[PATCH 1/2] lib: xarray: free unused spare node in xas_create_range()

[Andrew Morton <akpm@linux-foundation.org>]

From: Shardul Bankar <shardul.b@mpiricsoftware.com>

xas_create_range() is typically called in a retry loop that uses
xas_nomem() to handle -ENOMEM errors.  xas_nomem() may allocate a spare
xa_node and store it in xas->xa_alloc for use in the retry.

If the lock is dropped after xas_nomem(), another thread can expand the
xarray tree in the meantime.  On the next retry, xas_create_range() can
then succeed without consuming the spare node stored in xas->xa_alloc.  If
the function returns without freeing this spare node, it leaks.

xas_create_range() calls xas_create() multiple times in a loop for
different index ranges.  A spare node that isn't needed for one range
iteration might be needed for the next, so we cannot free it after each
xas_create() call.  We can only safely free it after xas_create_range()
completes.

Fix this by calling xas_destroy() at the end of xas_create_range() to free
any unused spare node.  This makes the API safer by default and prevents
callers from needing to remember cleanup.

This fixes a memory leak in mm/khugepaged.c and potentially other callers
that use xas_nomem() with xas_create_range().

Link: https://lore.kernel.org/20251204142625.1763372-1-shardul.b@mpiricsoftware.com
Link: https://syzkaller.appspot.com/bug?id=a274d65fc733448ed518ad15481ed575669dd98c
Link: https://lore.kernel.org/all/20251201074540.3576327-1-shardul.b@mpiricsoftware.com/ ("v3")
Fixes: cae106dd67b9 ("mm/khugepaged: refactor collapse_file control flow")
Signed-off-by: Shardul Bankar <shardul.b@mpiricsoftware.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: David Hildenbrand (ARM) <david@kernel.org>
Tested-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: <janak@mpiricsoftware.com>
Cc: Jinjiang Tu <tujinjiang@huawei.com>

 lib/xarray.c |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

--- a/lib/xarray.c~lib-xarray-free-unused-spare-node-in-xas_create_range
+++ a/lib/xarray.c
@@ -745,11 +745,16 @@ restore:
 	xas->xa_shift = shift;
 	xas->xa_sibs = sibs;
 	xas->xa_index = index;
-	return;
+	goto cleanup;
+
 success:
 	xas->xa_index = index;
 	if (xas->xa_node)
 		xas_set_offset(xas);
+
+cleanup:
+	/* Free any unused spare node from xas_nomem() */
+	xas_destroy(xas);
 }
 EXPORT_SYMBOL_GPL(xas_create_range);
 
_