Re: [PATCH 1/6] block: Add config option to not allow writing to mounted devices

Colin Walters
2023-07-04T15:57:14Z

On Tue, Jul 4, 2023, at 8:56 AM, Jan Kara wrote:
> Writing to mounted devices is dangerous and can lead to filesystem
> corruption as well as crashes. Furthermore syzbot comes with more and
> more involved examples how to corrupt block device under a mounted
> filesystem leading to kernel crashes and reports we can do nothing
> about. Add tracking of writers to each block device and a kernel cmdline
> argument which controls whether writes to block devices open with
> BLK_OPEN_BLOCK_WRITES flag are allowed. We will make filesystems use
> this flag for used devices.
>
> Syzbot can use this cmdline argument option to avoid uninteresting
> crashes. Also users whose userspace setup does not need writing to
> mounted block devices can set this option for hardening.
>
> Link:
> https://lore.kernel.org/all/60788e5d-5c7c-1142-e554-c21d709acfd9@linaro.org
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
> block/Kconfig | 16 ++++++++++
> block/bdev.c | 63 ++++++++++++++++++++++++++++++++++++++-
> include/linux/blk_types.h | 1 +
> include/linux/blkdev.h | 3 ++
> 4 files changed, 82 insertions(+), 1 deletion(-)
>
> diff --git a/block/Kconfig b/block/Kconfig
> index 86122e459fe0..8b4fa105b854 100644
> --- a/block/Kconfig
> +++ b/block/Kconfig
> @@ -77,6 +77,22 @@ config BLK_DEV_INTEGRITY_T10
> select CRC_T10DIF
> select CRC64_ROCKSOFT
>
> +config BLK_DEV_WRITE_MOUNTED
> + bool "Allow writing to mounted block devices"
> + default y
> + help
> + When a block device is mounted, writing to its buffer cache very likely

s/very/is very/

> + going to cause filesystem corruption. It is also rather easy to crash
> + the kernel in this way since the filesystem has no practical way of
> + detecting these writes to buffer cache and verifying its metadata
> + integrity. However there are some setups that need this capability
> + like running fsck on read-only mounted root device, modifying some
> + features on mounted ext4 filesystem, and similar. If you say N, the
> + kernel will prevent processes from writing to block devices that are
> + mounted by filesystems which provides some more protection from runaway
> + priviledged processes. If in doubt, say Y. The configuration can be

s/priviledged/privileged/

> + overridden with bdev_allow_write_mounted boot option.

s/with/with the/

> +/* open is exclusive wrt all other BLK_OPEN_WRITE opens to the device */
> +#define BLK_OPEN_BLOCK_WRITES ((__force blk_mode_t)(1 << 5))

Bikeshed but: I think BLK and BLOCK "stutter" here. The doc comment already uses the term "exclusive" so how about BLK_OPEN_EXCLUSIVE ?