Re: [PATCH bpf-next 1/2] bpf: allow rewriting to ports under ip_unprivileged_port_start

[Andrey Ignatov <rdna@fb.com>]

Stanislav Fomichev <sdf@google.com> [Wed, 2021-01-20 18:09 -0800]:
> At the moment, BPF_CGROUP_INET{4,6}_BIND hooks can rewrite user_port
> to the privileged ones (< ip_unprivileged_port_start), but it will
> be rejected later on in the __inet_bind or __inet6_bind.
>
> Let's export 'port_changed' event from the BPF program and bypass
> ip_unprivileged_port_start range check when we've seen that
> the program explicitly overrode the port. This is accomplished
> by generating instructions to set ctx->port_changed along with
> updating ctx->user_port.
> 
> Signed-off-by: Stanislav Fomichev <sdf@google.com>
> ---
...
> @@ -244,17 +245,27 @@ int bpf_percpu_cgroup_storage_update(struct bpf_map *map, void *key,
>  	if (cgroup_bpf_enabled(type))	{				       \
>  		lock_sock(sk);						       \
>  		__ret = __cgroup_bpf_run_filter_sock_addr(sk, uaddr, type,     \
> -							  t_ctx);	       \
> +							  t_ctx, NULL);	       \
>  		release_sock(sk);					       \
>  	}								       \
>  	__ret;								       \
>  })
>  
> -#define BPF_CGROUP_RUN_PROG_INET4_BIND_LOCK(sk, uaddr)			       \
> -	BPF_CGROUP_RUN_SA_PROG_LOCK(sk, uaddr, BPF_CGROUP_INET4_BIND, NULL)
> -
> -#define BPF_CGROUP_RUN_PROG_INET6_BIND_LOCK(sk, uaddr)			       \
> -	BPF_CGROUP_RUN_SA_PROG_LOCK(sk, uaddr, BPF_CGROUP_INET6_BIND, NULL)
> +#define BPF_CGROUP_RUN_PROG_INET_BIND_LOCK(sk, uaddr, type, flags)	       \
> +({									       \
> +	bool port_changed = false;					       \

I see the discussion with Martin in [0] on the program overriding the
port but setting exactly same value as it already contains. Commenting
on this patch since the code is here.

From what I understand there is no use-case to support overriding the
port w/o changing the value to just bypass the capability. In this case
the code can be simplified.

Here instead of introducing port_changed you can just remember the
original ((struct sockaddr_in *)uaddr)->sin_port or
((struct sockaddr_in6 *)uaddr)->sin6_port (they have same offset/size so
it can be simplified same way as in sock_addr_convert_ctx_access() for
user_port) ...

> +	int __ret = 0;							       \
> +	if (cgroup_bpf_enabled(type))	{				       \
> +		lock_sock(sk);						       \
> +		__ret = __cgroup_bpf_run_filter_sock_addr(sk, uaddr, type,     \
> +							  NULL,		       \
> +							  &port_changed);      \
> +		release_sock(sk);					       \
> +		if (port_changed)					       \

... and then just compare the original and the new ports here.

The benefits will be:
* no need to introduce port_changed field in struct bpf_sock_addr_kern;
* no need to do change program instructions;
* no need to think about compiler optimizing out those instructions;
* no need to think about multiple programs coordination, the flag will
  be set only if port has actually changed what is easy to reason about
  from user perspective.

wdyt?

> +			*flags |= BIND_NO_CAP_NET_BIND_SERVICE;		       \
> +	}								       \
> +	__ret;								       \
> +})
>  
>  #define BPF_CGROUP_PRE_CONNECT_ENABLED(sk)				       \
>  	((cgroup_bpf_enabled(BPF_CGROUP_INET4_CONNECT) ||		       \
> @@ -453,8 +464,7 @@ static inline int bpf_percpu_cgroup_storage_update(struct bpf_map *map,
>  #define BPF_CGROUP_RUN_PROG_INET_EGRESS(sk,skb) ({ 0; })
>  #define BPF_CGROUP_RUN_PROG_INET_SOCK(sk) ({ 0; })
>  #define BPF_CGROUP_RUN_PROG_INET_SOCK_RELEASE(sk) ({ 0; })
> -#define BPF_CGROUP_RUN_PROG_INET4_BIND_LOCK(sk, uaddr) ({ 0; })
> -#define BPF_CGROUP_RUN_PROG_INET6_BIND_LOCK(sk, uaddr) ({ 0; })
> +#define BPF_CGROUP_RUN_PROG_INET_BIND_LOCK(sk, uaddr, type, flags) ({ 0; })
>  #define BPF_CGROUP_RUN_PROG_INET4_POST_BIND(sk) ({ 0; })
>  #define BPF_CGROUP_RUN_PROG_INET6_POST_BIND(sk) ({ 0; })
>  #define BPF_CGROUP_RUN_PROG_INET4_CONNECT(sk, uaddr) ({ 0; })
...

[0] https://lore.kernel.org/bpf/20210121223330.pyk4ljtjirm2zlay@kafai-mbp/

-- 
Andrey Ignatov