[PATCH net,v3] tcp: fix a signed-integer-overflow bug in tcp_add_backlog()

The type of sk_rcvbuf and sk_sndbuf in struct sock is int, and
in tcp_add_backlog(), the variable limit is caculated by adding
sk_rcvbuf, sk_sndbuf and 64 * 1024, it may exceed the max value
of int and overflow. This patch reduces the limit budget by
halving the sndbuf to solve this issue since ACK packets are much
smaller than the payload.

Fixes: c9c3321257e1 ("tcp: add tcp_add_backlog()")
Signed-off-by: Lu Wei <redacted>
---
 net/ipv4/tcp_ipv4.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 7a250ef9d1b7..87d440f47a70 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c

@@ -1874,11 +1874,13 @@ bool tcp_add_backlog(struct sock *sk, struct sk_buff *skb,
 	__skb_push(skb, hdrlen);
 
 no_coalesce:
+	limit = (u32)READ_ONCE(sk->sk_rcvbuf) + (u32)(READ_ONCE(sk->sk_sndbuf) >> 1);
+
 	/* Only socket owner can try to collapse/prune rx queues
 	 * to reduce memory overhead, so add a little headroom here.
 	 * Few sockets backlog are possibly concurrently non empty.
 	 */
-	limit = READ_ONCE(sk->sk_rcvbuf) + READ_ONCE(sk->sk_sndbuf) + 64*1024;
+	limit += 64 * 1024;
 
 	if (unlikely(sk_add_backlog(sk, skb, limit))) {
 		bh_unlock_sock(sk);

-- 
2.31.1

On Thu, Oct 20, 2022 at 8:03 PM Lu Wei [off-list ref] wrote:

The type of sk_rcvbuf and sk_sndbuf in struct sock is int, and
in tcp_add_backlog(), the variable limit is caculated by adding
sk_rcvbuf, sk_sndbuf and 64 * 1024, it may exceed the max value
of int and overflow. This patch reduces the limit budget by
halving the sndbuf to solve this issue since ACK packets are much
smaller than the payload.

Fixes: c9c3321257e1 ("tcp: add tcp_add_backlog()")
Signed-off-by: Lu Wei <redacted>

Reviewed-by: Eric Dumazet <edumazet@google.com>

From:   Lu Wei <redacted>
Date:   Fri, 21 Oct 2022 12:06:22 +0800

The type of sk_rcvbuf and sk_sndbuf in struct sock is int, and
in tcp_add_backlog(), the variable limit is caculated by adding
sk_rcvbuf, sk_sndbuf and 64 * 1024, it may exceed the max value
of int and overflow. This patch reduces the limit budget by
halving the sndbuf to solve this issue since ACK packets are much
smaller than the payload.

Fixes: c9c3321257e1 ("tcp: add tcp_add_backlog()")
Signed-off-by: Lu Wei <redacted>

Acked-by: Kuniyuki Iwashima <redacted>

Hello:

This patch was applied to netdev/net.git (master)
by David S. Miller [off-list ref]:

On Fri, 21 Oct 2022 12:06:22 +0800 you wrote:

The type of sk_rcvbuf and sk_sndbuf in struct sock is int, and
in tcp_add_backlog(), the variable limit is caculated by adding
sk_rcvbuf, sk_sndbuf and 64 * 1024, it may exceed the max value
of int and overflow. This patch reduces the limit budget by
halving the sndbuf to solve this issue since ACK packets are much
smaller than the payload.

[...]

Here is the summary with links:
  - [net,v3] tcp: fix a signed-integer-overflow bug in tcp_add_backlog()
    https://git.kernel.org/netdev/net/c/ec791d8149ff

You are awesome, thank you!
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