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H A Dprotocol.cdiff d6692b3b97bdc165d150f4c1505751a323a80717 Tue Dec 14 17:16:02 CST 2021 Florian Westphal <fw@strlen.de> mptcp: clear 'kern' flag from fallback sockets

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>