linux-kernel - Re: [RFC PATCH] restrict /dev/mem to idle io memory ranges

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Date:	Fri, 20 Nov 2015 12:07:08 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Dan Williams <dan.j.williams@...el.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Russell King <linux@....linux.org.uk>,
	linux-nvdimm@...ts.01.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Will Deacon <will.deacon@....com>,
	Ingo Molnar <mingo@...hat.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH] restrict /dev/mem to idle io memory ranges

On Fri, Nov 20, 2015 at 12:00 PM, Arnd Bergmann <arnd@...db.de> wrote:
> On Friday 20 November 2015 09:31:33 Dan Williams wrote:
>> This effectively promotes IORESOURCE_BUSY to IORESOURCE_EXCLUSIVE
>> semantics by default.  If userspace really believes it is safe to access
>> the memory region it can also perform the extra step of disabling an
>> active driver.  This protects device address ranges with read side
>> effects and otherwise directs userspace to use the driver.
>>
>> Persistent memory presents a large "mistake surface" to /dev/mem as now
>> accidental writes can corrupt a filesystem.
>>
>> Cc: Kees Cook <keescook@...omium.org>
>> Cc: Russell King <linux@....linux.org.uk>
>> Cc: Catalin Marinas <catalin.marinas@....com>
>> Cc: Will Deacon <will.deacon@....com>
>> Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
>> Cc: Martin Schwidefsky <schwidefsky@...ibm.com>
>> Cc: Heiko Carstens <heiko.carstens@...ibm.com>
>> Cc: Thomas Gleixner <tglx@...utronix.de>
>> Cc: Ingo Molnar <mingo@...hat.com>
>> Cc: "H. Peter Anvin" <hpa@...or.com>
>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
>> Signed-off-by: Dan Williams <dan.j.williams@...el.com>
>>
>
> I like the idea.

Yes please! I was always surprised that IORESOURCE_BUSY was allowed
under STRICT_DEVMEM.

> Maybe split the change up into two patches, where the first one
> just does the trivial move of the Kconfig option, and the second
> one that changes behavior is small?

Agreed: consolidate the per-arch Kconfigs first.

> There is also a question of whether we actually need two options
> or if we can safely make the existing option stricter.

Right -- what actually breaks if we add _BUSY to getting blocked?

-Kees

-- 
Kees Cook
Chrome OS Security
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