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

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>

diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index 94c11cf0459d33bb12d26e246e7e9c66314e9e85..f34750691c5c0e24aef976f59041d44e8342d2a0 100644 (file)
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -2266,16 +2266,17 @@ 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);