[PATCH] idr: fix invalid ptr dereference on item delete

[Roman Kagan]

If an IDR contains a single entry at index==0, the underlying radix tree
has a single item in its root node, in which case
__radix_tree_lookup(index!=0) doesn't set its *@nodep argument (in
addition to returning NULL).

However, the tree itself is not empty, i.e. the tree root doesn't have
IDR_FREE tag.

As a result, on an attempt to remove an index!=0 entry from such an IDR,
radix_tree_delete_item doesn't return early and calls
__radix_tree_delete with invalid parameters which are then dereferenced.

Reported-by: syzbot+35666cba7f0a337e2e79@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Roman Kagan <rkagan@xxxxxxxxxxxxx>
---
 lib/radix-tree.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/lib/radix-tree.c b/lib/radix-tree.c
index da9e10c827df..10ff1bfae952 100644
--- a/lib/radix-tree.c
+++ b/lib/radix-tree.c
@@ -2040,8 +2040,9 @@ void *radix_tree_delete_item(struct radix_tree_root *root,
 	void *entry;
 
 	entry = __radix_tree_lookup(root, index, &node, &slot);
-	if (!entry && (!is_idr(root) || node_tag_get(root, node, IDR_FREE,
-						get_slot_offset(node, slot))))
+	if (!entry && (!is_idr(root) || !node ||
+		       node_tag_get(root, node, IDR_FREE,
+				    get_slot_offset(node, slot))))
 		return NULL;
 
 	if (item && entry != item)
-- 
2.17.0