Xarray: do not return sibling entries from xas_find_marked()

Patch series "Fixes and cleanups to xarray", v5.

This series contains some random fixes and cleanups to xarray. Patch 1-2
are fixes and patch 3-6 are cleanups. More details can be found in
respective patches.


This patch (of 5):

Similar to issue fixed in commit cbc02854331ed ("XArray: Do not return
sibling entries from xa_load()"), we may return sibling entries from
xas_find_marked as following:
Thread A: Thread B:
xa_store_range(xa, entry, 6, 7, gfp);
xa_set_mark(xa, 6, mark)
XA_STATE(xas, xa, 6);
xas_find_marked(&xas, 7, mark);
offset = xas_find_chunk(xas, advance, mark);
[offset is 6 which points to a valid entry]
xa_store_range(xa, entry, 4, 7, gfp);
entry = xa_entry(xa, node, 6);
[entry is a sibling of 4]
if (!xa_is_node(entry))
return entry;

Skip sibling entry like xas_find() does to protect caller from seeing
sibling entry from xas_find_marked() or caller may use sibling entry
as a valid entry and crash the kernel.

Besides, load_race() test is modified to catch mentioned issue and modified
load_race() only passes after this fix is merged.

Here is an example how this bug could be triggerred in tmpfs which
enables large folio in mapping:
Let's take a look at involved racer:
1. How pages could be created and dirtied in shmem file.
write
ksys_write
vfs_write
new_sync_write
shmem_file_write_iter
generic_perform_write
shmem_write_begin
shmem_get_folio
shmem_allowable_huge_orders
shmem_alloc_and_add_folios
shmem_alloc_folio
__folio_set_locked
shmem_add_to_page_cache
XA_STATE_ORDER(..., index, order)
xax_store()
shmem_write_end
folio_mark_dirty()

2. How dirty pages could be deleted in shmem file.
ioctl
do_vfs_ioctl
file_ioctl
ioctl_preallocate
vfs_fallocate
shmem_fallocate
shmem_truncate_range
shmem_undo_range
truncate_inode_folio
filemap_remove_folio
page_cache_delete
xas_store(&xas, NULL);

3. How dirty pages could be lockless searched
sync_file_range
ksys_sync_file_range
__filemap_fdatawrite_range
filemap_fdatawrite_wbc
do_writepages
writeback_use_writepage
writeback_iter
writeback_get_folio
filemap_get_folios_tag
find_get_entry
folio = xas_find_marked()
folio_try_get(folio)

Kernel will crash as following:
1.Create 2.Search 3.Delete
/* write page 2,3 */
write
...
shmem_write_begin
XA_STATE_ORDER(xas, i_pages, index = 2, order = 1)
xa_store(&xas, folio)
shmem_write_end
folio_mark_dirty()

/* sync page 2 and page 3 */
sync_file_range
...
find_get_entry
folio = xas_find_marked()
/* offset will be 2 */
offset = xas_find_chunk()

/* delete page 2 and page 3 */
ioctl
...
xas_store(&xas, NULL);

/* write page 0-3 */
write
...
shmem_write_begin
XA_STATE_ORDER(xas, i_pages, index = 0, order = 2)
xa_store(&xas, folio)
shmem_write_end
folio_mark_dirty(folio)

/* get sibling entry from offset 2 */
entry = xa_entry(.., 2)
/* use sibling entry as folio and crash kernel */
folio_try_get(folio)

Link: https://lkml.kernel.org/r/[e-mail]
Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Kemeng Shi <[e-mail]>
Cc: Mattew Wilcox <[e-mail]> [English fixes]
Signed-off-by: Andrew Morton <[e-mail]>

lib/xarray.c

···
 		entry = xa_entry(xas->xa, xas->xa_node, xas->xa_offset);
 		if (!entry && !(xa_track_free(xas->xa) && mark == XA_FREE_MARK))
 			continue;
+		if (xa_is_sibling(entry))
+			continue;
 		if (!xa_is_node(entry))
 			return entry;
 		xas->xa_node = xa_to_node(entry);

tools/testing/radix-tree/multiorder.c

···
 			unsigned long index = (3 << RADIX_TREE_MAP_SHIFT) -
 						(1 << order);
 			item_insert_order(tree, index, order);
+			xa_set_mark(tree, index, XA_MARK_1);
 			item_delete_rcu(tree, index);
 		}
 	}
···
 
 	rcu_register_thread();
 	while (!stop_iteration) {
+		unsigned long find_index = (2 << RADIX_TREE_MAP_SHIFT) + 1;
 		struct item *item = xa_load(ptr, index);
+		assert(!xa_is_internal(item));
+		item = xa_find(ptr, &find_index, index, XA_MARK_1);
 		assert(!xa_is_internal(item));
 	}
 	rcu_unregister_thread();