[PATCH net v3 2/2] net: Guard for gso_segs overflow in skb_segment

[Alice Mikityanska <alice.kernel@fastmail.im>]

From: Alice Mikityanska <alice@isovalent.com>

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 could generate
such a malformed packet until the previous patch.

Blocking malformed virtio_net packets was implemented in the previous
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.

In order to catch possible other cases of too many partial_segs, add a
DEBUG_NET_WARN_ON_ONCE when len / gso_size happens to be too big.

Signed-off-by: Alice Mikityanska <alice@isovalent.com>
---
 net/core/skbuff.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index d4382b68d56e..ceac295bb63d 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -4871,7 +4871,8 @@ 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;
+		DEBUG_NET_WARN_ON_ONCE(len / mss > GSO_MAX_SEGS);
+		partial_segs = min(len / mss, GSO_MAX_SEGS);
 		if (partial_segs > 1)
 			mss *= partial_segs;
 		else
-- 
2.55.0