Subject: Re: [PATCH bpf-next 4/8] bpf, sockmap: Don't let child socket inherit psock or its ops on copy

From: Jakub Sitnicki <[email-masked]>
Date: Tue, 17 Dec 2019 16:06:31 +0100

On Thu, Dec 12, 2019 at 08:23 PM CET, Martin Lau wrote:
> On Thu, Dec 12, 2019 at 12:27:19PM +0100, Jakub Sitnicki wrote:
>> On Wed, Dec 11, 2019 at 06:20 PM CET, Martin Lau wrote:
>> > On Tue, Dec 10, 2019 at 03:45:37PM +0100, Jakub Sitnicki wrote:
>> >> John, Martin,
>> >>
>> >> On Tue, Nov 26, 2019 at 07:36 PM CET, Jakub Sitnicki wrote:
>> >> > On Tue, Nov 26, 2019 at 06:16 PM CET, Martin Lau wrote:
>> >> >> On Tue, Nov 26, 2019 at 04:54:33PM +0100, Jakub Sitnicki wrote:
>> >> >>> On Mon, Nov 25, 2019 at 11:38 PM CET, Martin Lau wrote:
>> >> >>> > On Sat, Nov 23, 2019 at 12:07:47PM +0100, Jakub Sitnicki wrote:
>> >> >>> > [ ... ]
>> >> >>> >
>> >> >>> >> @@ -370,6 +378,11 @@ static inline void sk_psock_restore_proto(struct sock *sk,
>> >> >>> >>  			sk->sk_prot = psock->sk_proto;
>> >> >>> >>  		psock->sk_proto = NULL;
>> >> >>> >>  	}
>> >> >>> >> +
>> >> >>> >> +	if (psock->icsk_af_ops) {
>> >> >>> >> +		icsk->icsk_af_ops = psock->icsk_af_ops;
>> >> >>> >> +		psock->icsk_af_ops = NULL;
>> >> >>> >> +	}
>> >> >>> >>  }
>> >> >>> >
>> >> >>> > [ ... ]
>> >> >>> >
>> >> >>> >> +static struct sock *tcp_bpf_syn_recv_sock(const struct sock *sk,
>> >> >>> >> +					  struct sk_buff *skb,
>> >> >>> >> +					  struct request_sock *req,
>> >> >>> >> +					  struct dst_entry *dst,
>> >> >>> >> +					  struct request_sock *req_unhash,
>> >> >>> >> +					  bool *own_req)
>> >> >>> >> +{
>> >> >>> >> +	const struct inet_connection_sock_af_ops *ops;
>> >> >>> >> +	void (*write_space)(struct sock *sk);
>> >> >>> >> +	struct sk_psock *psock;
>> >> >>> >> +	struct proto *proto;
>> >> >>> >> +	struct sock *child;
>> >> >>> >> +
>> >> >>> >> +	rcu_read_lock();
>> >> >>> >> +	psock = sk_psock(sk);
>> >> >>> >> +	if (likely(psock)) {
>> >> >>> >> +		proto = psock->sk_proto;
>> >> >>> >> +		write_space = psock->saved_write_space;
>> >> >>> >> +		ops = psock->icsk_af_ops;
>> >> >>> > It is not immediately clear to me what ensure
>> >> >>> > ops is not NULL here.
>> >> >>> >
>> >> >>> > It is likely I missed something.  A short comment would
>> >> >>> > be very useful here.
>> >> >>>
>> >> >>> I can see the readability problem. Looking at it now, perhaps it should
>> >> >>> be rewritten, to the same effect, as:
>> >> >>>
>> >> >>> static struct sock *tcp_bpf_syn_recv_sock(...)
>> >> >>> {
>> >> >>> 	const struct inet_connection_sock_af_ops *ops = NULL;
>> >> >>>         ...
>> >> >>>
>> >> >>>         rcu_read_lock();
>> >> >>> 	psock = sk_psock(sk);
>> >> >>> 	if (likely(psock)) {
>> >> >>> 		proto = psock->sk_proto;
>> >> >>> 		write_space = psock->saved_write_space;
>> >> >>> 		ops = psock->icsk_af_ops;
>> >> >>> 	}
>> >> >>> 	rcu_read_unlock();
>> >> >>>
>> >> >>>         if (!ops)
>> >> >>> 		ops = inet_csk(sk)->icsk_af_ops;
>> >> >>>         child = ops->syn_recv_sock(sk, skb, req, dst, req_unhash, own_req);
>> >> >>>
>> >> >>> If psock->icsk_af_ops were NULL, it would mean we haven't initialized it
>> >> >>> properly. To double check what happens here:
>> >> >> I did not mean the init path.  The init path is fine since it init
>> >> >> eveything on psock before publishing the sk to the sock_map.
>> >> >>
>> >> >> I was thinking the delete path (e.g. sock_map_delete_elem).  It is not clear
>> >> >> to me what prevent the earlier pasted sk_psock_restore_proto() which sets
>> >> >> psock->icsk_af_ops to NULL from running in parallel with
>> >> >> tcp_bpf_syn_recv_sock()?  An explanation would be useful.
>> >> >
>> >> > Ah, I misunderstood. Nothing prevents the race, AFAIK.
>> >> >
>> >> > Setting psock->icsk_af_ops to null on restore and not checking for it
>> >> > here was a bad move on my side.  Also I need to revisit what to do about
>> >> > psock->sk_proto so the child socket doesn't end up with null sk_proto.
>> >> >
>> >> > This race should be easy enough to trigger. Will give it a shot.
>> >>
>> >> I've convinced myself that this approach is racy beyond repair.
>> >>
>> >> Once syn_recv_sock() has returned it is too late to reset the child
>> >> sk_user_data and restore its callbacks. It has been already inserted
>> >> into ehash and ingress path can invoke its callbacks.
>> >>
>> >> The race can be triggered with with a reproducer where:
>> >>
>> >> thread-1:
>> >>
>> >>         p = accept(s, ...);
>> >>         close(p);
>> >>
>> >> thread-2:
>> >>
>> >> 	bpf_map_update_elem(mapfd, &key, &s, BPF_NOEXIST);
>> >> 	bpf_map_delete_elem(mapfd, &key);
>> >>
>> >> This a dead-end because we can't have the parent and the child share the
>> >> psock state. Even though psock itself is refcounted, and potentially we
>> >> could grab a reference before cloning the parent, link into the map that
>> >> psock holds is not.
>> >>
>> >> Two ways out come to mind. Both involve touching TCP code, which I was
>> >> hoping to avoid:
>> >>
>> >> 1) reset sk_user_data when initializing the child
>> >>
>> >>    This is problematic because tcp_bpf callbacks are not designed to
>> >>    handle sockets with no psock _and_ with overridden sk_prot
>> >>    callbacks. (Although, I think they could if the fallback was directly
>> >>    on {tcp,tcpv6}_prot based on socket domain.)
>> >>
>> >>    Also, there are other sk_user_data users like DRBD which rely on
>> >>    sharing the sk_user_data pointer between parent and child, if I read
>> >>    the code correctly [0]. If anything, clearing the sk_user_data on
>> >>    clone would have to be guarded by a flag.
>> > Can the copy/not-to-copy sk_user_data decision be made in
>> > sk_clone_lock()?
>>
>> Yes, this could be pushed down to sk_clone_lock(), where we do similar
>> work (reset sk_reuseport_cb and clone bpf_sk_storage):
> aha.  I missed your eariler "clearing the sk_user_data on clone would have
> to be guarded by a flag..." part.  It turns out we were talking the same
> thing on (1).  sock_flag works better if there is still bit left (and it
> seems there is one),  although I was thinking more like adding
> something (e.g. a func ptr) to 'struct proto' to mangle sk_user_data
> before returning newsk....but not sure this kind of logic
> belongs to 'struct proto'

Sorry for late reply.

We have 4 bits left by my count. The multi-line comment for SOCK_NOFCS
is getting in the way of counting them line-for-bit.

A callback invoked on socket clone is something I was considering too.
I'm not sure either where it belongs. At risk of being too use-case
specific, perhaps it could live together with sk_user_data and sk_prot,
which it would mangle on sk_clone_lock():

struct sock {
        ...
	void			*sk_user_data;
	void			(*sk_clone)(struct sock *sk,
					    struct sock *newsk);
        ...
}

But, I feel adding a new sock field just for this wouldn't be justified.
I can get by with a sock flag. Unless we have other uses for it?

>
>>
>> 	/* User data can hold reference. Child must not
>> 	 * inherit the pointer without acquiring a reference.
>> 	 */
>> 	if (sock_flag(sk, SOCK_OWNS_USER_DATA)) {
>> 		sock_reset_flag(newsk, SOCK_OWNS_USER_DATA);
>> 		RCU_INIT_POINTER(newsk->sk_user_data, NULL);
>> 	}
>>
>> I belive this would still need to be guarded by a flag.  Do you see
>> value in clearing child sk_user_data on clone as opposed to dealying
>> that work until accept() time?
> It seems to me clearing things up front at the very beginning is more
> straight forward, such that it does not have to worry about the
> sk_user_data may be used in a wrong way before it gets a chance
> to be cleared in accept().
>
> Just something to consider, if it is obvious that there is no hole in
> clearing it in accept(), it is fine too.

Just when I thought I could get away with lazily clearing the
sk_user_data at accept() time, it occurred to me that it is not enough.

Listening socket could get deleted from sockmap before a child socket
that inherited a copy of sk_user_data pointer gets accept()'ed. In such
scenario the pointer would not get NULL'ed on accept(), because
listening socket would have it's sk_prot->accept restored by then.

I will need that flag after all...

-jkbs

>
>> >>
>> >> 2) Restore sk_prot callbacks on clone to {tcp,tcpv6}_prot
>> >>
>> >>    The simpler way out. tcp_bpf callbacks never get invoked on the child
>> >>    socket so the copied psock reference is no longer a problem. We can
>> >>    clear the pointer on accept().
>> >>
>> >>    So far I wasn't able poke any holes in it and it comes down to
>> >>    patching tcp_create_openreq_child() with:
>> >>
>> >> 	/* sk_msg and ULP frameworks can override the callbacks into
>> >> 	 * protocol. We don't assume they are intended to be inherited
>> >> 	 * by the child. Frameworks can re-install the callbacks on
>> >> 	 * accept() if needed.
>> >> 	 */
>> >> 	WRITE_ONCE(newsk->sk_prot, sk->sk_prot_creator);
>> >>
>> >>    That's what I'm going with for v2.
>> >>
>> >> Open to suggestions.
>> >>
>> >> Thanks,
>> >> Jakub
>> >>
>> >> BTW. Reading into kTLS code, I noticed it has been limited down to just
>> >> established sockets due to the same problem I'm struggling with here:
>> >>
>> >> static int tls_init(struct sock *sk)
>> >> {
>> >> ...
>> >> 	/* The TLS ulp is currently supported only for TCP sockets
>> >> 	 * in ESTABLISHED state.
>> >> 	 * Supporting sockets in LISTEN state will require us
>> >> 	 * to modify the accept implementation to clone rather then
>> >> 	 * share the ulp context.
>> >> 	 */
>> >> 	if (sk->sk_state != TCP_ESTABLISHED)
>> >> 		return -ENOTCONN;
>> >>
>> >> [0] https://urldefense.proofpoint.com/v2/url?u=https-3A__elixir.bootlin.com_linux_v5.5-2Drc1_source_drivers_block_drbd_drbd-5Freceiver.c-23L682&d=DwIBAg&c=5VD0RTtNlTh3ycd41b3MUw&r=VQnoQ7LvghIj0gVEaiQSUw&m=z2Cz1gEcqiw-8YqVOluxlUHh_CBs6PJWQN2vgirOyFk&s=WAiM0asZN0OkqrW02xm2mCMIzWhKQCc3KiY7pzMKNg4&e=