[PATCH 1/1] net: socket: set sock->sk to NULL after calling proto_ops::release()

[Tyler Hicks]

From: Eric Biggers <ebiggers at google.com>

Commit 9060cb719e61 ("net: crypto set sk to NULL when af_alg_release.")
fixed a use-after-free in sockfs_setattr() when an AF_ALG socket is
closed concurrently with fchownat().  However, it ignored that many
other proto_ops::release() methods don't set sock->sk to NULL and
therefore allow the same use-after-free:

    - base_sock_release
    - bnep_sock_release
    - cmtp_sock_release
    - data_sock_release
    - dn_release
    - hci_sock_release
    - hidp_sock_release
    - iucv_sock_release
    - l2cap_sock_release
    - llcp_sock_release
    - llc_ui_release
    - rawsock_release
    - rfcomm_sock_release
    - sco_sock_release
    - svc_release
    - vcc_release
    - x25_release

Rather than fixing all these and relying on every socket type to get
this right forever, just make __sock_release() set sock->sk to NULL
itself after calling proto_ops::release().

Reproducer that produces the KASAN splat when any of these socket types
are configured into the kernel:

    #include <pthread.h>
    #include <stdlib.h>
    #include <sys/socket.h>
    #include <unistd.h>

    pthread_t t;
    volatile int fd;

    void *close_thread(void *arg)
    {
        for (;;) {
            usleep(rand() % 100);
            close(fd);
        }
    }

    int main()
    {
        pthread_create(&t, NULL, close_thread, NULL);
        for (;;) {
            fd = socket(rand() % 50, rand() % 11, 0);
            fchownat(fd, "", 1000, 1000, 0x1000);
            close(fd);
        }
    }

Fixes: 86741ec25462 ("net: core: Add a UID field to struct sock.")
Signed-off-by: Eric Biggers <ebiggers at google.com>
Acked-by: Cong Wang <xiyou.wangcong at gmail.com>
Signed-off-by: David S. Miller <davem at davemloft.net>

CVE-2019-8912

(cherry picked from commit ff7b11aa481f682e0e9711abfeb7d03f5cd612bf linux-next)
Signed-off-by: Tyler Hicks <tyhicks at canonical.com>
---
 net/socket.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/socket.c b/net/socket.c
index f2957aa205e6..874656a19eac 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -605,6 +605,7 @@ static void __sock_release(struct socket *sock, struct inode *inode)
 		if (inode)
 			inode_lock(inode);
 		sock->ops->release(sock);
+		sock->sk = NULL;
 		if (inode)
 			inode_unlock(inode);
 		sock->ops = NULL;
-- 
2.7.4