On 2024/12/13 20:25, Kemeng Shi wrote:
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
Seems not a good example, IIUC, tmpfs doesn't support writeback
(mapping_can_writeback() will return false), right?
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)
