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 ad6bf88a6c ("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 2903381fce ("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>
dcacbc1242