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

From: Kees Cook
Date: Fri Nov 20 2015 - 15:07:19 EST

On Fri, Nov 20, 2015 at 12:00 PM, Arnd Bergmann <arnd@xxxxxxxx> 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@xxxxxxxxxxxx>
>> Cc: Russell King <linux@xxxxxxxxxxxxxxxx>
>> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
>> Cc: Will Deacon <will.deacon@xxxxxxx>
>> Cc: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
>> Cc: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
>> Cc: Heiko Carstens <heiko.carstens@xxxxxxxxxx>
>> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
>> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
>> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
>> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> I like the idea.

Yes please! I was always surprised that IORESOURCE_BUSY was allowed

> 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 Cook
Chrome OS Security
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