Re: [PATCH v3 net-next 14/15] net: Reference bpf_redirect_info via task_struct on PREEMPT_RT.

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Sebastian Andrzej Siewior <bigeasy@linutronix.de> writes:

> The XDP redirect process is two staged:
> - bpf_prog_run_xdp() is invoked to run a eBPF program which inspects the
>   packet and makes decisions. While doing that, the per-CPU variable
>   bpf_redirect_info is used.
>
> - Afterwards xdp_do_redirect() is invoked and accesses bpf_redirect_info
>   and it may also access other per-CPU variables like xskmap_flush_list.
>
> At the very end of the NAPI callback, xdp_do_flush() is invoked which
> does not access bpf_redirect_info but will touch the individual per-CPU
> lists.
>
> The per-CPU variables are only used in the NAPI callback hence disabling
> bottom halves is the only protection mechanism. Users from preemptible
> context (like cpu_map_kthread_run()) explicitly disable bottom halves
> for protections reasons.
> Without locking in local_bh_disable() on PREEMPT_RT this data structure
> requires explicit locking.
>
> PREEMPT_RT has forced-threaded interrupts enabled and every
> NAPI-callback runs in a thread. If each thread has its own data
> structure then locking can be avoided.
>
> Create a struct bpf_net_context which contains struct bpf_redirect_info.
> Define the variable on stack, use bpf_net_ctx_set() to save a pointer to
> it. Use the __free() annotation to automatically reset the pointer once
> function returns.
> The bpf_net_ctx_set() may nest. For instance a function can be used from
> within NET_RX_SOFTIRQ/ net_rx_action which uses bpf_net_ctx_set() and
> NET_TX_SOFTIRQ which does not. Therefore only the first invocations
> updates the pointer.
> Use bpf_net_ctx_get_ri() as a wrapper to retrieve the current struct
> bpf_redirect_info.
>
> On PREEMPT_RT the pointer to bpf_net_context is saved task's
> task_struct. On non-PREEMPT_RT builds the pointer saved in a per-CPU
> variable (which is always NODE-local memory). Using always the
> bpf_net_context approach has the advantage that there is almost zero
> differences between PREEMPT_RT and non-PREEMPT_RT builds.
>
> Cc: Alexei Starovoitov <ast@kernel.org>
> Cc: Andrii Nakryiko <andrii@kernel.org>
> Cc: Eduard Zingerman <eddyz87@gmail.com>
> Cc: Hao Luo <haoluo@google.com>
> Cc: Jesper Dangaard Brouer <hawk@kernel.org>
> Cc: Jiri Olsa <jolsa@kernel.org>
> Cc: John Fastabend <john.fastabend@gmail.com>
> Cc: KP Singh <kpsingh@kernel.org>
> Cc: Martin KaFai Lau <martin.lau@linux.dev>
> Cc: Song Liu <song@kernel.org>
> Cc: Stanislav Fomichev <sdf@google.com>
> Cc: Toke Høiland-Jørgensen <toke@redhat.com>
> Cc: Yonghong Song <yonghong.song@linux.dev>
> Cc: bpf@vger.kernel.org
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

[...]
> @@ -240,12 +240,14 @@ static int cpu_map_bpf_prog_run(struct bpf_cpu_map_entry *rcpu, void **frames,
>  				int xdp_n, struct xdp_cpumap_stats *stats,
>  				struct list_head *list)
>  {
> +	struct bpf_net_context __bpf_net_ctx, *bpf_net_ctx;
>  	int nframes;

I think we need to zero-initialise all the context objects we allocate
on the stack.

The reason being that an XDP program can return XDP_REDIRECT without
calling any of the redirect helpers first; which will lead to
xdp_do_redirect() being called without any of the fields in struct
bpf_redirect_info having being set. This can lead to a crash if the
values happen to be the wrong value; and if we're not initialising the
stack space used by this struct, we have no guarantees about what value
they will end up with.

We fixed a similar bug relatively recently, see:

5bcf0dcbf906 ("xdp: use flags field to disambiguate broadcast redirect")

>  void bpf_clear_redirect_map(struct bpf_map *map)
>  {
> -	struct bpf_redirect_info *ri;
> -	int cpu;
> -
> -	for_each_possible_cpu(cpu) {
> -		ri = per_cpu_ptr(&bpf_redirect_info, cpu);
> -		/* Avoid polluting remote cacheline due to writes if
> -		 * not needed. Once we pass this test, we need the
> -		 * cmpxchg() to make sure it hasn't been changed in
> -		 * the meantime by remote CPU.
> -		 */
> -		if (unlikely(READ_ONCE(ri->map) == map))
> -			cmpxchg(&ri->map, map, NULL);
> -	}
> +	/* ri->map is assigned in __bpf_xdp_redirect_map() from within a eBPF
> +	 * program/ during NAPI callback. It is used during
> +	 * xdp_do_generic_redirect_map()/ __xdp_do_redirect_frame() from the
> +	 * redirect callback afterwards. ri->map is cleared after usage.
> +	 * The path has no explicit RCU read section but the local_bh_disable()
> +	 * is also a RCU read section which makes the complete softirq callback
> +	 * RCU protected. This in turn makes ri->map RCU protected and it is
> +	 * sufficient to wait a grace period to ensure that no "ri->map == map"
> +	 * exists. dev_map_free() removes the map from the list and then
> +	 * invokes synchronize_rcu() after calling this function.
> +	 */
>  }

With the zeroing of the stack variable mentioned above, I agree that
this is not needed anymore, but I think we should just get rid of the
function entirely and put a comment in devmap.c instead of the call to
the (now empty) function.

-Toke