fix misuses of f_count() in ppp and netlink

we used to check for "nobody else could start doing anything with
that opened file" by checking that refcount was 2 or less - one
for descriptor table and one we'd acquired in fget() on the way to
wherever we are.  That was race-prone (somebody else might have
had a reference to descriptor table and do fget() just as we'd
been checking) and it had become flat-out incorrect back when
we switched to fget_light() on those codepaths - unlike fget(),
it doesn't grab an extra reference unless the descriptor table
is shared.  The same change allowed a race-free check, though -
we are safe exactly when refcount is less than 2.

It was a long time ago; pre-2.6.12 for ioctl() (the codepath leading
to ppp one) and 2.6.17 for sendmsg() (netlink one).  OTOH,
netlink hadn't grown that check until 3.9 and ppp used to live
in drivers/net, not drivers/net/ppp until 3.1.  The bug existed
well before that, though, and the same fix used to apply in old
location of file.

Cc: stable@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 1 insertions, 1 deletions

diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index c416725d28c4..7a186e74b1b3 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -715,7 +715,7 @@ static int netlink_mmap_sendmsg(struct sock *sk, struct msghdr *msg,
          * after validation, the socket and the ring may only be used by a
          * single process, otherwise we fall back to copying.
          */
-        if (atomic_long_read(&sk->sk_socket->file->f_count) > 2 ||
+        if (atomic_long_read(&sk->sk_socket->file->f_count) > 1 ||
             atomic_read(&nlk->mapped) > 1)
                 excl = false;