Re: [PATCH v2] net: rds: acquire refcount on TCP sockets

On Tue, May 3, 2022 at 6:27 AM David Laight [off-list ref] wrote:

From: Paolo Abeni

quoted

Sent: 03 May 2022 10:03

Hello,

On Mon, 2022-05-02 at 10:40 +0900, Tetsuo Handa wrote:

quoted

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")

quoted

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);
+           sock_inuse_add(net, 1);
+   }
    if (rtn->sndbuf_size > 0) {
            sk->sk_sndbuf = rtn->sndbuf_size;
            sk->sk_userlocks |= SOCK_SNDBUF_LOCK;

This looks equivalent to the fix presented here:

https://lore.kernel.org/all/CANn89i+484ffqb93aQm1N-tjxxvb3WDKX0EbD7318RwRgsatjw@mail.gmail.com/ (local)

but the latter looks a more generic solution. @Tetsuo could you please
test the above in your setup?

Wouldn't a more generic solution be to add a flag to sock_create_kern()
so that it acquires a reference to the namespace?
This could be a bit on one of the existing parameters - like SOCK_NONBLOCK.

I've a driver that uses __sock_create() in order to get that reference.
I'm pretty sure the extra 'security' check will never fail.

This would be silly really.

Definition of a 'kernel socket' is that it does not hold a reference
to the namespace.
(otherwise a netns could not be destroyed by user space)

A kernel layer using kernel sockets needs to properly dismantle them
when a namespace is destroyed.

In the RDS case, the socket was a user socket, or RDS lacked proper
tracking of all the sockets
so that they can be dismantled properly.