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.

Signed-off-by David Jeffery <[email protected]>

fs/lockd/svclock.c +4 -1

@@ -951,6 +951,7 @@ 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);
 

@@ -960,6 +961,7 @@ 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);

@@ -969,7 +971,9 @@ 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;
 }
--

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