[PATCH net v2] net/mlx5e: SHAMPO, Fix IP length overflow on large HW GRO sessions

[Tianyu Zuo <cosmosocket@gmail.com>]

mlx5e_hw_gro_skb_has_enough_space() bounds a HW GRO session by the
payload held in the skb fragments only. The L3/L4 headers that
header-data split placed in the linear area are not accounted for, and
the limit is inclusive of GRO_LEGACY_MAX_SIZE.

On a 4K page system a session can therefore grow to 16 full page
fragments (65536 bytes) plus the 40 bytes of IPv4/TCP headers in the
linear part, giving skb->len = 65576.

mlx5e_shampo_update_hdr() writes the IP length itself:

	__be16 newlen = htons(skb->len - nhoff);
	csum_replace2(&ipv4->check, ipv4->tot_len, newlen);
	ipv4->tot_len = newlen;

The result wraps. ip_rcv_core() then trims the 64KB skb down to the
wrapped tot_len, silently dropping the payload. The IPv6 path wraps
identically in ipv6hdr->payload_len.

Both branches of the check are inclusive of GRO_LEGACY_MAX_SIZE, so a
session can reach skb->len == 65536 and wrap tot_len to zero. In the
page_size >= GRO_LEGACY_MAX_SIZE branch skb->len already covers the
linear area, so that off by one is the only problem there. The fragment
based branch additionally omits the linear area entirely.

Account for skb_headlen() in the fragment based branch and make both
comparisons strictly less than GRO_LEGACY_MAX_SIZE, so that skb->len can
never exceed 65535. The new bound is strictly tighter than the old one,
so the implicit limit on the fragment count (at most 65536 / page_size,
well below MAX_SKB_FRAGS) is preserved.

Fixes: 92552d3abd32 ("net/mlx5e: HW_GRO cqe handler implementation")
Signed-off-by: Tianyu Zuo <cosmosocket@gmail.com>
---
v2: commit message only, per review from Dragos and Tariq.

v1: https://lore.kernel.org/netdev/20260729204745.166584-1-cosmosocket@gmail.com/

 drivers/net/ethernet/mellanox/mlx5/core/en_rx.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c b/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
index 6fbc0441c4b8..2e9676305439 100644
--- a/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
+++ b/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
@@ -2222,9 +2222,10 @@ static bool mlx5e_hw_gro_skb_has_enough_space(struct sk_buff *skb,
 	int nr_frags = skb_shinfo(skb)->nr_frags;
 
 	if (page_size >= GRO_LEGACY_MAX_SIZE)
-		return skb->len + data_bcnt <= GRO_LEGACY_MAX_SIZE;
+		return skb->len + data_bcnt < GRO_LEGACY_MAX_SIZE;
 	else
-		return page_size * nr_frags + data_bcnt <= GRO_LEGACY_MAX_SIZE;
+		return skb_headlen(skb) + page_size * nr_frags + data_bcnt <
+			GRO_LEGACY_MAX_SIZE;
 }
 
 static void mlx5e_handle_rx_cqe_mpwrq_shampo(struct mlx5e_rq *rq, struct mlx5_cqe64 *cqe)

base-commit: 51b093a7ba27476e1f639455f005e8d2e75390e4
-- 
2.34.1