null_blk: fix UBSAN shift-out-of-bounds when zone_size is 0 or overflows

null_zone_no() does sect >> ilog2(dev->zone_size_sects). When
zone_size_sects is 0, ilog2(0) returns -1, producing shift exponent -1
which UBSAN reports as shift-out-of-bounds.

  UBSAN: shift-out-of-bounds in drivers/block/null_blk/zoned.c:21:14
  shift exponent -1 is negative
  Call Trace:
   null_zone_no drivers/block/null_blk/zoned.c:21 [inline]
   null_process_zoned_cmd+0xf76/0xf80 drivers/block/null_blk/zoned.c:728
   null_handle_cmd drivers/block/null_blk/main.c:1455 [inline]
   null_queue_rq+0x8bc/0xe70 drivers/block/null_blk/main.c:1703
   __blk_mq_issue_directly block/blk-mq.c:2694 [inline]
   blk_mq_try_issue_directly+0x3f4/0x880 block/blk-mq.c:2754
   blk_mq_submit_bio+0x20c0/0x2a40 block/blk-mq.c:3208
   submit_bio_noacct_nocheck+0x2f4/0xa40 block/blk-core.c:790
   block_read_full_folio+0x7a6/0x810 fs/buffer.c:2463
   filemap_read_folio+0x12c/0x3a0 mm/filemap.c:2510
   read_part_sector+0xb6/0x2b0 block/partitions/core.c:724
   adfspart_check_ICS+0xb1/0x960 block/partitions/acorn.c:357
   check_partition block/partitions/core.c:143 [inline]
   blk_add_partitions block/partitions/core.c:591 [inline]
   bdev_disk_changed+0x851/0x17a0 block/partitions/core.c:695
   blkdev_get_whole+0x372/0x510 block/bdev.c:751
   add_disk_final block/genhd.c:412 [inline]
   add_disk_fwnode+0x24b/0x3a0 block/genhd.c:606
   null_add_dev+0x130b/0x1d70 drivers/block/null_blk/main.c:2052
   nullb_device_power_store+0x240/0x380 drivers/block/null_blk/main.c:501
   configfs_write_iter+0x337/0x430 fs/configfs/file.c:229

Syzkaller triggers this by creating a zoned null_blk device via
configfs. The Call Trace shows configfs_write_iter in configfs/file.c
handling a write to power file, which calls nullb_device_power_store in
main.c, which calls null_add_dev in main.c, which calls add_disk in
genhd.c, which triggers partition scan via bdev_disk_changed in
partitions/core.c.

A zoned null_blk device with zone_size 0 should not be legal. Existing
code tries to reject it via is_power_of_2() check in zoned.c and
!zone_size check in main.c, but syzkaller can still reach
null_zone_no() with zone_size_sects 0 via two paths:

1. Direct 0 via configfs: zone_size attribute store in main.c has
NULLB_DEVICE_ATTR(zone_size, ulong, NULL) with no validation callback,
so echo 0 > zone_size succeeds before power store. If zoned is false
at power store time, the !zone_size check in main.c is skipped, and
later zoned set true leaves zone_size 0.

2. Large value overflow: mb_to_sects() in zoned.c does
(sector_t)mb * SZ_1M >> SECTOR_SHIFT which is mb * 2048. If mb is
1UL << 53 (9PB), mb * 2048 overflows 64-bit to 0. The value is
power-of-two so is_power_of_2() passes, but mb_to_sects() returns 0.

Check for zero zone_size explicitly in null_init_zoned_dev() in
zoned.c, returning -EINVAL with "must be non-zero power-of-two".
Check for zero zone_size_sects after mb_to_sects() conversion,
returning -EINVAL for overflow case. Keep defensive check in
null_zone_no() returning 0 for zero sectors to avoid shift out-of-bounds
even if  zero slips through.

This change should be safe because zone_size is set once in
null_init_zoned_dev() under device lock and never changes after, and 0
is never valid for a zoned device. Returning -EINVAL at init time fails
device creation early with clear error, while defensive return 0 in
null_zone_no() makes zoned command fail via offline zone check.
No new locking is introduced.

Reported-by: syzbot+abd6a8dca0f2b7726060@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=abd6a8dca0f2b7726060
Link: https://lore.kernel.org/all/6a75205c.01d0871a.3a0d52.0033.GAE@google.com/
Fixes: 8a3cf049af68 ("null_blk: add zoned block device emulation")
Cc: stable@vger.kernel.org
Assisted-by: Hermes:muse-spark-1.2 syzkaller
Signed-off-by: Rik van Riel <riel@surriel.com>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Link: https://patch.msgid.link/20260808114239.69167f68@fangorn
Signed-off-by: Jens Axboe <axboe@kernel.dk>

0 files changed, 0 insertions, 0 deletions