[PATCH net] qede: fix off-by-one in BD ring consumption on build_skb failure

[Shigeru Yoshida <syoshida@redhat.com>]

qede_rx_build_skb() and qede_tpa_rx_build_skb() do not check for a
NULL return from qede_build_skb(). When it returns NULL under memory
pressure, the functions still consume a BD from the ring before
returning NULL. The callers then recycle additional BDs, resulting in
one extra BD being consumed (off-by-one). This desynchronizes the BD
ring, which can corrupt DMA page reference counts and lead to SLUB
freelist corruption.

Commit 4e910dbe3650 ("qede: confirm skb is allocated before using")
added a NULL check inside qede_build_skb() to prevent a NULL pointer
dereference, but did not address the missing NULL checks in the
callers, making this off-by-one reachable.

Fix this by adding NULL checks for the return value of
qede_build_skb() in both qede_rx_build_skb() and
qede_tpa_rx_build_skb(), returning NULL immediately before any BD ring
manipulation.

Fixes: 4e910dbe3650 ("qede: confirm skb is allocated before using")
Signed-off-by: Shigeru Yoshida <syoshida@redhat.com>
---
 drivers/net/ethernet/qlogic/qede/qede_fp.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/net/ethernet/qlogic/qede/qede_fp.c b/drivers/net/ethernet/qlogic/qede/qede_fp.c
index 33e18bb69774..c11e0d8f98aa 100644
--- a/drivers/net/ethernet/qlogic/qede/qede_fp.c
+++ b/drivers/net/ethernet/qlogic/qede/qede_fp.c
@@ -765,6 +765,9 @@ qede_tpa_rx_build_skb(struct qede_dev *edev,
 	struct sk_buff *skb;
 
 	skb = qede_build_skb(rxq, bd, len, pad);
+	if (unlikely(!skb))
+		return NULL;
+
 	bd->page_offset += rxq->rx_buf_seg_size;
 
 	if (bd->page_offset == PAGE_SIZE) {
@@ -812,6 +815,8 @@ qede_rx_build_skb(struct qede_dev *edev,
 	}
 
 	skb = qede_build_skb(rxq, bd, len, pad);
+	if (unlikely(!skb))
+		return NULL;
 
 	if (unlikely(qede_realloc_rx_buffer(rxq, bd))) {
 		/* Incr page ref count to reuse on allocation failure so
-- 
2.54.0