Subject: RE: [PATCH bpf] bpf, sockmap: Reject sk_msg egress redirects to non-TCP sockets

From: John Fastabend <[email-masked]>
Date: Mon, 25 Sep 2023 11:27:32 -0700

Jakub Sitnicki wrote:
> With a SOCKMAP/SOCKHASH map and an sk_msg program user can steer messages
> sent from one TCP socket (s1) to actually egress from another TCP
> socket (s2):
> 
> tcp_bpf_sendmsg(s1)		// = sk_prot->sendmsg
>   tcp_bpf_send_verdict(s1)	// __SK_REDIRECT case
>     tcp_bpf_sendmsg_redir(s2)
>       tcp_bpf_push_locked(s2)
> 	tcp_bpf_push(s2)
> 	  tcp_rate_check_app_limited(s2) // expects tcp_sock
> 	  tcp_sendmsg_locked(s2)	 // ditto
> 
> There is a hard-coded assumption in the call-chain, that the egress
> socket (s2) is a TCP socket.
> 
> However in commit 122e6c79efe1 ("sock_map: Update sock type checks for
> UDP") we have enabled redirects to non-TCP sockets. This was done for the
> sake of BPF sk_skb programs. There was no indention to support sk_msg
> send-to-egress use case.
> 
> As a result, attempts to send-to-egress through a non-TCP socket lead to a
> crash due to invalid downcast from sock to tcp_sock:
> 
>  BUG: kernel NULL pointer dereference, address: 000000000000002f
>  ...
>  Call Trace:
>   <TASK>
>   ? show_regs+0x60/0x70
>   ? __die+0x1f/0x70
>   ? page_fault_oops+0x80/0x160
>   ? do_user_addr_fault+0x2d7/0x800
>   ? rcu_is_watching+0x11/0x50
>   ? exc_page_fault+0x70/0x1c0
>   ? asm_exc_page_fault+0x27/0x30
>   ? tcp_tso_segs+0x14/0xa0
>   tcp_write_xmit+0x67/0xce0
>   __tcp_push_pending_frames+0x32/0xf0
>   tcp_push+0x107/0x140
>   tcp_sendmsg_locked+0x99f/0xbb0
>   tcp_bpf_push+0x19d/0x3a0
>   tcp_bpf_sendmsg_redir+0x55/0xd0
>   tcp_bpf_send_verdict+0x407/0x550
>   tcp_bpf_sendmsg+0x1a1/0x390
>   inet_sendmsg+0x6a/0x70
>   sock_sendmsg+0x9d/0xc0
>   ? sockfd_lookup_light+0x12/0x80
>   __sys_sendto+0x10e/0x160
>   ? syscall_enter_from_user_mode+0x20/0x60
>   ? __this_cpu_preempt_check+0x13/0x20
>   ? lockdep_hardirqs_on+0x82/0x110
>   __x64_sys_sendto+0x1f/0x30
>   do_syscall_64+0x38/0x90
>   entry_SYSCALL_64_after_hwframe+0x63/0xcd
> 
> Reject selecting a non-TCP sockets as redirect target from a BPF sk_msg
> program to prevent the crash. When attempted, user will receive an EACCES
> error from send/sendto/sendmsg() syscall.
> 
> Fixes: 122e6c79efe1 ("sock_map: Update sock type checks for UDP")
> Signed-off-by: Jakub Sitnicki <[email-masked]>
> ---
> FYI, I'm working on revamping the sockmap_listen selftest, which exercises
> some of redirect combinations, to cover the whole combination matrix so
> that we can catch these kinds of problems early on.

Yes this would be appreciated.

> 
>  net/core/sock_map.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/net/core/sock_map.c b/net/core/sock_map.c
> index cb11750b1df5..4292c2ed1828 100644
> --- a/net/core/sock_map.c
> +++ b/net/core/sock_map.c
> @@ -668,6 +668,8 @@ BPF_CALL_4(bpf_msg_redirect_map, struct sk_msg *, msg,
>  	sk = __sock_map_lookup_elem(map, key);
>  	if (unlikely(!sk || !sock_map_redirect_allowed(sk)))
>  		return SK_DROP;
> +	if (!(flags & BPF_F_INGRESS) && !sk_is_tcp(sk))
> +		return SK_DROP;
>  
>  	msg->flags = flags;
>  	msg->sk_redir = sk;
> @@ -1267,6 +1269,8 @@ BPF_CALL_4(bpf_msg_redirect_hash, struct sk_msg *, msg,
>  	sk = __sock_hash_lookup_elem(map, key);
>  	if (unlikely(!sk || !sock_map_redirect_allowed(sk)))
>  		return SK_DROP;
> +	if (!(flags & BPF_F_INGRESS) && !sk_is_tcp(sk))
> +		return SK_DROP;

As a stop gap I think this is fine. If anyone wants to add support though
I do think as a use case it would make sense to redirect TCP into an
AF_UNIX socket and vice versa.

Acked-by: John Fastabend <[email-masked]>