loop: block changing lo_offset/lo_sizelimit on mounted device#705
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LOOP_SET_STATUS{64} allows changing lo_offset and shrinking
lo_sizelimit while a filesystem is mounted on the loop device.
This effectively mutates the data visible to the mounted filesystem,
which is equivalent to writing directly to the block device.
When CONFIG_BLK_DEV_WRITE_MOUNTED is disabled, direct writes to a
mounted block device are blocked. However, LOOP_SET_STATUS{64}
bypasses this protection because it modifies the loop configuration
through an ioctl rather than opening the block device for writing.
Fix this by checking bdev_writes_blocked() before allowing changes
to lo_offset or shrinking lo_sizelimit. If the loop device has
writes blocked, return -EBUSY. Increasing lo_sizelimit is still
allowed since growing the device is harmless and has legitimate
use cases such as online resize.
Move bdev_writes_blocked() from block/bdev.c to
include/linux/blk_types.h as a static inline function so it can
be used from the loop driver without exporting a symbol.
Suggested-by: Theodore Ts'o <tytso@mit.edu>
Reported-by: syzbot+fb32afec111a7d61b939@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=fb32afec111a7d61b939
Tested-by: syzbot+fb32afec111a7d61b939@syzkaller.appspotmail.com
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
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Pull request for series with
subject: loop: block changing lo_offset/lo_sizelimit on mounted device
version: 2
url: https://patchwork.kernel.org/project/linux-block/list/?series=1074954