netdev - [PATCH net] mptcp: fix data stream corruption

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Message-Id: <95cee926051dae0afe4d39072f446e1cad17008a.1620720059.git.pabeni@redhat.com>
Date:   Tue, 11 May 2021 10:22:08 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     netdev@...r.kernel.org
Cc:     Mat Martineau <mathew.j.martineau@...ux.intel.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, mptcp@...ts.linux.dev,
        Maxim Galaganov <max@...ernet.ru>
Subject: [PATCH net] mptcp: fix data stream corruption

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.

Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/178
Reported-and-tested-by: Maxim Galaganov <max@...ernet.ru>
Signed-off-by: Paolo Abeni <pabeni@...hat.com>
---
 net/mptcp/protocol.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 29a2d690d8d5..2d21a4793d9d 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -879,12 +879,18 @@ static bool mptcp_skb_can_collapse_to(u64 write_seq,
 	       !mpext->frozen;
 }
 
+/* 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->size - pfrag->offset > 0 &&
+		pfrag->offset == (df->offset + df->data_len) &&
 		df->data_seq + df->data_len == msk->write_seq;
 }
 
-- 
2.26.2