[PATCH net v2] tcp: make probe0 timer handle expired user timeout

[Altan Hacigumus]

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: 344db93ae3ee ("tcp: make TCP_USER_TIMEOUT accurate for zero window probes")
Link: https://lore.kernel.org/r/20260414013634.43997-1-ahacigu.linux@xxxxxxxxx
Signed-off-by: Altan Hacigumus <ahacigu.linux@xxxxxxxxx>
---
v2:
- Use the existing TCP_TIMEOUT_MIN clamping along with signed @remaining
---
 net/ipv4/tcp_timer.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index 8d791a954cd6..322db13333c7 100644
--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c
@@ -50,7 +50,8 @@ 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);
@@ -61,7 +62,7 @@ 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);
 }
-- 
2.43.0