On Tue, Mar 10, 2020 at 06:47 PM CET, Lorenz Bauer wrote:
> Allow callers with CAP_NET_ADMIN to retrieve file descriptors from a
> sockmap and sockhash. O_CLOEXEC is enforced on all fds.
>
> Without this, it's difficult to resize or otherwise rebuild existing
> sockmap or sockhashes.
>
> Suggested-by: Jakub Sitnicki <jakub@xxxxxxxxxxxxxx>
> Signed-off-by: Lorenz Bauer <lmb@xxxxxxxxxxxxxx>
> ---
> net/core/sock_map.c | 19 +++++++++++++++++++
> 1 file changed, 19 insertions(+)
>
> diff --git a/net/core/sock_map.c b/net/core/sock_map.c
> index 03e04426cd21..3228936aa31e 100644
> --- a/net/core/sock_map.c
> +++ b/net/core/sock_map.c
> @@ -347,12 +347,31 @@ static void *sock_map_lookup(struct bpf_map *map, void *key)
> static int __sock_map_copy_value(struct bpf_map *map, struct sock *sk,
> void *value)
> {
> + struct file *file;
> + int fd;
> +
> switch (map->value_size) {
> case sizeof(u64):
> sock_gen_cookie(sk);
> *(u64 *)value = atomic64_read(&sk->sk_cookie);
> return 0;
>
> + case sizeof(u32):
> + if (!capable(CAP_NET_ADMIN))
> + return -EPERM;
> +
> + fd = get_unused_fd_flags(O_CLOEXEC);
> + if (unlikely(fd < 0))
> + return fd;
> +
> + read_lock_bh(&sk->sk_callback_lock);
> + file = get_file(sk->sk_socket->file);
I think this deserves a second look.
We don't lock the sock, so what if tcp_close orphans it before we enter
this critical section? Looks like sk->sk_socket might be NULL.
I'd find a test that tries to trigger the race helpful, like:
thread A: loop in lookup FD from map
thread B: loop in insert FD into map, close FD
> + read_unlock_bh(&sk->sk_callback_lock);
> +
> + fd_install(fd, file);
> + *(u32 *)value = fd;
> + return 0;
> +
> default:
> return -ENOSPC;
> }
