Commit 649932fc authored Apr 28, 2026 by Gao Xiang

erofs: fix managed cache race for unaligned extents



After unaligned compressed extents were introduced, the following race
could occur:

[Thread 1]                                   [Thread 2]
(z_erofs_fill_bio_vec)
<handle a Z_EROFS_PREALLOCATED_FOLIO folio>
...
filemap_add_folio (1)
                                             (z_erofs_bind_cache)
                                             <the same folio is found..>
                                             ..
                                             ..
folio_attach_private (2)
                                             filemap_add_folio (3) again

Since (1) is executed but (2) hasn't been executed yet, it's possible
that another thread finds the same managed folio in z_erofs_bind_cache()
for a different pcluster and calls filemap_add_folio() again since
folio->private is still Z_EROFS_PREALLOCATED_FOLIO.

Fix this by explicitly clearing folio->private before making the folio
visible in the managed cache so that another pcluster can simply wait
on the locked managed folio as what we did for other shared cases [1].

This only impacts unaligned data compression (`-E48bit` with zstd,
for example).

[1] Commit 9e2f9d34 ("erofs: handle overlapped pclusters out of
 crafted images properly") was originally introduced to handle crafted
 overlapped extents, but it addresses unaligned extents as well.

Fixes: 7361d1e3 ("erofs: support unaligned encoded data")
Reported-by: Arseniy Krasnov <avkrasnov@salutedevices.com>
Closes: https://lore.kernel.org/r/4a2f3801-fac1-42fe-ae75-da315822e088@salutedevices.com


Tested-by: Arseniy Krasnov <avkrasnov@salutedevices.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

parent 254f4963

fs/erofs/zdata.c

+8 −7

Original line number	Diff line number	Diff line
		@@ -1509,8 +1509,15 @@ static void z_erofs_fill_bio_vec(struct bio_vec *bvec,
		DBG_BUGON(z_erofs_is_shortlived_page(bvec->bv_page));

		folio = page_folio(zbv.page);
		/* For preallocated managed folios, add them to page cache here */
		/*
		* Preallocated folios are added to the managed cache here rather than
		* in z_erofs_bind_cache() in order to keep these folios locked in
		* increasing (physical) address order.
		* Clear folio->private before these folios become visible to others in
		* the managed cache to avoid duplicate additions for unaligned extents.
		*/
		if (folio->private == Z_EROFS_PREALLOCATED_FOLIO) {
		folio->private = NULL;
		tocache = true;
		goto out_tocache;
		}
		@@ -1546,14 +1553,8 @@ static void z_erofs_fill_bio_vec(struct bio_vec *bvec,
		}
		return;
		}
		/*
		* Already linked with another pcluster, which only appears in
		* crafted images by fuzzers for now. But handle this anyway.
		*/
		tocache = false; /* use temporary short-lived pages */
		} else {
		DBG_BUGON(1); /* referenced managed folios can't be truncated */
		tocache = true;
		}
		folio_unlock(folio);
		folio_put(folio);