5.15-stable review patch. If anyone has any objections, please let me know.
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From: Christoph Paasch cpaasch@openai.com
commit ccab044697980c6c01ab51f43f48f13b8a3e5c33 upstream.
When skb_ext_add(skb, SKB_EXT_MPTCP) fails in mptcp_incoming_options(), we used to return true, letting the segment proceed through the TCP receive path without a DSS mapping. Such segments can leave inconsistent mapping state and trigger a mid-stream fallback to TCP, which in testing collapsed (by artificially forcing failures in skb_ext_add) throughput to zero.
Return false instead so the TCP input path drops the skb (see tcp_data_queue() and step-7 processing). This is the safer choice under memory pressure: it preserves MPTCP correctness and provides backpressure to the sender.
Control packets remain unaffected: ACK updates and DATA_FIN handling happen before attempting the extension allocation, and tcp_reset() continues to ignore the return value.
With this change, MPTCP continues to work at high throughput if we artificially inject failures into skb_ext_add.
Fixes: 6787b7e350d3 ("mptcp: avoid processing packet if a subflow reset") Cc: stable@vger.kernel.org Signed-off-by: Christoph Paasch cpaasch@openai.com Reviewed-by: Matthieu Baerts (NGI0) matttbe@kernel.org Signed-off-by: Matthieu Baerts (NGI0) matttbe@kernel.org Link: https://patch.msgid.link/20250815-net-mptcp-misc-fixes-6-17-rc2-v1-1-521fe99... Signed-off-by: Jakub Kicinski kuba@kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- net/mptcp/options.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-)
--- a/net/mptcp/options.c +++ b/net/mptcp/options.c @@ -1074,7 +1074,9 @@ static bool add_addr_hmac_valid(struct m return hmac == mp_opt->ahmac; }
-/* Return false if a subflow has been reset, else return true */ +/* Return false in case of error (or subflow has been reset), + * else return true. + */ bool mptcp_incoming_options(struct sock *sk, struct sk_buff *skb) { struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(sk); @@ -1170,7 +1172,7 @@ bool mptcp_incoming_options(struct sock
mpext = skb_ext_add(skb, SKB_EXT_MPTCP); if (!mpext) - return true; + return false;
memset(mpext, 0, sizeof(*mpext));