On 02/21/2019 02:13 PM, Eric Biggers wrote:
> From: Eric Biggers <ebigg...@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:
>
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: <sta...@vger.kernel.org> # v4.10+
> Signed-off-by: Eric Biggers <ebigg...@google.com>
> ---
>
> 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;
>
