[PATCH] net: tcp: avoid division by zero in __tcp_select_window

[Evan Li <evan.li@linux.alibaba.com>]

We discovered a division-by-zero bug in __tcp_select_window() since
commit ae155060247b ("mptcp: fix duplicate reset on fastclose").

Under certain conditions during MPTCP fastclose, the mss value passed to
__tcp_select_window can be zero. The existing logic attempts to perform
rounddown(free_space, mss) without validating mss, leading to a division
operation in the helper (via do_div() or inline assembly) that triggers a
UBSAN overflow and kernel oops:

UBSAN: division-overflow in net/ipv4/tcp_output.c:3333:13
division by zero
RIP: __tcp_select_window+0x58a/0x1240
Call Trace:
 __tcp_transmit_skb+0xca3/0x38b0
 tcp_send_active_reset+0x422/0x7e0
 mptcp_do_fastclose+0x158/0x1e0
 ...

The issue occurs when tcp_send_active_reset() is called on a subflow with
an unset or zero mss, which can happen during fastclose teardown due to
earlier state transitions.

This patch adds a guard to return 0 immediately if mss == 0, preventing
the unsafe rounding operation. This is safe because a zero MSS implies
invalid or uninitialized state, and returning zero window reflects that no
reliable data transmission can proceed.

Fixes: ae155060247b ("mptcp: fix duplicate reset on fastclose")
Reported-by: kitta <kitta@linux.alibaba.com>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=220820
Co-developed-by: kitta <kitta@linux.alibaba.com>
Signed-off-by: Evan Li <evan.li@linux.alibaba.com>
---
 net/ipv4/tcp_output.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index b94efb3050d2..e6d2851a0ae9 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -3329,9 +3329,11 @@ u32 __tcp_select_window(struct sock *sk)
 		 * We also don't do any window rounding when the free space
 		 * is too small.
 		 */
-		if (window <= free_space - mss || window > free_space)
+		if (window <= free_space - mss || window > free_space) {
+			if (unlikely(mss == 0))
+				return 0;  /* Prevent division by zero */
 			window = rounddown(free_space, mss);
-		else if (mss == full_space &&
+		} else if (mss == full_space &&
 			 free_space > window + (full_space >> 1))
 			window = free_space;
 	}
-- 
2.43.7