netdev - RE: [RFC net] net/tls: clear SG markings on encryption error

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Date:   Fri, 22 Nov 2019 19:25:13 -0800
From:   John Fastabend <john.fastabend@...il.com>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>, borisp@...lanox.com,
        aviadye@...lanox.com, john.fastabend@...il.com,
        daniel@...earbox.net
Cc:     netdev@...r.kernel.org,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        syzbot+df0d4ec12332661dd1f9@...kaller.appspotmail.com
Subject: RE: [RFC net] net/tls: clear SG markings on encryption error

Jakub Kicinski wrote:
> When tls_do_encryption() fails the SG lists are left with the
> SG_END and SG_CHAIN marks in place. One could hope that once
> encryption fails we will never see the record again, but that
> is in fact not true. Commit d3b18ad31f93 ("tls: add bpf support
> to sk_msg handling") added special handling to ENOMEM and ENOSPC
> errors which mean we may see the same record re-submitted.
> 
> In all honesty I don't understand why we need the ENOMEM handling.
> Waiting for socket memory without setting SOCK_NOSPACE on any
> random memory allocation failure seems slightly ill advised.
> 
> Having said that, undoing the SG markings seems wise regardless.
> 
> Reported-by: syzbot+df0d4ec12332661dd1f9@...kaller.appspotmail.com
> Fixes: 130b392c6cd6 ("net: tls: Add tls 1.3 support")
> Fixes: d3b18ad31f93 ("tls: add bpf support to sk_msg handling")
> Signed-off-by: Jakub Kicinski <jakub.kicinski@...ronome.com>
> ---
> John, I'm sending this mostly to ask if we can safely remove
> the ENOMEM handling? :)
> 
What ENOMEM are you asking about here? The return code handling
from bpf_exec_tx_verdict?


	ret = bpf_exec_tx_verdict(msg_pl, sk, full_record,
				  record_type, &copied,
				  msg->msg_flags);
	if (ret) {
		if (ret == -EINPROGRESS)
			num_async++;
		else if (ret == -ENOMEM)
			goto wait_for_memory;
		else if (ret == -ENOSPC)
			goto rollback_iter;
		else if (ret != -EAGAIN)
			goto send_end;
	}

I would want to run it through some of our tests but I don't think
there is anything specific about BPF that needs it to be handled.
I was just trying to handle the error case gracefully and
wait_for_memory seems like the right behavior to me. What is ill
advised here?

> I was going to try the sockmap tests myself, but looks like the current
> LLVM 10 build I get from their debs just segfaults when trying to build
> selftest :/
> 
> Also there's at least one more bug in this piece of code, TLS 1.3
> can't assume there's at least one free SG entry.

There should always be one free SG entry at the end of the ring
that is used for chaining.

>From sk_msg_sg{}

	/* The extra element is used for chaining the front and sections when
	 * the list becomes partitioned (e.g. end < start). The crypto APIs
	 * require the chaining.
	 */
	struct scatterlist		data[MAX_MSG_FRAGS + 1];

Can we use that element in that case? Otherwise probably can
add an extra element there if needed, data[MAX_MSG_FRAGS + 2].

> 
>  net/tls/tls_sw.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/net/tls/tls_sw.c b/net/tls/tls_sw.c
> index 24161750a737..4a0ea87b20cf 100644
> --- a/net/tls/tls_sw.c
> +++ b/net/tls/tls_sw.c
> @@ -737,6 +737,19 @@ static int tls_push_record(struct sock *sk, int flags,
>  	if (rc < 0) {
>  		if (rc != -EINPROGRESS) {
>  			tls_err_abort(sk, EBADMSG);
> +
> +			i = msg_pl->sg.end;
> +			if (prot->version == TLS_1_3_VERSION) {
> +				sg_mark_end(sk_msg_elem(msg_pl, i));
> +				sg_unmark_end(sk_msg_elem(msg_pl, i));
> +			}
> +			sk_msg_iter_var_prev(i);
> +			sg_unmark_end(sk_msg_elem(msg_pl, i));
> +
> +			i = msg_en->sg.end;
> +			sk_msg_iter_var_prev(i);
> +			sg_unmark_end(sk_msg_elem(msg_en, i));
> +
>  			if (split) {
>  				tls_ctx->pending_open_record_frags = true;
>  				tls_merge_open_record(sk, rec, tmp, orig_end);
> -- 
> 2.23.0
> 

Can you copy the tls_push_record() error handling from BPF side instead of
embedding more into tls_push_record itself?

	err = tls_push_record(sk, flags, record_type);
	if (err < 0) {
		*copied -= sk_msg_free(sk, msg);
		tls_free_open_rec(sk);
		goto out_err;
	}

If the BPF program is not installed I guess you can skip the copied part
because you wont have the 'more_data' case.

So something like (untested/compiled/etc)

diff --git a/net/tls/tls_sw.c b/net/tls/tls_sw.c
index 141da093ff04..0469eb73bc88 100644
--- a/net/tls/tls_sw.c
+++ b/net/tls/tls_sw.c
@@ -771,8 +771,14 @@ static int bpf_exec_tx_verdict(struct sk_msg *msg, struct sock *sk,
 
 	policy = !(flags & MSG_SENDPAGE_NOPOLICY);
 	psock = sk_psock_get(sk);
-	if (!psock || !policy)
-		return tls_push_record(sk, flags, record_type);
+	if (!psock || !policy) {
+		err = tls_push_record(sk, flags, record_type);
+		if (err) {
+			sk_msg_free(sk, msg);
+			tls_free_open_rec(sk); // might not be needed in noBPF
+		}
+		return err;
+	}
 more_data:
 	enospc = sk_msg_full(msg);
 	if (psock->eval == __SK_NONE) {