[PATCH v2] mm: workingset: fix NULL ptr dereference

[Minchan Kim]

It assumes shadow entries of radix tree rely on the init state
that node->private_list allocated newly is list_empty state
for the working. Currently, it's initailized in SLAB constructor
which means node of radix tree would be initialized only when
*slub allocates new page*, not *slub alloctes new object*.

If some FS or subsystem pass gfp_mask to __GFP_ZERO, that means
newly allocated node can have !list_empty(node->private_list)
by memset of slab allocator. It ends up calling NULL deference
at workingset_update_node by failing list_empty check.

This patch fixes it.

Fixes: 449dd6984d0e ("mm: keep page cache radix tree nodes in check")
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Jaegeuk Kim <jaegeuk@xxxxxxxxxx>
Cc: Chao Yu <yuchao0@xxxxxxxxxx>
Cc: Christopher Lameter <cl@xxxxxxxxx>
Cc: linux-fsdevel@xxxxxxxxxxxxxxx
Reported-by: Chris Fries <cfries@xxxxxxxxxx>
Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
---
 lib/radix-tree.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/lib/radix-tree.c b/lib/radix-tree.c
index da9e10c827df..9d68f2a7888e 100644
--- a/lib/radix-tree.c
+++ b/lib/radix-tree.c
@@ -511,6 +511,16 @@ int radix_tree_preload(gfp_t gfp_mask)
 {
 	/* Warn on non-sensical use... */
 	WARN_ON_ONCE(!gfpflags_allow_blocking(gfp_mask));
+	/*
+	 * New allocate node must have node->private_list as INIT_LIST_HEAD
+	 * state by workingset shadow memory implementation.
+	 * If user pass  __GFP_ZERO by mistake, slab allocator will clear
+	 * node->private_list, which makes a BUG. Rather than going Oops,
+	 * just fix and warn about it.
+	 */
+	if (WARN_ON(gfp_mask & __GFP_ZERO))
+		gfp_mask &= ~GFP_ZERO
+
 	return __radix_tree_preload(gfp_mask, RADIX_TREE_PRELOAD_SIZE);
 }
 EXPORT_SYMBOL(radix_tree_preload);
@@ -522,7 +532,7 @@ EXPORT_SYMBOL(radix_tree_preload);
  */
 int radix_tree_maybe_preload(gfp_t gfp_mask)
 {
-	if (gfpflags_allow_blocking(gfp_mask))
+	if (gfpflags_allow_blocking(gfp_mask) && !(gfp_mask & __GFP_ZERO))
 		return __radix_tree_preload(gfp_mask, RADIX_TREE_PRELOAD_SIZE);
 	/* Preloading doesn't help anything with this gfp mask, skip it */
 	preempt_disable();
-- 
2.17.0.484.g0c8726318c-goog