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

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

[Tianyu Zuo]

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;

With nhoff == 0 this stores tot_len = 40 and updates the header checksum
to match, so the corruption is self-consistent. The GRO stack does not
repair it: the header is written before napi_gro_receive(), and
inet_gro_complete() only runs for skbs that the GRO engine holds on its
gro_list. HW GRO sessions are typically flushed on TCP_FLAG_PSH, which
makes tcp_gro_receive() set NAPI_GRO_CB(skb)->flush, so dev_gro_receive()
hands the skb over via GRO_NORMAL and the gro_complete() callbacks are
never invoked. The length check in inet_gro_receive() cannot catch it
either, since tot_len and skb_gro_len() are compared modulo 64K.

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.

Triggering this requires the payload of the aggregated session to reach
GRO_LEGACY_MAX_SIZE with every fragment fully populated, since
page_size * nr_frags otherwise overestimates the data actually present
and the session is flushed earlier. In practice this needs an MSS that
is a multiple of the page size (for example 8192 on a 4K page host with
jumbo frames) together with a page aligned start of the session.

Account for skb_headlen() and make both checks strictly less than
GRO_LEGACY_MAX_SIZE so that skb->len can never exceed 65535.

The check is strictly more conservative than before, so the implicit
bound on the fragment count is preserved: page_size * nr_frags +
data_bcnt <= 65535 gives nr_frags + data_bcnt / page_size <=
65536 / page_size - 1, and a single CQE adds at most
data_bcnt / page_size + 1 fragments, for a total of at most
65536 / page_size (16 on 4K pages), well below MAX_SKB_FRAGS.

Fixes: 92552d3abd32 ("net/mlx5e: HW_GRO cqe handler implementation")
Signed-off-by: Tianyu Zuo <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