[PATCH 5.7 186/265] bcache: check and adjust logical block size for backing devices

Sasha Levin
2020-06-29T21:49:10Z

From: Mauricio Faria de Oliveira <mfo@canonical.com>

[ Upstream commit dcacbc1242c71e18fa9d2eadc5647e115c9c627d ]

It's possible for a block driver to set logical block size to
a value greater than page size incorrectly; e.g. bcache takes
the value from the superblock, set by the user w/ make-bcache.

This causes a BUG/NULL pointer dereference in the path:

__blkdev_get()
-> set_init_blocksize() // set i_blkbits based on ...
-> bdev_logical_block_size()
-> queue_logical_block_size() // ... this value
-> bdev_disk_changed()
...
-> blkdev_readpage()
-> block_read_full_page()
-> create_page_buffers() // size = 1 << i_blkbits
-> create_empty_buffers() // give size/take pointer
-> alloc_page_buffers() // return NULL
.. BUG!

Because alloc_page_buffers() is called with size > PAGE_SIZE,
thus it initializes head = NULL, skips the loop, return head;
then create_empty_buffers() gets (and uses) the NULL pointer.

This has been around longer than commit ad6bf88a6c19 ("block:
fix an integer overflow in logical block size"); however, it
increased the range of values that can trigger the issue.

Previously only 8k/16k/32k (on x86/4k page size) would do it,
as greater values overflow unsigned short to zero, and queue_
logical_block_size() would then use the default of 512.

Now the range with unsigned int is much larger, and users w/
the 512k value, which happened to be zero'ed previously and
work fine, started to hit this issue -- as the zero is gone,
and queue_logical_block_size() does return 512k (>PAGE_SIZE.)

Fix this by checking the bcache device's logical block size,
and if it's greater than page size, fallback to the backing/
cached device's logical page size.

This doesn't affect cache devices as those are still checked
for block/page size in read_super(); only the backing/cached
devices are not.

Apparently it's a regression from commit 2903381fce71 ("bcache:
Take data offset from the bdev superblock."), moving the check
into BCACHE_SB_VERSION_CDEV only. Now that we have superblocks
of backing devices out there with this larger value, we cannot
refuse to load them (i.e., have a similar check in _BDEV.)

Ideally perhaps bcache should use all values from the backing
device (physical/logical/io_min block size)? But for now just
fix the problematic case.

Test-case:

# IMG=/root/disk.img
# dd if=/dev/zero of=$IMG bs=1 count=0 seek=1G
# DEV=$(losetup --find --show $IMG)
# make-bcache --bdev $DEV --block 8k
< see dmesg >

Before:

# uname -r
5.7.0-rc7

[ 55.944046] BUG: kernel NULL pointer dereference, address: 0000000000000000
...
[ 55.949742] CPU: 3 PID: 610 Comm: bcache-register Not tainted 5.7.0-rc7 #4
...
[ 55.952281] RIP: 0010:create_empty_buffers+0x1a/0x100
...
[ 55.966434] Call Trace:
[ 55.967021] create_page_buffers+0x48/0x50
[ 55.967834] block_read_full_page+0x49/0x380
[ 55.972181] do_read_cache_page+0x494/0x610
[ 55.974780] read_part_sector+0x2d/0xaa
[ 55.975558] read_lba+0x10e/0x1e0
[ 55.977904] efi_partition+0x120/0x5a6
[ 55.980227] blk_add_partitions+0x161/0x390
[ 55.982177] bdev_disk_changed+0x61/0xd0
[ 55.982961] __blkdev_get+0x350/0x490
[ 55.983715] __device_add_disk+0x318/0x480
[ 55.984539] bch_cached_dev_run+0xc5/0x270
[ 55.986010] register_bcache.cold+0x122/0x179
[ 55.987628] kernfs_fop_write+0xbc/0x1a0
[ 55.988416] vfs_write+0xb1/0x1a0
[ 55.989134] ksys_write+0x5a/0xd0
[ 55.989825] do_syscall_64+0x43/0x140
[ 55.990563] entry_SYSCALL_64_after_hwframe+0x44/0xa9
[ 55.991519] RIP: 0033:0x7f7d60ba3154
...

After:

# uname -r
5.7.0.bcachelbspgsz

[ 31.672460] bcache: bcache_device_init() bcache0: sb/logical block size (8192) greater than page size (4096) falling back to device logical block size (512)
[ 31.675133] bcache: register_bdev() registered backing device loop0

# grep ^ /sys/block/bcache0/queue/*_block_size
/sys/block/bcache0/queue/logical_block_size:512
/sys/block/bcache0/queue/physical_block_size:8192

Reported-by: Ryan Finnie <ryan@finnie.org>
Reported-by: Sebastian Marsching <sebastian@marsching.com>
Signed-off-by: Mauricio Faria de Oliveira <mfo@canonical.com>
Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/md/bcache/super.c | 22 +++++++++++++++++++---
1 file changed, 19 insertions(+), 3 deletions(-)

diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
index 4d8bf731b118c..a2e5a0fcd7d5c 100644
--- a/drivers/md/bcache/super.c
+++ b/drivers/md/bcache/super.c
@@ -819,7 +819,8 @@ static void bcache_device_free(struct bcache_device *d)
}

static int bcache_device_init(struct bcache_device *d, unsigned int block_size,
- sector_t sectors, make_request_fn make_request_fn)
+ sector_t sectors, make_request_fn make_request_fn,
+ struct block_device *cached_bdev)
{
struct request_queue *q;
const size_t max_stripes = min_t(size_t, INT_MAX,
@@ -885,6 +886,21 @@ static int bcache_device_init(struct bcache_device *d, unsigned int block_size,
q->limits.io_min = block_size;
q->limits.logical_block_size = block_size;
q->limits.physical_block_size = block_size;
+
+ if (q->limits.logical_block_size > PAGE_SIZE && cached_bdev) {
+ /*
+ * This should only happen with BCACHE_SB_VERSION_BDEV.
+ * Block/page size is checked for BCACHE_SB_VERSION_CDEV.
+ */
+ pr_info("%s: sb/logical block size (%u) greater than page size "
+ "(%lu) falling back to device logical block size (%u)",
+ d->disk->disk_name, q->limits.logical_block_size,
+ PAGE_SIZE, bdev_logical_block_size(cached_bdev));
+
+ /* This also adjusts physical block size/min io size if needed */
+ blk_queue_logical_block_size(q, bdev_logical_block_size(cached_bdev));
+ }
+
blk_queue_flag_set(QUEUE_FLAG_NONROT, d->disk->queue);
blk_queue_flag_clear(QUEUE_FLAG_ADD_RANDOM, d->disk->queue);
blk_queue_flag_set(QUEUE_FLAG_DISCARD, d->disk->queue);
@@ -1342,7 +1358,7 @@ static int cached_dev_init(struct cached_dev *dc, unsigned int block_size)

ret = bcache_device_init(&dc->disk, block_size,
dc->bdev->bd_part->nr_sects - dc->sb.data_offset,
- cached_dev_make_request);
+ cached_dev_make_request, dc->bdev);
if (ret)
return ret;

@@ -1455,7 +1471,7 @@ static int flash_dev_run(struct cache_set *c, struct uuid_entry *u)
kobject_init(&d->kobj, &bch_flash_dev_ktype);

if (bcache_device_init(d, block_bytes(c), u->sectors,
- flash_dev_make_request))
+ flash_dev_make_request, NULL))
goto err;

bcache_device_attach(d, c, u - c->uuids);
--
2.25.1