[PATCH] erofs: complete fscache pseudo-bio once when a read is split

[Michael Bommarito]

In fscache mode a compressed read uses one pseudo-bio whose io->end_io is
erofs_fscache_bio_endio(). When prepare_ondemand_read() splits the read at
a cached/uncached boundary, erofs_fscache_read_io_async() issues several
fscache subreads on the same bio and erofs_fscache_bio_endio() calls
bio_endio() on each. The pseudo-bio is not chained, so z_erofs_endio()
runs once per subread while z_erofs_submit_queue() counted the bio only
once, underflowing pending_bios: the reader hangs in D state, or, on async
completion, the first completion frees the decompress queue and the rest
are use-after-free.

Hold a bio_inc_remaining() reference per issued subread and drop the
submitter's initial reference with one bio_endio() once submission
finishes, so the bio completes exactly once. The request path
(erofs_fscache_req_end_io) is unaffected; it uses its own refcount and
never calls bio_endio().

Fixes: a1bafc3109d7 ("erofs: support compressed inodes over fscache")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Michael Bommarito <michael.bommarito@xxxxxxxxx>
Assisted-by: Claude:claude-opus-4-7
---

Reproduced on x86-64 with KASAN via the erofs-on-demand path (a cachefiles
ondemand daemon serving a crafted compressed image that splits a pcluster
read). Found with the help of an automated review tool.

Without this patch a stock kernel either hangs the reader:

  task:dd  state:D
  filemap_get_pages / erofs_file_read_iter

or, when completion is asynchronous, faults:

  BUG: KASAN: slab-use-after-free in z_erofs_endio
  Kernel panic - not syncing: Fatal exception in interrupt

With this patch the same daemon, image and reads complete cleanly: no
hang, no KASAN report, no panic. Harness and full logs available on
request.
 fs/erofs/fscache.c | 27 ++++++++++++++++++---------
 1 file changed, 18 insertions(+), 9 deletions(-)

diff --git a/fs/erofs/fscache.c b/fs/erofs/fscache.c
index 685c68774379b..0cce498faa32d 100644
--- a/fs/erofs/fscache.c
+++ b/fs/erofs/fscache.c
@@ -29,6 +29,14 @@ struct erofs_fscache_rq {
 	refcount_t		ref;
 };
 
+struct erofs_fscache_bio {
+	struct erofs_fscache_io io;
+	struct bio bio;		/* w/o bdev to share bio_add_page/endio() */
+	struct bio_vec bvecs[BIO_MAX_VECS];
+};
+
+static void erofs_fscache_bio_endio(void *priv, ssize_t transferred_or_error);
+
 static bool erofs_fscache_io_put(struct erofs_fscache_io *io)
 {
 	if (!refcount_dec_and_test(&io->ref))
@@ -122,8 +130,13 @@ static int erofs_fscache_read_io_async(struct fscache_cookie *cookie,
 	enum netfs_io_source source;
 	struct netfs_cache_resources *cres = &io->cres;
 	struct iov_iter *iter = &io->iter;
+	struct bio *bio = NULL;
 	int ret;
 
+	/* Chain-account a split pseudo-bio so it completes only once. */
+	if (io->end_io == erofs_fscache_bio_endio)
+		bio = &container_of(io, struct erofs_fscache_bio, io)->bio;
+
 	ret = fscache_begin_read_operation(cres, cookie);
 	if (ret)
 		return ret;
@@ -143,6 +156,8 @@ static int erofs_fscache_read_io_async(struct fscache_cookie *cookie,
 
 		iov_iter_truncate(iter, len);
 		refcount_inc(&io->ref);
+		if (bio)
+			bio_inc_remaining(bio);
 		ret = fscache_read(cres, pstart, iter, NETFS_READ_HOLE_FAIL,
 				   io->end_io, io);
 		if (ret == -EIOCBQUEUED)
@@ -160,12 +175,6 @@ static int erofs_fscache_read_io_async(struct fscache_cookie *cookie,
 	return 0;
 }
 
-struct erofs_fscache_bio {
-	struct erofs_fscache_io io;
-	struct bio bio;		/* w/o bdev to share bio_add_page/endio() */
-	struct bio_vec bvecs[BIO_MAX_VECS];
-};
-
 static void erofs_fscache_bio_endio(void *priv, ssize_t transferred_or_error)
 {
 	struct erofs_fscache_bio *io = priv;
@@ -200,9 +209,9 @@ void erofs_fscache_submit_bio(struct bio *bio)
 	ret = erofs_fscache_read_io_async(io->io.private,
 				bio->bi_iter.bi_sector << 9, &io->io);
 	erofs_fscache_io_put(&io->io);
-	if (!ret)
-		return;
-	bio->bi_status = errno_to_blk_status(ret);
+	if (ret)
+		bio->bi_status = errno_to_blk_status(ret);
+	/* Drop the submitter's ref; subread chain refs complete the bio. */
 	bio_endio(bio);
 }
 
-- 
2.53.0