btrfs: prevent use-after-free on folio private data in btrfs_subpage_clear_uptodate()

This is a stable-only patch. The issue was inadvertently fixed in 6.17 [0] as part of a refactoring, but this patch serves as a minimal targeted fix for prior kernels.

Users of filemap_lock_folio() need to guard against the situation where release_folio() has been invoked during reclaim but the folio was ultimately not removed from the page cache. This patch covers one location that was overlooked.

After acquiring the folio, use set_folio_extent_mapped() to ensure the folio private state is valid. This is especially important in the subpage case, where the private field is an allocated struct containing bitmap and lock data.

Without this protection, the race below is possible:

[mm] page cache reclaim path [fs] relocation in subpage mode
shrink_folio_list()
folio_trylock() /* lock acquired */ filemap_release_folio()
mapping->a_ops->release_folio()
btrfs_release_folio()
__btrfs_release_folio()
clear_folio_extent_mapped()
btrfs_detach_subpage()
subpage = folio_detach_private(folio) btrfs_free_subpage(subpage)
kfree(subpage) /* point A */

prealloc_file_extent_cluster()
filemap_lock_folio()
folio_try_get() /* inc refcount */ folio_lock() /* wait for lock */

if (...)

    ...

else if (!mapping || !__remove_mapping(..))
/*
* __remove_mapping() returns zero when
* folio_ref_freeze(folio, refcount) fails /* point B */ */
goto keep_locked /* folio remains in cache */

keep_locked:
folio_unlock(folio) /* lock released */

                                   /* lock acquired */
                                   btrfs_subpage_clear_uptodate()
                                     /* use-after-free */
                                     subpage = folio_get_private(folio)

[0] 4e346baee95f ("btrfs: reloc: unconditionally invalidate the page cache for each cluster")
---
v2:
- comment text formatting
- renamed subject from "prevent use-after-free prealloc_file_extent_cluster()"

v1:
- https://lore.kernel.org/all/[email protected]/

 fs/btrfs/relocation.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

index 0d5a3846811a..43e8c331168e 100644
@@ -2811,6 +2811,20 @@ static noinline_for_stack int prealloc_file_extent_cluster(struct reloc_control
 		 * will re-read the whole page anyway.
 		 */
 		if (!IS_ERR(folio)) {
+			/*
+			 * release_folio() could have cleared the folio private data
+			 * while we were not holding the lock. Reset the mapping if
+			 * needed so subpage operations can access a valid private
+			 * folio state.
+			 */
+			ret = set_folio_extent_mapped(folio);
+			if (ret) {
+				folio_unlock(folio);
+				folio_put(folio);
+
+				return ret;
+			}
+
 			btrfs_subpage_clear_uptodate(fs_info, folio, i_size,
 					round_up(i_size, PAGE_SIZE) - i_size);
 			folio_unlock(folio);

2.52.0