[ 3.5.y.z extended stable ] Patch "lockd: protect nlm_blocked access in nlmsvc_retry_blocked" has been added to staging queue

[Luis Henriques]

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

    lockd: protect nlm_blocked access in nlmsvc_retry_blocked

to the linux-3.5.y-queue branch of the 3.5.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.5.y-queue

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

For more information about the 3.5.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From bde3109d409c10538a606df33326050449d41be8 Mon Sep 17 00:00:00 2001
From: David Jeffery <djeffery at redhat.com>
Date: Wed, 10 Jul 2013 13:19:50 -0400
Subject: [PATCH] lockd: protect nlm_blocked access in nlmsvc_retry_blocked

commit 1c327d962fc420aea046c16215a552710bde8231 upstream.

In nlmsvc_retry_blocked, the check that the list is non-empty and acquiring
the pointer of the first entry is unprotected by any lock.  This allows a rare
race condition when there is only one entry on the list.  A function such as
nlmsvc_grant_callback() can be called, which will temporarily remove the entry
from the list.  Between the list_empty() and list_entry(),the list may become
empty, causing an invalid pointer to be used as an nlm_block, leading to a
possible crash.

This patch adds the nlm_block_lock around these calls to prevent concurrent
use of the nlm_blocked list.

This was a regression introduced by
f904be9cc77f361d37d71468b13ff3d1a1823dea  "lockd: Mostly remove BKL from
the server".

Cc: Bryan Schumaker <bjschuma at netapp.com>
Signed-off-by: David Jeffery <djeffery at redhat.com>
Signed-off-by: J. Bruce Fields <bfields at redhat.com>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 fs/lockd/svclock.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/lockd/svclock.c b/fs/lockd/svclock.c
index e46353f..aff1c61 100644
--- a/fs/lockd/svclock.c
+++ b/fs/lockd/svclock.c
@@ -941,6 +941,7 @@ nlmsvc_retry_blocked(void)
 	unsigned long	timeout = MAX_SCHEDULE_TIMEOUT;
 	struct nlm_block *block;

+	spin_lock(&nlm_blocked_lock);
 	while (!list_empty(&nlm_blocked) && !kthread_should_stop()) {
 		block = list_entry(nlm_blocked.next, struct nlm_block, b_list);

@@ -950,6 +951,7 @@ nlmsvc_retry_blocked(void)
 			timeout = block->b_when - jiffies;
 			break;
 		}
+		spin_unlock(&nlm_blocked_lock);

 		dprintk("nlmsvc_retry_blocked(%p, when=%ld)\n",
 			block, block->b_when);
@@ -959,7 +961,9 @@ nlmsvc_retry_blocked(void)
 			retry_deferred_block(block);
 		} else
 			nlmsvc_grant_blocked(block);
+		spin_lock(&nlm_blocked_lock);
 	}
+	spin_unlock(&nlm_blocked_lock);

 	return timeout;
 }
--
1.8.1.2