taprio: Fix sending packets without dequeueing them

There was a bug that was causing packets to be sent to the driver
without first calling dequeue() on the "child" qdisc. And the KASAN
report below shows that sending a packet without calling dequeue()
leads to bad results.

The problem is that when checking the last qdisc "child" we do not set
the returned skb to NULL, which can cause it to be sent to the driver,
and so after the skb is sent, it may be freed, and in some situations a
reference to it may still be in the child qdisc, because it was never
dequeued.

The crash log looks like this:

[   19.937538] ==================================================================
[   19.938300] BUG: KASAN: use-after-free in taprio_dequeue_soft+0x620/0x780
[   19.938968] Read of size 4 at addr ffff8881128628cc by task swapper/1/0
[   19.939612]
[   19.939772] CPU: 1 PID: 0 Comm: swapper/1 Not tainted 5.6.0-rc3+ #97
[   19.940397] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.0-59-gc9ba5276e321-prebuilt.qe4
[   19.941523] Call Trace:
[   19.941774]  <IRQ>
[   19.941985]  dump_stack+0x97/0xe0
[   19.942323]  print_address_description.constprop.0+0x3b/0x60
[   19.942884]  ? taprio_dequeue_soft+0x620/0x780
[   19.943325]  ? taprio_dequeue_soft+0x620/0x780
[   19.943767]  __kasan_report.cold+0x1a/0x32
[   19.944173]  ? taprio_dequeue_soft+0x620/0x780
[   19.944612]  kasan_report+0xe/0x20
[   19.944954]  taprio_dequeue_soft+0x620/0x780
[   19.945380]  __qdisc_run+0x164/0x18d0
[   19.945749]  net_tx_action+0x2c4/0x730
[   19.946124]  __do_softirq+0x268/0x7bc
[   19.946491]  irq_exit+0x17d/0x1b0
[   19.946824]  smp_apic_timer_interrupt+0xeb/0x380
[   19.947280]  apic_timer_interrupt+0xf/0x20
[   19.947687]  </IRQ>
[   19.947912] RIP: 0010:default_idle+0x2d/0x2d0
[   19.948345] Code: 00 00 41 56 41 55 65 44 8b 2d 3f 8d 7c 7c 41 54 55 53 0f 1f 44 00 00 e8 b1 b2 c5 fd e9 07 00 3
[   19.950166] RSP: 0018:ffff88811a3efda0 EFLAGS: 00000282 ORIG_RAX: ffffffffffffff13
[   19.950909] RAX: 0000000080000000 RBX: ffff88811a3a9600 RCX: ffffffff8385327e
[   19.951608] RDX: 1ffff110234752c0 RSI: 0000000000000000 RDI: ffffffff8385262f
[   19.952309] RBP: ffffed10234752c0 R08: 0000000000000001 R09: ffffed10234752c1
[   19.953009] R10: ffffed10234752c0 R11: ffff88811a3a9607 R12: 0000000000000001
[   19.953709] R13: 0000000000000001 R14: 0000000000000000 R15: 0000000000000000
[   19.954408]  ? default_idle_call+0x2e/0x70
[   19.954816]  ? default_idle+0x1f/0x2d0
[   19.955192]  default_idle_call+0x5e/0x70
[   19.955584]  do_idle+0x3d4/0x500
[   19.955909]  ? arch_cpu_idle_exit+0x40/0x40
[   19.956325]  ? _raw_spin_unlock_irqrestore+0x23/0x30
[   19.956829]  ? trace_hardirqs_on+0x30/0x160
[   19.957242]  cpu_startup_entry+0x19/0x20
[   19.957633]  start_secondary+0x2a6/0x380
[   19.958026]  ? set_cpu_sibling_map+0x18b0/0x18b0
[   19.958486]  secondary_startup_64+0xa4/0xb0
[   19.958921]
[   19.959078] Allocated by task 33:
[   19.959412]  save_stack+0x1b/0x80
[   19.959747]  __kasan_kmalloc.constprop.0+0xc2/0xd0
[   19.960222]  kmem_cache_alloc+0xe4/0x230
[   19.960617]  __alloc_skb+0x91/0x510
[   19.960967]  ndisc_alloc_skb+0x133/0x330
[   19.961358]  ndisc_send_ns+0x134/0x810
[   19.961735]  addrconf_dad_work+0xad5/0xf80
[   19.962144]  process_one_work+0x78e/0x13a0
[   19.962551]  worker_thread+0x8f/0xfa0
[   19.962919]  kthread+0x2ba/0x3b0
[   19.963242]  ret_from_fork+0x3a/0x50
[   19.963596]
[   19.963753] Freed by task 33:
[   19.964055]  save_stack+0x1b/0x80
[   19.964386]  __kasan_slab_free+0x12f/0x180
[   19.964830]  kmem_cache_free+0x80/0x290
[   19.965231]  ip6_mc_input+0x38a/0x4d0
[   19.965617]  ipv6_rcv+0x1a4/0x1d0
[   19.965948]  __netif_receive_skb_one_core+0xf2/0x180
[   19.966437]  netif_receive_skb+0x8c/0x3c0
[   19.966846]  br_handle_frame_finish+0x779/0x1310
[   19.967302]  br_handle_frame+0x42a/0x830
[   19.967694]  __netif_receive_skb_core+0xf0e/0x2a90
[   19.968167]  __netif_receive_skb_one_core+0x96/0x180
[   19.968658]  process_backlog+0x198/0x650
[   19.969047]  net_rx_action+0x2fa/0xaa0
[   19.969420]  __do_softirq+0x268/0x7bc
[   19.969785]
[   19.969940] The buggy address belongs to the object at ffff888112862840
[   19.969940]  which belongs to the cache skbuff_head_cache of size 224
[   19.971202] The buggy address is located 140 bytes inside of
[   19.971202]  224-byte region [ffff888112862840, ffff888112862920)
[   19.972344] The buggy address belongs to the page:
[   19.972820] page:ffffea00044a1800 refcount:1 mapcount:0 mapping:ffff88811a2bd1c0 index:0xffff8881128625c0 compo0
[   19.973930] flags: 0x8000000000010200(slab|head)
[   19.974388] raw: 8000000000010200 ffff88811a2ed650 ffff88811a2ed650 ffff88811a2bd1c0
[   19.975151] raw: ffff8881128625c0 0000000000190013 00000001ffffffff 0000000000000000
[   19.975915] page dumped because: kasan: bad access detected
[   19.976461] page_owner tracks the page as allocated
[   19.976946] page last allocated via order 2, migratetype Unmovable, gfp_mask 0xd20c0(__GFP_IO|__GFP_FS|__GFP_NO)
[   19.978332]  prep_new_page+0x24b/0x330
[   19.978707]  get_page_from_freelist+0x2057/0x2c90
[   19.979170]  __alloc_pages_nodemask+0x218/0x590
[   19.979619]  new_slab+0x9d/0x300
[   19.979948]  ___slab_alloc.constprop.0+0x2f9/0x6f0
[   19.980421]  __slab_alloc.constprop.0+0x30/0x60
[   19.980870]  kmem_cache_alloc+0x201/0x230
[   19.981269]  __alloc_skb+0x91/0x510
[   19.981620]  alloc_skb_with_frags+0x78/0x4a0
[   19.982043]  sock_alloc_send_pskb+0x5eb/0x750
[   19.982476]  unix_stream_sendmsg+0x399/0x7f0
[   19.982904]  sock_sendmsg+0xe2/0x110
[   19.983262]  ____sys_sendmsg+0x4de/0x6d0
[   19.983660]  ___sys_sendmsg+0xe4/0x160
[   19.984032]  __sys_sendmsg+0xab/0x130
[   19.984396]  do_syscall_64+0xe7/0xae0
[   19.984761] page last free stack trace:
[   19.985142]  __free_pages_ok+0x432/0xbc0
[   19.985533]  qlist_free_all+0x56/0xc0
[   19.985907]  quarantine_reduce+0x149/0x170
[   19.986315]  __kasan_kmalloc.constprop.0+0x9e/0xd0
[   19.986791]  kmem_cache_alloc+0xe4/0x230
[   19.987182]  prepare_creds+0x24/0x440
[   19.987548]  do_faccessat+0x80/0x590
[   19.987906]  do_syscall_64+0xe7/0xae0
[   19.988276]  entry_SYSCALL_64_after_hwframe+0x49/0xbe
[   19.988775]
[   19.988930] Memory state around the buggy address:
[   19.989402]  ffff888112862780: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
[   19.990111]  ffff888112862800: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb
[   19.990822] >ffff888112862880: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
[   19.991529]                                               ^
[   19.992081]  ffff888112862900: fb fb fb fb fc fc fc fc fc fc fc fc fc fc fc fc
[   19.992796]  ffff888112862980: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc

Fixes: 5a781ccbd19e ("tc: Add support for configuring the taprio scheduler")
Reported-by: Michael Schmidt <michael.schmidt@eti.uni-siegen.de>
Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Acked-by: Andre Guedes <andre.guedes@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/sched/sch_taprio.c b/net/sched/sch_taprio.c
index ee717e0..b1eb12d 100644
--- a/net/sched/sch_taprio.c
+++ b/net/sched/sch_taprio.c
@@ -564,8 +564,10 @@
 		prio = skb->priority;
 		tc = netdev_get_prio_tc_map(dev, prio);
 
-		if (!(gate_mask & BIT(tc)))
+		if (!(gate_mask & BIT(tc))) {
+			skb = NULL;
 			continue;
+		}
 
 		len = qdisc_pkt_len(skb);
 		guard = ktime_add_ns(taprio_get_time(q),
@@ -575,13 +577,17 @@
 		 * guard band ...
 		 */
 		if (gate_mask != TAPRIO_ALL_GATES_OPEN &&
-		    ktime_after(guard, entry->close_time))
+		    ktime_after(guard, entry->close_time)) {
+			skb = NULL;
 			continue;
+		}
 
 		/* ... and no budget. */
 		if (gate_mask != TAPRIO_ALL_GATES_OPEN &&
-		    atomic_sub_return(len, &entry->budget) < 0)
+		    atomic_sub_return(len, &entry->budget) < 0) {
+			skb = NULL;
 			continue;
+		}
 
 		skb = child->ops->dequeue(child);
 		if (unlikely(!skb))