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

[Eric Biggers <ebiggers@kernel.org>]

Hi Jan,

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>

Can you make it clear that the important thing this patch prevents is writes to
the block device's buffer cache, not writes to the underlying storage?  It's
super important not to confuse the two cases.

Related to this topic, I wonder if there is any value in providing an option
that would allow O_DIRECT writes but forbid buffered writes?  Would that be
useful for any of the known use cases for writing to mounted block devices?

- Eric