Commit 1c327d96 authored Jul 10, 2013 by David Jeffery Committed by J. Bruce Fields Jul 11, 2013

lockd: protect nlm_blocked access in nlmsvc_retry_blocked



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
f904be9c  "lockd: Mostly remove BKL from
the server".

Cc: Bryan Schumaker <bjschuma@netapp.com>
Cc: stable@vger.kernel.org
Signed-off-by: David Jeffery <djeffery@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

parent d1091481

fs/lockd/svclock.c

+4 −0

Original line number	Diff line number	Diff line
		@@ -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;
		}