On 2 May 2022, at 03:40, Tetsuo Handa [off-list ref] wrote:

syzbot is reporting use-after-free read in tcp_retransmit_timer() [1],
for TCP socket used by RDS is accessing sock_net() without acquiring a
refcount on net namespace. Since TCP's retransmission can happen after
a process which created net namespace terminated, we need to explicitly
acquire a refcount.

Link: https://syzkaller.appspot.com/bug?extid=694120e1002c117747ed [1]
Reported-by: syzbot <redacted>
Fixes: 26abe14379f8e2fa ("net: Modify sk_alloc to not reference count the netns of kernel sockets.")
Fixes: 8a68173691f03661 ("net: sk_clone_lock() should only do get_net() if the parent is not a kernel socket")
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Tested-by: syzbot <redacted>
---
Changes in v2:
 Add Fixes: tag.
 Move to inside lock_sock() section.

I chose 26abe14379f8e2fa and 8a68173691f03661 which went to 4.2 for Fixes: tag,
for refcount was implicitly taken when 70041088e3b97662 ("RDS: Add TCP transport
to RDS") was added to 2.6.32.

net/rds/tcp.c | 8 ++++++++
1 file changed, 8 insertions(+)

diff --git a/net/rds/tcp.c b/net/rds/tcp.c
index 5327d130c4b5..2f638f8b7b1e 100644
--- a/net/rds/tcp.c
+++ b/net/rds/tcp.c

@@ -495,6 +495,14 @@ void rds_tcp_tune(struct socket *sock)

	tcp_sock_set_nodelay(sock->sk);
	lock_sock(sk);
+	/* TCP timer functions might access net namespace even after
+	 * a process which created this net namespace terminated.
+	 */
+	if (!sk->sk_net_refcnt) {
+		sk->sk_net_refcnt = 1;
+		get_net_track(net, &sk->ns_tracker, GFP_KERNEL);