[PATCH 6.1.y v2] block: make bio_check_eod work for zero sized devices

["Miguel García" <miguelgarciaroman8@gmail.com>]

From: Christoph Hellwig <hch@lst.de>

commit 3eb96946f0be6bf447cbdf219aba22bc42672f92 upstream.

Since the dawn of time bio_check_eod has a check for a non-zero size of
the device.  This doesn't really make any sense as we never want to send
I/O to a device that's been set to zero size, or never moved out of that.

I am a bit surprised we haven't caught this for a long time, but the
removal of the extra validation inside of zram caused syzbot to trip
over this issue recently.  I've added a Fixes tag for that commit, but
the issue really goes back way before git history.

Fixes: 9fe95babc742 ("zram: remove valid_io_request")
Reported-by: syzbot+b8d61a58b7c7ebd2c8e0@syzkaller.appspotmail.com
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20230524060538.1593686-1-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Backport note for 6.1.y:

The same underlying bug affects 6.1.y even though 9fe95babc742 is not in
that tree.  The syzbot reproducer attaches a socket to /dev/nbd0 without
setting its capacity, then mounts it as BEFS.  BEFS calls sb_bread() while
bdev_nr_sectors() is zero.  The maxsector guard makes bio_check_eod() accept
that read, which reaches NBD and waits for a response instead of failing
with -EIO.  syzbot reproduced the resulting __bread_gfp() hang on 6.1.y and
identified this upstream commit by fix bisection.

Reported-by: syzbot+2aca91e1d3ae43aef10c@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=2aca91e1d3ae43aef10c
Signed-off-by: Miguel García <miguelgarciaroman8@gmail.com>
---
Changes in v2:
- Explain the 6.1.y NBD and BEFS failure path, as requested by Greg:
  https://lore.kernel.org/r/2025041215-grudge-rearrange-7123@gregkh
- Preserve both syzbot reports and the upstream commit metadata.
- Refresh against v6.1.186.

Older stable trees are not included here because this backport and its local
validation are scoped to the syzbot report for 6.1.y.

v1: https://lore.kernel.org/r/20250412102424.56383-1-miguelgarciaroman8@gmail.com

 block/blk-core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index 25b4733..1fcd80a 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -515,7 +515,7 @@ static inline int bio_check_eod(struct bio *bio)
 	sector_t maxsector = bdev_nr_sectors(bio->bi_bdev);
 	unsigned int nr_sectors = bio_sectors(bio);
 
-	if (nr_sectors && maxsector &&
+	if (nr_sectors &&
 	    (nr_sectors > maxsector ||
 	     bio->bi_iter.bi_sector > maxsector - nr_sectors)) {
 		pr_info_ratelimited("%s: attempt to access beyond end of device\n"
-- 
2.43.0