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

["Darrick J. Wong" <djwong@kernel.org>]

On Mon, Aug 19, 2024 at 07:56:27PM -0300, 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.
> 
> Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
> ---
>  Documentation/admin-guide/kernel-parameters.txt | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 09126bb8cc9f..709d1ee342db 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -517,6 +517,16 @@
>  			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
> +			devices' page cache, i.e., allow / disallow writes that
> +			bypasses the FS. This was implemented as a means to
> +			prevent fuzzers to crash the kernel by breaking the
> +			filesystem without its awareness, through direct block
> +			device writes. Default is Y and can be changed through
> +			the Kconfig option CONFIG_BLK_DEV_WRITE_MOUNTED.

Can we mention that this also solves the problem of naïve storage
management tools (aka the ones that don't use O_EXCL) writing over a
mounted filesystem and trashing it?

--D

> +
>  	bert_disable	[ACPI]
>  			Disable BERT OS support on buggy BIOSes.
>  
> -- 
> 2.45.2
> 
>