[PATCH v2] xen/netback: avoid entering xenvif_rx_next_skb() with an empty rx queue

[Juergen Gross <jgross@xxxxxxxx>]

xenvif_rx_next_skb() is expecting the rx queue not being empty, but
in case the loop in xenvif_rx_action() is doing multiple iterations,
the availability of another skb in the rx queue is not being checked.

This can lead to crashes:

[40072.537261] BUG: unable to handle kernel NULL pointer dereference at 
0000000000000080
[40072.537407] IP: xenvif_rx_skb+0x23/0x590 [xen_netback]
[40072.537534] PGD 0 P4D 0
[40072.537644] Oops: 0000 [#1] SMP NOPTI
[40072.537749] CPU: 0 PID: 12505 Comm: v1-c40247-q2-gu Not tainted 
4.12.14-122.121-default #1 SLE12-SP5
[40072.537867] Hardware name: HP ProLiant DL580 Gen9/ProLiant DL580 Gen9, BIOS 
U17 11/23/2021
[40072.537999] task: ffff880433b38100 task.stack: ffffc90043d40000
[40072.538112] RIP: e030:xenvif_rx_skb+0x23/0x590 [xen_netback]
[40072.538217] RSP: e02b:ffffc90043d43de0 EFLAGS: 00010246
[40072.538319] RAX: 0000000000000000 RBX: ffffc90043cd7cd0 RCX: 00000000000000f7
[40072.538430] RDX: 0000000000000000 RSI: 0000000000000006 RDI: ffffc90043d43df8
[40072.538531] RBP: 000000000000003f R08: 000077ff80000000 R09: 0000000000000008
[40072.538644] R10: 0000000000007ff0 R11: 00000000000008f6 R12: ffffc90043ce2708
[40072.538745] R13: 0000000000000000 R14: ffffc90043d43ed0 R15: ffff88043ea748c0
[40072.538861] FS: 0000000000000000(0000) GS:ffff880484600000(0000) 
knlGS:0000000000000000
[40072.538988] CS: e033 DS: 0000 ES: 0000 CR0: 0000000080050033
[40072.539088] CR2: 0000000000000080 CR3: 0000000407ac8000 CR4: 0000000000040660
[40072.539211] Call Trace:
[40072.539319] xenvif_rx_action+0x71/0x90 [xen_netback]
[40072.539429] xenvif_kthread_guest_rx+0x14a/0x29c [xen_netback]

Fix that by stopping the loop in case the rx queue becomes empty.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 98f6d57ced73 ("xen-netback: process guest rx packets in batches")
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
Reviewed-by: Paul Durrant <paul@xxxxxxx>
---
V2:
- modified patch title (Jan Beulich)
- added Fixes: (Jan Beulich)
---
 drivers/net/xen-netback/rx.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/net/xen-netback/rx.c b/drivers/net/xen-netback/rx.c
index dbac4c03d21a..a0335407be42 100644
--- a/drivers/net/xen-netback/rx.c
+++ b/drivers/net/xen-netback/rx.c
@@ -495,6 +495,7 @@ void xenvif_rx_action(struct xenvif_queue *queue)
        queue->rx_copy.completed = &completed_skbs;
 
        while (xenvif_rx_ring_slots_available(queue) &&
+              !skb_queue_empty(&queue->rx_queue) &&
               work_done < RX_BATCH_SIZE) {
                xenvif_rx_skb(queue);
                work_done++;
-- 
2.35.3