[PATCH net-next v2 1/2] net: Guard for gso_segs overflow in skb_segment

From: Alice Mikityanska <[email protected]>

skb_segment calculates 32-bit partial_segs as len / gso_size, and then
assigns it to the 16-bit gso_segs field. The division might overflow in
some edge cases where the SKB is BIG TCP (65536 <= len <= 8*65535), and
gso_size < TCP_MIN_GSO_SIZE = 8. While normally this can't happen due to
TCP_MIN_GSO_SIZE, an AF_PACKET PACKET_VNET_HDR socket can generate such
a malformed packet.

Blocking malformed virtio_net packets is implemented in the next patch,
but this patch clamps partial_segs in skb_segment itself for more
generic robustness. Should len / gso_size happen to be bigger than
65535 in partial GSO, skb_segment will now just produce more than two
output SKBs, all of which will be valid with gso_segs <= 65535.

Signed-off-by: Alice Mikityanska <[email protected]>
---
 net/core/skbuff.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index c82a1472a5ea..439cbfeb02bd 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -4860,7 +4860,7 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
 		 * doesn't fit into an MSS sized block, so take care of that
 		 * now.
 		 */
-		partial_segs = len / mss;
+		partial_segs = min(len / mss, GSO_MAX_SEGS);
 		if (partial_segs > 1)
 			mss *= partial_segs;
 		else
-- 
2.55.0