netlink: do not set cb_running if dump's start() errs

[ Upstream commit 41c87425a1ac9b633e0fcc78eb1f19640c8fb5a0 ]

It turns out that multiple places can call netlink_dump(), which means
it's still possible to dereference partially initialized values in
dump() that were the result of a faulty returned start().

This fixes the issue by calling start() _before_ setting cb_running to
true, so that there's no chance at all of hitting the dump() function
through any indirect paths.

It also moves the call to start() to be when the mutex is held. This has
the nice side effect of serializing invocations to start(), which is
likely desirable anyway. It also prevents any possible other races that
might come out of this logic.

In testing this with several different pieces of tricky code to trigger
these issues, this commit fixes all avenues that I'm aware of.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Cc: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 7 insertions, 6 deletions

diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index 2a5775f8a6ca..a1dca3b169a1 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -2207,16 +2207,17 @@ int __netlink_dump_start(struct sock *ssk, struct sk_buff *skb,
 	cb->min_dump_alloc = control->min_dump_alloc;
 	cb->skb = skb;
 
+	if (cb->start) {
+		ret = cb->start(cb);
+		if (ret)
+			goto error_unlock;
+	}
+
 	nlk->cb_running = true;
 
 	mutex_unlock(nlk->cb_mutex);
 
-	ret = 0;
-	if (cb->start)
-		ret = cb->start(cb);
-
-	if (!ret)
-		ret = netlink_dump(sk);
+	ret = netlink_dump(sk);
 
 	sock_put(sk);