atlantic: fix double-free in aq_ring_tx_clean

We found this bug while fuzzing the device driver. Using and freeing
the dangling pointer buff->skb would cause use-after-free and
double-free.

This bug is triggerable with compromised/malfunctioning devices. We
found the bug with QEMU emulation and tested the patch by emulation.
We did NOT test on a real device.

Attached is the bug report.

BUG: KASAN: double-free or invalid-free in consume_skb+0x6c/0x1c0

Call Trace:
 dump_stack+0x76/0xa0
 print_address_description.constprop.0+0x16/0x200
 ? consume_skb+0x6c/0x1c0
 kasan_report_invalid_free+0x61/0xa0
 ? consume_skb+0x6c/0x1c0
 __kasan_slab_free+0x15e/0x170
 ? consume_skb+0x6c/0x1c0
 kfree+0x8c/0x230
 consume_skb+0x6c/0x1c0
 aq_ring_tx_clean+0x5c2/0xa80 [atlantic]
 aq_vec_poll+0x309/0x5d0 [atlantic]
 ? _sub_I_65535_1+0x20/0x20 [atlantic]
 ? __next_timer_interrupt+0xba/0xf0
 net_rx_action+0x363/0xbd0
 ? call_timer_fn+0x240/0x240
 ? __switch_to_asm+0x34/0x70
 ? napi_busy_loop+0x520/0x520
 ? net_tx_action+0x379/0x720
 __do_softirq+0x18c/0x634
 ? takeover_tasklets+0x5f0/0x5f0
 run_ksoftirqd+0x15/0x20
 smpboot_thread_fn+0x2f1/0x6b0
 ? smpboot_unregister_percpu_thread+0x160/0x160
 ? __kthread_parkme+0x80/0x100
 ? smpboot_unregister_percpu_thread+0x160/0x160
 kthread+0x2b5/0x3b0
 ? kthread_create_on_node+0xd0/0xd0
 ret_from_fork+0x22/0x40

Reported-by: Brendan Dolan-Gavitt <brendandg@nyu.edu>
Signed-off-by: Zekun Shen <bruceshenzk@gmail.com>
Reviewed-by: Igor Russkikh <irusskikh@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/aquantia/atlantic/aq_ring.c

@@ -298,13 +298,14 @@ bool aq_ring_tx_clean(struct aq_ring_s *self)
 			}
 		}
 
-		if (unlikely(buff->is_eop)) {
+		if (unlikely(buff->is_eop && buff->skb)) {
 			u64_stats_update_begin(&self->stats.tx.syncp);
 			++self->stats.tx.packets;
 			self->stats.tx.bytes += buff->skb->len;
 			u64_stats_update_end(&self->stats.tx.syncp);
 
 			dev_kfree_skb_any(buff->skb);
+			buff->skb = NULL;
 		}
 		buff->pa = 0U;
 		buff->eop_index = 0xffffU;