Fix NULL deref in balance_pgdat()

Be careful not to unconditionally clear the PF_MEMALLOC bit in the task structure. It may have already been set when entering zpl_putpage() in which case it must remain set on exit. In particular the kswapd thread will have PF_MEMALLOC set in order to prevent it from entering direct reclaim. By clearing it we allow the following NULL deref to potentially occur. BUG: unable to handle kernel NULL pointer dereference at (null) IP: [<ffffffff8109c7ab>] balance_pgdat+0x25b/0x4ff Signed-off-by: Brian Behlendorf <[email protected]> Issue openzfs#287
behlendorf · Nov 1, 2011 · 80c97dd · 80c97dd
1 parent c475167
commit 80c97dd
Showing 1 changed file with 7 additions and 3 deletions.
diff --git a/module/zfs/zpl_file.c b/module/zfs/zpl_file.c
@@ -324,9 +324,13 @@ zpl_putpage(struct page *pp, struct writeback_control *wbc, void *data)
 	 * the VM might try to write out additional pages by calling
 	 * zpl_putpage() again resulting in a deadlock.
 	 */
-	current->flags |= PF_MEMALLOC;
-	(void) zfs_putpage(mapping->host, pp, wbc);
-	current->flags &= ~PF_MEMALLOC;
+	if (current->flags & PF_MEMALLOC) {
+		(void) zfs_putpage(mapping->host, pp, wbc);
+	} else {
+		current->flags |= PF_MEMALLOC;
+		(void) zfs_putpage(mapping->host, pp, wbc);
+		current->flags &= ~PF_MEMALLOC;
+	}
 
 	return (0);
 }