tcp: make probe0 timer handle expired user timeout

[ Upstream commit 2b9f6f7 ]

tcp_clamp_probe0_to_user_timeout() computes remaining time in jiffies
using subtraction with an unsigned lvalue.  If elapsed probing time
exceeds the configured TCP_USER_TIMEOUT, the underflow yields a large
value.

This ends up re-arming the probe timer for a full backoff interval
instead of expiring immediately, delaying connection teardown beyond
the configured timeout.

Fix this by preventing underflow so user-set timeout expiration is
handled correctly without extending the probe timer.

Fixes: 344db93 ("tcp: make TCP_USER_TIMEOUT accurate for zero window probes")
Link: https://lore.kernel.org/r/20260414013634.43997-1-ahacigu.linux@gmail.com
Signed-off-by: Altan Hacigumus <ahacigu.linux@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260424014639.54110-1-ahacigu.linux@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: ahacigu-linux

net/ipv4/tcp_timer.c

@@ -49,7 +49,8 @@ static u32 tcp_clamp_rto_to_user_timeout(const struct sock *sk)
 u32 tcp_clamp_probe0_to_user_timeout(const struct sock *sk, u32 when)
 {
 	const struct inet_connection_sock *icsk = inet_csk(sk);
-	u32 remaining, user_timeout;
+	u32 user_timeout;
+	s32 remaining;
 	s32 elapsed;
 
 	user_timeout = READ_ONCE(icsk->icsk_user_timeout);
@@ -60,7 +61,7 @@ u32 tcp_clamp_probe0_to_user_timeout(const struct sock *sk, u32 when)
 	if (unlikely(elapsed < 0))
 		elapsed = 0;
 	remaining = msecs_to_jiffies(user_timeout) - elapsed;
-	remaining = max_t(u32, remaining, TCP_TIMEOUT_MIN);
+	remaining = max_t(int, remaining, TCP_TIMEOUT_MIN);
 
 	return min_t(u32, remaining, when);
 }