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Subject[PATCH 1/1] xen-netback: process malformed sk_buff correctly to avoid BUG_ON()
Date
The "BUG_ON(!frag_iter)" in function xenvif_rx_next_chunk() is triggered if
the received sk_buff is malformed, that is, when the sk_buff has pattern
(skb->data_len && !skb_shinfo(skb)->nr_frags). Below is a sample call
stack:

[ 438.652658] ------------[ cut here ]------------
[ 438.652660] kernel BUG at drivers/net/xen-netback/rx.c:325!
[ 438.652714] invalid opcode: 0000 [#1] SMP NOPTI
[ 438.652813] CPU: 0 PID: 2492 Comm: vif1.0-q0-guest Tainted: G O 4.16.0-rc6+ #1
[ 438.652896] RIP: e030:xenvif_rx_skb+0x3c2/0x5e0 [xen_netback]
[ 438.652926] RSP: e02b:ffffc90040877dc8 EFLAGS: 00010246
[ 438.652956] RAX: 0000000000000160 RBX: 0000000000000022 RCX: 0000000000000001
[ 438.652993] RDX: ffffc900402890d0 RSI: 0000000000000000 RDI: ffffc90040889000
[ 438.653029] RBP: ffff88002b460040 R08: ffffc90040877de0 R09: 0100000000000000
[ 438.653065] R10: 0000000000007ff0 R11: 0000000000000002 R12: ffffc90040889000
[ 438.653100] R13: ffffffff80000000 R14: 0000000000000022 R15: 0000000080000000
[ 438.653149] FS: 00007f15603778c0(0000) GS:ffff880030400000(0000) knlGS:0000000000000000
[ 438.653188] CS: e033 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 438.653219] CR2: 0000000001832a08 CR3: 0000000029c12000 CR4: 0000000000042660
[ 438.653262] Call Trace:
[ 438.653284] ? xen_hypercall_event_channel_op+0xa/0x20
[ 438.653313] xenvif_rx_action+0x41/0x80 [xen_netback]
[ 438.653341] xenvif_kthread_guest_rx+0xb2/0x2a8 [xen_netback]
[ 438.653374] ? __schedule+0x352/0x700
[ 438.653398] ? wait_woken+0x80/0x80
[ 438.653421] kthread+0xf3/0x130
[ 438.653442] ? xenvif_rx_action+0x80/0x80 [xen_netback]
[ 438.653470] ? kthread_destroy_worker+0x40/0x40
[ 438.653497] ret_from_fork+0x35/0x40

The issue is hit by xen-netback when there is bug with other networking
interface (e.g., dom0 physical NIC), who has generated and forwarded
malformed sk_buff to dom0 vifX.Y. It is possible to reproduce the issue on
purpose with below sample code in a kernel module:

skb->dev = dev; // dev of vifX.Y
skb->len = 386;
skb->data_len = 352;
skb->tail = 98;
skb->end = 384;
dev->netdev_ops->ndo_start_xmit(skb, dev);

This patch stops processing sk_buff immediately if it is detected as
malformed, that is, pkt->frag_iter is NULL but there is still remaining
pkt->remaining_len.

Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
---
drivers/net/xen-netback/rx.c | 8 ++++++++
1 file changed, 8 insertions(+)

diff --git a/drivers/net/xen-netback/rx.c b/drivers/net/xen-netback/rx.c
index b1cf7c6..289cc82 100644
--- a/drivers/net/xen-netback/rx.c
+++ b/drivers/net/xen-netback/rx.c
@@ -369,6 +369,14 @@ static void xenvif_rx_data_slot(struct xenvif_queue *queue,
offset += len;
pkt->remaining_len -= len;

+ if (unlikely(!pkt->frag_iter && pkt->remaining_len)) {
+ pkt->remaining_len = 0;
+ pkt->extra_count = 0;
+ pr_err_ratelimited("malformed sk_buff at %s\n",
+ queue->name);
+ break;
+ }
+
} while (offset < XEN_PAGE_SIZE && pkt->remaining_len > 0);

if (pkt->remaining_len > 0)
--
2.7.4
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