[3.16.y-ckt stable] Patch "fix misuses of f_count() in ppp and netlink" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    fix misuses of f_count() in ppp and netlink

to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.16.y-queue

This patch is scheduled to be released in version 3.16.7-ckt2.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.16.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From a0649e6d56ff026292a1d6da0853c81b2ca8536c Mon Sep 17 00:00:00 2001
From: Al Viro <viro at zeniv.linux.org.uk>
Date: Wed, 8 Oct 2014 23:44:00 -0400
Subject: fix misuses of f_count() in ppp and netlink

commit 24dff96a37a2ca319e75a74d3929b2de22447ca6 upstream.

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.

Signed-off-by: Al Viro <viro at zeniv.linux.org.uk>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 drivers/net/ppp/ppp_generic.c | 2 +-
 net/netlink/af_netlink.c      | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c
index d5b77ef3a210..a75a411b1c27 100644
--- a/drivers/net/ppp/ppp_generic.c
+++ b/drivers/net/ppp/ppp_generic.c
@@ -594,7 +594,7 @@ static long ppp_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 			if (file == ppp->owner)
 				ppp_shutdown_interface(ppp);
 		}
-		if (atomic_long_read(&file->f_count) <= 2) {
+		if (atomic_long_read(&file->f_count) < 2) {
 			ppp_release(NULL, file);
 			err = 0;
 		} else
diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index 48fc607a211e..d158e1c888d8 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -707,7 +707,7 @@ 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;

--
2.1.0