linux-kernel - Re: [PATCH v2] drivers/kmem: disable on arm64

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Date:   Mon, 19 Jun 2017 16:03:23 +0100
From:   Will Deacon <will.deacon@....com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        arnd@...db.de, mark.rutland@....com, gregkh@...uxfoundation.org
Subject: Re: [PATCH v2] drivers/kmem: disable on arm64

On Mon, Jun 19, 2017 at 04:37:24PM +0200, Ard Biesheuvel wrote:
> On arm64, the /dev/kmem driver barely works, given that it assumes that
> VMALLOC_START > PAGE_OFFSET, which is not the case on arm64. Due to the

Probably worth being explicit about the failure mode here (the commit
message you had in your RFC was much better ;).

Will

> dubious value of kmem on an architecture such as arm64, which is fairly
> new and is not as forgiving as other architectures when it comes to
> userland accesses to kernel memory using mismatched attributes*, let's
> disable it altogether if CONFIG_ARM64=y.
> 
> * Mappings in the vmalloc region will be accessed using vread/vwrite when
>   reading/writing to/from /dev/kmem, which will perform these accesses by
>   kmap()'ing each page using writeback cacheable attributes. This will be
>   correct in most cases, but this is not guaranteed.
> 
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> ---
>  drivers/char/Kconfig | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/char/Kconfig b/drivers/char/Kconfig
> index 31adbebf812e..8102ee7b3247 100644
> --- a/drivers/char/Kconfig
> +++ b/drivers/char/Kconfig
> @@ -17,6 +17,8 @@ config DEVMEM
>  
>  config DEVKMEM
>  	bool "/dev/kmem virtual device support"
> +	# On arm64, VMALLOC_START < PAGE_OFFSET, which confuses kmem read/write
> +	depends on !ARM64
>  	help
>  	  Say Y here if you want to support the /dev/kmem device. The
>  	  /dev/kmem device is rarely used, but can be used for certain
> -- 
> 2.7.4
>