[PATCH net] tcp: fix icsk_ack.ato bitfield overflow
From: Jiayuan Chen <jiayuan.chen@linux.dev>
Date: 2026-08-07 01:45:26
Also in:
lkml
Subsystem:
networking [general], networking [tcp], the rest · Maintainers:
"David S. Miller", Eric Dumazet, Jakub Kicinski, Paolo Abeni, Neal Cardwell, Linus Torvalds
On cross-region connections we observed delayed ACKs suddenly turning
into immediate ACKs plus a TCP_MAX_QUICKACKS burst, as if the
connection had just received its first data segment.
Commit 95b9a87c6a6b ("tcp: record last received ipv6 flowlabel")
squeezed icsk_ack.ato into 8 bits, sized for TCP_DELACK_MAX. But both
writers still bound ato by icsk_rto, which can be well above 255
jiffies, so the bitfield assignment silently wraps mod 256: repeated
delack timer misses double ato up to icsk_rto, storing 320 as 64 and
256 as 0, and ato == 0 is the "first data packet" sentinel in
tcp_event_data_recv().
Clamp both writers to TCP_DELACK_MAX, which the static_assert already
guarantees to fit and tcp_send_delayed_ack() effectively caps ato at
anyway.
Fixes: 95b9a87c6a6b ("tcp: record last received ipv6 flowlabel")
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
net/ipv4/tcp_input.c | 6 +++---
net/ipv4/tcp_timer.c | 4 +++-
2 files changed, 6 insertions(+), 4 deletions(-)
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index daff93d51342..c0d2d3c0d40a 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c@@ -1039,9 +1039,9 @@ static void tcp_event_data_recv(struct sock *sk, struct sk_buff *skb) /* The fastest case is the first. */ icsk->icsk_ack.ato = (icsk->icsk_ack.ato >> 1) + TCP_ATO_MIN / 2; } else if (m < icsk->icsk_ack.ato) { - icsk->icsk_ack.ato = (icsk->icsk_ack.ato >> 1) + m; - if (icsk->icsk_ack.ato > icsk->icsk_rto) - icsk->icsk_ack.ato = icsk->icsk_rto; + icsk->icsk_ack.ato = min3((icsk->icsk_ack.ato >> 1) + (u32)m, + icsk->icsk_rto, + (u32)TCP_DELACK_MAX); } else if (m > icsk->icsk_rto) { /* Too long gap. Apparently sender failed to * restart window, so that we send ACKs quickly.
diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index f7215d53bbda..1038e7ba9c2e 100644
--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c@@ -334,7 +334,9 @@ void tcp_delack_timer_handler(struct sock *sk) if (inet_csk_ack_scheduled(sk)) { if (!inet_csk_in_pingpong_mode(sk)) { /* Delayed ACK missed: inflate ATO. */ - icsk->icsk_ack.ato = min_t(u32, icsk->icsk_ack.ato << 1, icsk->icsk_rto); + icsk->icsk_ack.ato = min3((u32)icsk->icsk_ack.ato << 1, + icsk->icsk_rto, + (u32)TCP_DELACK_MAX); } else { /* Delayed ACK missed: leave pingpong mode and * deflate ATO.
--
2.43.0