From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Florian Westphal <fw@strlen.de>,
        Mat Martineau <mathew.j.martineau@linux.intel.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Sasha Levin <sashal@kernel.org>
Subject: [PATCH 5.10 48/99] mptcp: clear kern flag from fallback sockets
Date:   Mon, 20 Dec 2021 15:34:21 +0100
Message-Id: <20211220143030.996564528@linuxfoundation.org>
In-Reply-To: <20211220143029.352940568@linuxfoundation.org>
References: <20211220143029.352940568@linuxfoundation.org>
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From: Florian Westphal <fw@strlen.de>

[ Upstream commit d6692b3b97bdc165d150f4c1505751a323a80717 ]

The mptcp ULP extension relies on sk->sk_sock_kern being set correctly:
It prevents setsockopt(fd, IPPROTO_TCP, TCP_ULP, "mptcp", 6); from
working for plain tcp sockets (any userspace-exposed socket).

But in case of fallback, accept() can return a plain tcp sk.
In such case, sk is still tagged as 'kernel' and setsockopt will work.

This will crash the kernel, The subflow extension has a NULL ctx->conn
mptcp socket:

BUG: KASAN: null-ptr-deref in subflow_data_ready+0x181/0x2b0
Call Trace:
 tcp_data_ready+0xf8/0x370
 [..]

Fixes: cf7da0d66cc1 ("mptcp: Create SUBFLOW socket for incoming connections")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 net/mptcp/protocol.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 3ca8b359e399a..8123c79e27913 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -2149,7 +2149,7 @@ static struct sock *mptcp_accept(struct sock *sk, int flags, int *err,
 		 */
 		if (WARN_ON_ONCE(!new_mptcp_sock)) {
 			tcp_sk(newsk)->is_mptcp = 0;
-			return newsk;
+			goto out;
 		}
 
 		/* acquire the 2nd reference for the owning socket */
@@ -2174,6 +2174,8 @@ static struct sock *mptcp_accept(struct sock *sk, int flags, int *err,
 				MPTCP_MIB_MPCAPABLEPASSIVEFALLBACK);
 	}
 
+out:
+	newsk->sk_kern_sock = kern;
 	return newsk;
 }
 
-- 
2.33.0