[PATCH v2] erofs: fix folio reuse from a different address_space in erofs_bread()

[Binglei Wang]

erofs_bread() reuses a cached folio on page index match without
checking folio->mapping.

xattr.c calls erofs_init_metabuf() twice on the same buffer without
erofs_put_metabuf(); the two in_metabox sources differ, so buf->mapping
can switch while buf->page still belongs to the old address_space.
The next erofs_bread() then reads the wrong mapping, dropping shared
xattrs with METABOX.

We require folio->mapping == buf->mapping in the reuse check; otherwise
drop the cached folio and re-read via the slow path.

Fixes: 414091322c63 ("erofs: implement metadata compression")
Cc: Bo Liu (OpenAnolis) <liubo03@xxxxxxxxxx>
Signed-off-by: Binglei Wang <l3b2w1@xxxxxxxxx>
---
Changes since v1:
 - Drop the redundant erofs_put_metabuf() call; setting folio to NULL is
   enough since the slow path already releases the cached folio (Gao Xiang).
 - Rewrap the commit message to 72 columns.

 fs/erofs/data.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/erofs/data.c b/fs/erofs/data.c
index be63b89f0862..4b9fe670fdda 100644
--- a/fs/erofs/data.c
+++ b/fs/erofs/data.c
@@ -33,8 +33,12 @@ void *erofs_bread(struct erofs_buf *buf, erofs_off_t offset, bool need_kmap)
 
 	if (buf->page) {
 		folio = page_folio(buf->page);
-		if (folio_file_page(folio, index) != buf->page)
+		if (folio->mapping != buf->mapping) {
+			/* the cached folio belongs to another address_space */
+			folio = NULL;
+		} else if (folio_file_page(folio, index) != buf->page) {
 			erofs_unmap_metabuf(buf);
+		}
 	}
 	if (!folio || !folio_contains(folio, index)) {
 		erofs_put_metabuf(buf);