Re: [PATCH V3] Documentation: Document the kernel flag bdev_allow_write_mounted

[Jens Axboe <axboe@kernel.dk>]

On 8/23/24 12:05 PM, Guilherme G. Piccoli wrote:
> Commit ed5cc702d311 ("block: Add config option to not allow writing to mounted
> devices") added a Kconfig option along with a kernel command-line tuning to
> control writes to mounted block devices, as a means to deal with fuzzers like
> Syzkaller, that provokes kernel crashes by directly writing on block devices
> bypassing the filesystem (so the FS has no awareness and cannot cope with that).
> 
> The patch just missed adding such kernel command-line option to the kernel
> documentation, so let's fix that.
> 
> Cc: Bart Van Assche <bvanassche@acm.org>
> Cc: Darrick J. Wong <djwong@kernel.org>
> Cc: Jan Kara <jack@suse.cz>
> Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
> ---
> 
> V3: Dropped reference to page cache (thanks Bart!).
> 
> V2 link: https://lore.kernel.org/r/20240823142840.63234-1-gpiccoli@igalia.com
> 
> 
>  Documentation/admin-guide/kernel-parameters.txt | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 09126bb8cc9f..58b9455baf4a 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -517,6 +517,18 @@
>  			Format: <io>,<irq>,<mode>
>  			See header of drivers/net/hamradio/baycom_ser_hdx.c.
>  
> +	bdev_allow_write_mounted=
> +			Format: <bool>
> +			Control the ability of directly writing to mounted block

Since we're nit picking...

Control the ability to directly write [...]

The directly may be a bit confusing ("does it mean O_DIRECT?"), so maybe
just

Control the ability to write [...]

would be better and more clear.

> +			devices, i.e., allow / disallow writes that bypasses the

Since we're nit picking, s/bypasses/bypass

> +			FS. This was implemented as a means to prevent fuzzers
> +			from crashing the kernel by overwriting the metadata
> +			underneath a mounted FS without its awareness. This
> +			also prevents destructive formatting of mounted
> +			filesystems by naive storage tooling that don't use
> +			O_EXCL. Default is Y and can be changed through the
> +			Kconfig option CONFIG_BLK_DEV_WRITE_MOUNTED.
> +
>  	bert_disable	[ACPI]
>  			Disable BERT OS support on buggy BIOSes.

-- 
Jens Axboe