[PATCH net v3 4/5] xsk: drain continuation descs on invalid descriptor in __xsk_generic_xmit()

[Jason Xing <kerneljasonxing@gmail.com>]

From: Jason Xing <kernelxing@tencent.com>

When the TX loop in __xsk_generic_xmit() encounters an invalid
descriptor mid-packet (e.g. an out-of-bounds address), the partial
skb is dropped and the offending descriptor is released. However,
remaining continuation descriptors belonging to the same multi-buffer
packet still sit in the TX ring. Since xs->skb becomes NULL after the
drop, the next iteration treats the leftover continuation fragment as
a brand-new packet, corrupting the packet stream.

Fix this by setting the drain_cont flag when the released descriptor
has XDP_PKT_CONTD set. On the next call to __xsk_generic_xmit(), the
drain logic introduced in the previous patch handles the remaining
fragments with normal CQ backpressure.

There is one subtle case: if a subsequent continuation descriptor also
has an invalid address, xskq_cons_peek_desc() rejects it and the
while loop is never entered, so the in-loop drain path cannot clear
drain_cont. The post-loop code already handles this: it sees
xskq_has_descs() is true (the failed descriptor was read but not
released by peek), releases it, and checks its XDP_PKT_CONTD flag.
Add an else branch so that when the released descriptor is the
last fragment (no XDP_PKT_CONTD), drain_cont is cleared. This
prevents the next valid packet from being incorrectly drained.

Fixes: cf24f5a5feea ("xsk: add support for AF_XDP multi-buffer on Tx path")
Signed-off-by: Jason Xing <kernelxing@tencent.com>
---
 net/xdp/xsk.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
index 298194b7335e..cd451b285645 100644
--- a/net/xdp/xsk.c
+++ b/net/xdp/xsk.c
@@ -1122,6 +1122,7 @@ static int __xsk_generic_xmit(struct sock *sk)
 		if (xs->skb)
 			xsk_drop_skb(xs->skb);
 		xskq_cons_release(xs->tx);
+		xs->drain_cont = xp_mb_desc(&desc);
 	}
 
 out:
-- 
2.43.7