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

[Eric Biggers <ebiggers@kernel.org>]

On Tue, Jul 04, 2023 at 02:56:49PM +0200, 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
> +	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
> +	overridden with bdev_allow_write_mounted boot option.

Does this prevent the underlying storage from being written to?  Say if the
mounted block device is /dev/sda1 and someone tries to write to /dev/sda in the
region that contains sda1.

I *think* the answer is no, writes to /dev/sda are still allowed since the goal
is just to prevent writes to the buffer cache of mounted block devices, not
writes to the underlying storage.  That is really something that should be
stated explicitly, though.

- Eric