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 <jakub@xxxxxxxxxxxxxx>
> ---
> 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 <john.fastabend@xxxxxxxxx>
