[RFC / RFT net 4/7] tls: rx: strp: fix determining record length in copy mode

[Jakub Kicinski <kuba@kernel.org>]

We call tls_rx_msg_size(skb) before doing skb->len += chunk.
So the tls_rx_msg_size() code will see old skb->len, most
likely leading to an over-read.

Worst case we will over read an entire record, next iteration
will try to trim the skb but may end up turning frag len negative
or discarding the subsequent record (since we already told TCP
we've read it during previous read but now we'll trim it out of
the skb).

Fixes: 84c61fe1a75b ("tls: rx: do not use the standard strparser")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
---
 net/tls/tls_strp.c | 21 +++++++++++++++------
 1 file changed, 15 insertions(+), 6 deletions(-)

diff --git a/net/tls/tls_strp.c b/net/tls/tls_strp.c
index 9a125c28da88..7b0c8145ace6 100644
--- a/net/tls/tls_strp.c
+++ b/net/tls/tls_strp.c
@@ -210,19 +210,28 @@ static int tls_strp_copyin(read_descriptor_t *desc, struct sk_buff *in_skb,
 					   skb_frag_size(frag),
 					   chunk));
 
-		sz = tls_rx_msg_size(strp, strp->anchor);
+		skb->len += chunk;
+		skb->data_len += chunk;
+		skb_frag_size_add(frag, chunk);
+
+		sz = tls_rx_msg_size(strp, skb);
 		if (sz < 0) {
 			desc->error = sz;
 			return 0;
 		}
 
 		/* We may have over-read, sz == 0 is guaranteed under-read */
-		if (sz > 0)
-			chunk =	min_t(size_t, chunk, sz - skb->len);
+		if (unlikely(sz && sz < skb->len)) {
+			int over = skb->len - sz;
+
+			WARN_ON_ONCE(over > chunk);
+			skb->len -= over;
+			skb->data_len -= over;
+			skb_frag_size_add(frag, -over);
+
+			chunk -= over;
+		}
 
-		skb->len += chunk;
-		skb->data_len += chunk;
-		skb_frag_size_add(frag, chunk);
 		frag++;
 		len -= chunk;
 		offset += chunk;
-- 
2.40.1