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

[Eric Dumazet]

On 02/21/2019 02:13 PM, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@xxxxxxxxxx>
> 
> 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:
> 

I fail to see how setting a pointer to NULL can avoid races.


We lack some kind of protection, rcu or something, if another thread can change sock->sk at anytime
while sockfs_setattr() is used.

sockfs_setattr()
...
     if (sock->sk)

// even if sock->sk was not NULL for the if (...).

// it can be NULL right now, compiler could read sock->sk a second time and catch a NULL.

        sock->sk->sk_uid = iattr->ia_uid;



>     - 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.")
> Cc: <stable@xxxxxxxxxxxxxxx> # v4.10+
> Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
> ---
> 
> NOTE: I am not an expert in the networking code, so please carefully
> check that I haven't missed some reason why this simple fix won't do.
> 
>  net/socket.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/net/socket.c b/net/socket.c
> index d80d87a395ea..320f51b22b19 100644
> --- a/net/socket.c
> +++ b/net/socket.c
> @@ -577,6 +577,7 @@ 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;
>