tcp: Avoid overlapping memcpy() in DUP_ACK handling

When handling the DUP_ACK flag, we copy all the buffers making up the ack
frame.  However, all our frames share the same buffer for the Ethernet
header (tcp4_eth_src or tcp6_eth_src), so copying the TCP_IOV_ETH will
result in a (perfectly) overlapping memcpy().  This seems to have been
harmless so far, but overlapping ranges to memcpy() is undefined behaviour,
so we really should avoid it.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

1 files changed, 7 insertions, 3 deletions

diff --git a/tcp_buf.c b/tcp_buf.c
index 2e044b2..1a39846 100644
--- a/tcp_buf.c
+++ b/tcp_buf.c
@@ -332,9 +332,13 @@ int tcp_buf_send_flag(struct ctx *c, struct tcp_tap_conn *conn, int flags)
 		else
 			dup_iov = tcp6_l2_flags_iov[tcp6_flags_used++];
 
-		for (i = 0; i < TCP_NUM_IOVS; i++)
-			memcpy(dup_iov[i].iov_base, iov[i].iov_base,
-			       iov[i].iov_len);
+		for (i = 0; i < TCP_NUM_IOVS; i++) {
+			/* All frames share the same ethernet header buffer */
+			if (i != TCP_IOV_ETH) {
+				memcpy(dup_iov[i].iov_base, iov[i].iov_base,
+				       iov[i].iov_len);
+			}
+		}
 		dup_iov[TCP_IOV_PAYLOAD].iov_len = iov[TCP_IOV_PAYLOAD].iov_len;
 	}