net: mctp: hold key reference when looking up a general key

net: mctp: hold key reference when looking up a general key

[ Upstream commit 6e54ea37e344f145665c2dc3cc534b92529e8de5 ]

Currently, we have a race where we look up a sock through a "general"
(ie, not directly associated with the (src,dest,tag) tuple) key, then
drop the key reference while still holding the key's sock.

This change expands the key reference until we've finished using the
sock, and hence the sock reference too.

Commit message changes from Jeremy Kerr <jk@codeconstruct.com.au>.

Reported-by: Noam Rathaus <noamr@ssd-disclosure.com>
Fixes: c88d6fe021f9 ("mctp: locking, lifetime and validity changes for sk_keys")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>