netdev - RE: [PATCH 4/5] bpf: sockmap, sockhash: return file descriptors from privileged lookup

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Date:   Wed, 11 Mar 2020 16:27:41 -0700
From:   John Fastabend <john.fastabend@...il.com>
To:     Lorenz Bauer <lmb@...udflare.com>,
        John Fastabend <john.fastabend@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jakub Sitnicki <jakub@...udflare.com>,
        Lorenz Bauer <lmb@...udflare.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>
Cc:     kernel-team@...udflare.com, netdev@...r.kernel.org,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: RE: [PATCH 4/5] bpf: sockmap, sockhash: return file descriptors from
 privileged lookup

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@...udflare.com>
> Signed-off-by: Lorenz Bauer <lmb@...udflare.com>
> ---
>  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);
> +		read_unlock_bh(&sk->sk_callback_lock);
> +
> +		fd_install(fd, file);
> +		*(u32 *)value = fd;
> +		return 0;
> +

Hi Lorenz, Can you say something about what happens if the sk
is deleted from the map or the sock is closed/unhashed ideally
in the commit message so we have it for later reference. I guess
because we are in an rcu block here the sk will be OK and psock
reference will exist until after the rcu block at least because
of call_rcu(). If the psock is destroyed from another path then
the fd will still point at the sock. correct?

Thanks.