb->next = NULL;
 			skb->prev = NULL;
 			skb->tstamp = netem_skb_cb(skb)->tstamp_save;
+			/* skb->dev shares skb->rbnode area,
+			 * we need to restore its value.
+			 */
+			skb->dev = qdisc_dev(sch);
 
 #ifdef CONFIG_NET_CLS_ACT
 			/*
-- 
2.16.2