linux-kernel - [PATCH net] sctp: Fix undefined behavior in left shift operation

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Message-ID: <20250218081217.3468369-1-eleanor15x@gmail.com>
Date: Tue, 18 Feb 2025 16:12:16 +0800
From: Yu-Chun Lin <eleanor15x@...il.com>
To: marcelo.leitner@...il.com,
	lucien.xin@...il.com,
	davem@...emloft.net,
	edumazet@...gle.com,
	kuba@...nel.org,
	pabeni@...hat.com
Cc: linux-sctp@...r.kernel.org,
	netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	jserv@...s.ncku.edu.tw,
	visitorckw@...il.com,
	Yu-Chun Lin <eleanor15x@...il.com>
Subject: [PATCH net] sctp: Fix undefined behavior in left shift operation

According to the C11 standard (ISO/IEC 9899:2011, 6.5.7):
"If E1 has a signed type and E1 x 2^E2 is not representable in the result
type, the behavior is undefined."

Shifting 1 << 31 causes signed integer overflow, which leads to undefined
behavior.

Fix this by explicitly using '1U << 31' to ensure the shift operates on
an unsigned type, avoiding undefined behavior.

Signed-off-by: Yu-Chun Lin <eleanor15x@...il.com>
---
 net/sctp/stream.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/sctp/stream.c b/net/sctp/stream.c
index c241cc552e8d..bfcff6d6a438 100644
--- a/net/sctp/stream.c
+++ b/net/sctp/stream.c
@@ -735,7 +735,7 @@ struct sctp_chunk *sctp_process_strreset_tsnreq(
 	 *     value SHOULD be the smallest TSN not acknowledged by the
 	 *     receiver of the request plus 2^31.
 	 */
-	init_tsn = sctp_tsnmap_get_ctsn(&asoc->peer.tsn_map) + (1 << 31);
+	init_tsn = sctp_tsnmap_get_ctsn(&asoc->peer.tsn_map) + (1U << 31);
 	sctp_tsnmap_init(&asoc->peer.tsn_map, SCTP_TSN_MAP_INITIAL,
 			 init_tsn, GFP_ATOMIC);
 
-- 
2.43.0