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

From: Altan Hacigumus <[email-masked]>
Date: Mon, 13 Apr 2026 18:36:34 -0700

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

Handle this expiration case similarly to tcp_clamp_rto_to_user_timeout().

Fixes: 344db93ae3ee ("tcp: make TCP_USER_TIMEOUT accurate for zero window probes")
Signed-off-by: Altan Hacigumus <[email-masked]>
---
 net/ipv4/tcp_timer.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index 5a14a53a3c9e..4a43356a4e06 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);
@@ -61,6 +62,8 @@ 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;
+	if (remaining <= 0)
+		return 1;
 	remaining = max_t(u32, remaining, TCP_TIMEOUT_MIN);
 
 	return min_t(u32, remaining, when);
-- 
2.43.0