[072/251] lockd: protect nlm_blocked access in nlmsvc_retry_blocked

[Steven Rostedt]

3.6.11.9-rc1 stable review patch.
If anyone has any objections, please let me know.

------------------

From: David Jeffery <djeffery@xxxxxxxxxx>

[ Upstream commit 1c327d962fc420aea046c16215a552710bde8231 ]

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@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: David Jeffery <djeffery@xxxxxxxxxx>
Signed-off-by: J. Bruce Fields <bfields@xxxxxxxxxx>
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
---
 fs/lockd/svclock.c |    4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/lockd/svclock.c b/fs/lockd/svclock.c
index 8d80c99..57a3922 100644
--- a/fs/lockd/svclock.c
+++ b/fs/lockd/svclock.c
@@ -939,6 +939,7 @@ 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);
 
@@ -948,6 +949,7 @@ 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);
@@ -957,7 +959,9 @@ 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;
 }
-- 
1.7.10.4


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