Subject: Re: [PATCH bpf-next 5/8] bpf: Allow selecting reuseport socket from a SOCKMAP

From: Jakub Sitnicki <[email-masked]>
Date: Mon, 25 Nov 2019 11:40:41 +0100

On Mon, Nov 25, 2019 at 05:17 AM CET, John Fastabend wrote:
> Alexei Starovoitov wrote:
>> On Sat, Nov 23, 2019 at 12:07:48PM +0100, Jakub Sitnicki wrote:
>> > SOCKMAP now supports storing references to listening sockets. Nothing keeps
>> > us from using it as an array of sockets to select from in SK_REUSEPORT
>> > programs.
>> >
>> > Whitelist the map type with the BPF helper for selecting socket. However,
>> > impose a restriction that the selected socket needs to be a listening TCP
>> > socket or a bound UDP socket (connected or not).
>> >
>> > The only other map type that works with the BPF reuseport helper,
>> > REUSEPORT_SOCKARRAY, has a corresponding check in its update operation
>> > handler.
>> >
>> > Signed-off-by: Jakub Sitnicki <[email-masked]>
>> > ---
>
> [...]
>
>> > diff --git a/net/core/filter.c b/net/core/filter.c
>> > index 49ded4a7588a..e3fb77353248 100644
>> > --- a/net/core/filter.c
>> > +++ b/net/core/filter.c
>> > @@ -8723,6 +8723,8 @@ BPF_CALL_4(sk_select_reuseport, struct sk_reuseport_kern *, reuse_kern,
>> >  	selected_sk = map->ops->map_lookup_elem(map, key);
>> >  	if (!selected_sk)
>> >  		return -ENOENT;
>> > +	if (!sock_flag(selected_sk, SOCK_RCU_FREE))
>> > +		return -EINVAL;
>>
>> hmm. I wonder whether this breaks existing users...
>
> There is already this check in reuseport_array_update_check()
>
> 	/*
> 	 * sk must be hashed (i.e. listening in the TCP case or binded
> 	 * in the UDP case) and
> 	 * it must also be a SO_REUSEPORT sk (i.e. reuse cannot be NULL).
> 	 *
> 	 * Also, sk will be used in bpf helper that is protected by
> 	 * rcu_read_lock().
> 	 */
> 	if (!sock_flag(nsk, SOCK_RCU_FREE) || !sk_hashed(nsk) || !nsk_reuse)
> 		return -EINVAL;
>
> So I believe it should not cause any problems with existing users. Perhaps
> we could consolidate the checks a bit or move it into the update paths if we
> wanted. I assume Jakub was just ensuring we don't get here with SOCK_RCU_FREE
> set from any of the new paths now. I'll let him answer though.

That was exactly my thinking here.

REUSEPORT_SOCKARRAY can't be populated with sockets that don't have
SOCK_RCU_FREE set. This makes the flag check in sk_select_reuseport BPF
helper redundant for this map type.

SOCKMAP, OTOH, allows storing established TCP sockets, which don't have
SOCK_RCU_FREE flag and shouldn't be used as reuseport targets. The newly
added check protects us against it.

I have a couple tests in the last patch for it -
test_sockmap_reuseport_select_{listening,connected}. Admittedly, UDP is
not covered.

Not sure how we could go about moving the checks to the update path for
SOCKMAP. At update time we don't know if the map will be used with a
reuseport or a sk_{skb,msg} program.

-Jakub

>
>> Martin,
>> what do you think?
>
> More eyes the better.
>
>> Could you also take a look at other patches too?
>> In particular patch 7?
>>
>
> Agreed would be good to give 7/8 a look I'm not too familiar with the
> selftests there.