[PATCH 5.10 129/252] mptcp: fix data stream corruption

[Greg Kroah-Hartman]

From: Paolo Abeni <pabeni@xxxxxxxxxx>

commit 29249eac5225429b898f278230a6ca2baa1ae154 upstream.

Maxim reported several issues when forcing a TCP transparent proxy
to use the MPTCP protocol for the inbound connections. He also
provided a clean reproducer.

The problem boils down to 'mptcp_frag_can_collapse_to()' assuming
that only MPTCP will use the given page_frag.

If others - e.g. the plain TCP protocol - allocate page fragments,
we can end-up re-using already allocated memory for mptcp_data_frag.

Fix the issue ensuring that the to-be-expanded data fragment is
located at the current page frag end.

v1 -> v2:
 - added missing fixes tag (Mat)

Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/178
Reported-and-tested-by: Maxim Galaganov <max@xxxxxxxxxxx>
Fixes: 18b683bff89d ("mptcp: queue data for mptcp level retransmission")
Signed-off-by: Paolo Abeni <pabeni@xxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 net/mptcp/protocol.c |    6 ++++++
 1 file changed, 6 insertions(+)

--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -758,11 +758,17 @@ static bool mptcp_skb_can_collapse_to(u6
 	return mpext && mpext->data_seq + mpext->data_len == write_seq;
 }
 
+/* we can append data to the given data frag if:
+ * - there is space available in the backing page_frag
+ * - the data frag tail matches the current page_frag free offset
+ * - the data frag end sequence number matches the current write seq
+ */
 static bool mptcp_frag_can_collapse_to(const struct mptcp_sock *msk,
 				       const struct page_frag *pfrag,
 				       const struct mptcp_data_frag *df)
 {
 	return df && pfrag->page == df->page &&
+		pfrag->offset == (df->offset + df->data_len) &&
 		df->data_seq + df->data_len == msk->write_seq;
 }