[PATCH 5.15 091/177] mptcp: clear kern flag from fallback sockets

Greg Kroah-Hartman
2021-12-20T14:58:02Z

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 421fa62ce5cdf..fdff811c9a0da 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -2901,7 +2901,7 @@ 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 */
@@ -2913,6 +2913,8 @@ 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