[PATCH RFC] XArray: fix index jumping backwards in xas_find()

A bug in the XArray iterator xas_find() causes the iterator's index
(xas->xa_index) to jump backwards when iterating over a multi-index entry
(like a THP) that resides in a non-leaf node and is concurrently split.

When iterating over a multi-index entry in a non-leaf node, xas_load() sets
xas->xa_offset to the base offset of the entry, but leaves xas->xa_index at
the requested index. When the caller subsequently wants to advance to the
next entry, xas_find() is called. xas_find() attempts to synchronize
xas->xa_offset with xas->xa_index before advancing. However, the fixup
logic was incorrectly restricted to leaf nodes (!xas->xa_node->shift).
Because the THP resides in a non-leaf node, the fixup is skipped.

As a result, xas_find() simply increments xas->xa_offset and recalculates
xas->xa_index based on this new offset. This causes xas->xa_index to jump
backwards. If the THP was concurrently split, the entry at the new offset
is a node pointer, so xas_find() descends into it and returns the folio at
the backwards index. The caller (filemap_map_pages()) then calculates the
PTE pointer based on this backwards index, resulting in an out-of-bounds
memory access.

For example, a KASAN slab-out-of-bounds read can be triggered in ptep_get()
via filemap_map_pages():

BUG: KASAN: slab-out-of-bounds in ptep_get include/linux/pgtable.h:495
[inline]
BUG: KASAN: slab-out-of-bounds in filemap_map_folio_range mm/filemap.c:3820
[inline]
BUG: KASAN: slab-out-of-bounds in filemap_map_pages+0x1100/0x2090
mm/filemap.c:3955
Read of size 8 at addr ffff88811b3b4a00 by task syz-executor411/5876

To fix this, remove the !xas->xa_node->shift restriction and use the
get_offset() helper to correctly calculate the offset for any node shift.
This ensures that xas->xa_offset is correctly synchronized with
xas->xa_index before advancing, regardless of the node's depth in the tree.

Fixes: b803b42823d0 ("xarray: Add XArray iterators")
Assisted-by: Gemini:gemini-3.6-flash Gemini:gemini-3.1-pro-preview syzbot
Reported-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=b72767277f29b6407083
Link: https://syzkaller.appspot.com/ai_job?id=608570d2-1765-472d-a79b-ea5b0b3a226a
To: "Andrew Morton" <[email protected]>
To: <[email protected]>
To: <[email protected]>
To: "Matthew Wilcox" <[email protected]>
Cc: <[email protected]>

---
diff --git a/lib/xarray.c b/lib/xarray.c
index 9a8b49165..5695618bf 100644
--- a/lib/xarray.c
+++ b/lib/xarray.c
@@ -1406,9 +1406,8 @@ void *xas_find(struct xa_state *xas, unsigned long max)
 		entry = xas_load(xas);
 		if (entry || xas_not_node(xas->xa_node))
 			return entry;
-	} else if (!xas->xa_node->shift &&
-		    xas->xa_offset != (xas->xa_index & XA_CHUNK_MASK)) {
-		xas->xa_offset = ((xas->xa_index - 1) & XA_CHUNK_MASK) + 1;
+	} else if (xas->xa_offset != get_offset(xas->xa_index, xas->xa_node)) {
+		xas->xa_offset = get_offset(xas->xa_index - 1, xas->xa_node) + 1;
 	}
 
 	xas_next_offset(xas);


base-commit: 8d3ae59288f1e7d58d76558a6ee96d533bc5019f
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