net/tls: Don't recursively call push_record during tls_write_space callbacks

It is reported that in some cases, write_space may be called in
do_tcp_sendpages, such that we recursively invoke do_tcp_sendpages again:

[  660.468802]  ? do_tcp_sendpages+0x8d/0x580
[  660.468826]  ? tls_push_sg+0x74/0x130 [tls]
[  660.468852]  ? tls_push_record+0x24a/0x390 [tls]
[  660.468880]  ? tls_write_space+0x6a/0x80 [tls]
...

tls_push_sg already does a loop over all sending sg's, so ignore
any tls_write_space notifications until we are done sending.
We then have to call the previous write_space to wake up
poll() waiters after we are done with the send loop.
Reported-by: NAndre Tomt <andre@tomt.net>
Signed-off-by: NDave Watson <davejwatson@fb.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/tls.h

@@ -148,6 +148,7 @@ struct tls_context {
 	struct scatterlist *partially_sent_record;
 	u16 partially_sent_offset;
 	unsigned long flags;
+	bool in_tcp_sendpages;
 
 	u16 pending_open_record_frags;
 	int (*push_pending_record)(struct sock *sk, int flags);

net/tls/tls_main.c

@@ -114,6 +114,7 @@ int tls_push_sg(struct sock *sk,
 	size = sg->length - offset;
 	offset += sg->offset;
 
+	ctx->in_tcp_sendpages = true;
 	while (1) {
 		if (sg_is_last(sg))
 			sendpage_flags = flags;
@@ -148,6 +149,8 @@ int tls_push_sg(struct sock *sk,
 	}
 
 	clear_bit(TLS_PENDING_CLOSED_RECORD, &ctx->flags);
+	ctx->in_tcp_sendpages = false;
+	ctx->sk_write_space(sk);
 
 	return 0;
 }
@@ -217,6 +220,10 @@ static void tls_write_space(struct sock *sk)
 {
 	struct tls_context *ctx = tls_get_ctx(sk);
 
+	/* We are already sending pages, ignore notification */
+	if (ctx->in_tcp_sendpages)
+		return;
+
 	if (!sk->sk_write_pending && tls_is_pending_closed_record(ctx)) {
 		gfp_t sk_allocation = sk->sk_allocation;
 		int rc;