[PATCH] net/sctp: fix race condition in sctp_destroy_sock

[PATCH] net/sctp: fix race condition in sctp_destroy_sock

[Or Cohen]

If sctp_destroy_sock is called without sock_net(sk)->sctp.addr_wq_lock
held and sp->do_auto_asconf is true, then an element is removed
from the auto_asconf_splist without any proper locking.

This can happen in the following functions:
1. In sctp_accept, if sctp_sock_migrate fails.
2. In inet_create or inet6_create, if there is a bpf program
   attached to BPF_CGROUP_INET_SOCK_CREATE which denies
   creation of the sctp socket.

The bug is fixed by acquiring addr_wq_lock in sctp_destroy_sock
instead of sctp_close.

Reported-by: Or Cohen <orcohen@paloaltonetworks.com>
Reviewed-by: Xin Long <lucien.xin@gmail.com>
Fixes: 610236587600 ("bpf: Add new cgroup attach type to enable sock modifications")
Signed-off-by: Or Cohen <orcohen@paloaltonetworks.com>
---
 net/sctp/socket.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/net/sctp/socket.c b/net/sctp/socket.c
index a710917c5ac7..9af232d4fb6b 100644
--- a/net/sctp/socket.c
+++ b/net/sctp/socket.c
@@ -1520,11 +1520,9 @@ static void sctp_close(struct sock *sk, long timeout)
 
 	/* Supposedly, no process has access to the socket, but
 	 * the net layers still may.
-	 * Also, sctp_destroy_sock() needs to be called with addr_wq_lock
-	 * held and that should be grabbed before socket lock.
 	 */
-	spin_lock_bh(&net->sctp.addr_wq_lock);
-	bh_lock_sock_nested(sk);
+	local_bh_disable();
+	bh_lock_sock(sk);
 
 	/* Hold the sock, since sk_common_release() will put sock_put()
 	 * and we have just a little more cleanup.
@@ -1533,7 +1531,7 @@ static void sctp_close(struct sock *sk, long timeout)
 	sk_common_release(sk);
 
 	bh_unlock_sock(sk);
-	spin_unlock_bh(&net->sctp.addr_wq_lock);
+	local_bh_enable();
 
 	sock_put(sk);
 
@@ -5030,7 +5028,9 @@ static void sctp_destroy_sock(struct sock *sk)
 
 	if (sp->do_auto_asconf) {
 		sp->do_auto_asconf = 0;
+		spin_lock_bh(&sock_net(sk)->sctp.addr_wq_lock);
 		list_del(&sp->auto_asconf_list);
+		spin_unlock_bh(&sock_net(sk)->sctp.addr_wq_lock);
 	}
 	sctp_endpoint_free(sp->ep);
 	local_bh_disable();
-- 
2.7.4

Re: [PATCH] net/sctp: fix race condition in sctp_destroy_sock

[Marcelo Ricardo Leitner]

On Tue, Apr 13, 2021 at 12:31:53PM +0300, Or Cohen wrote:
> +++ b/net/sctp/socket.c
> @@ -1520,11 +1520,9 @@ static void sctp_close(struct sock *sk, long timeout)
>  
>  	/* Supposedly, no process has access to the socket, but
>  	 * the net layers still may.
> -	 * Also, sctp_destroy_sock() needs to be called with addr_wq_lock
> -	 * held and that should be grabbed before socket lock.
>  	 */

Please also update the following comment in sctp_init_sock():
          /* Nothing can fail after this block, otherwise
           * sctp_destroy_sock() will be called without addr_wq_lock held
           */

Other than this, LGTM. Thanks.

  Marcelo