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

[Kees Cook <keescook@chromium.org>]

On Fri, Nov 20, 2015 at 12:00 PM, Arnd Bergmann <arnd@arndb.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@chromium.org>
>> Cc: Russell King <linux@arm.linux.org.uk>
>> Cc: Catalin Marinas <catalin.marinas@arm.com>
>> Cc: Will Deacon <will.deacon@arm.com>
>> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
>> Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
>> Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
>> Cc: Thomas Gleixner <tglx@linutronix.de>
>> Cc: Ingo Molnar <mingo@redhat.com>
>> Cc: "H. Peter Anvin" <hpa@zytor.com>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
>> Signed-off-by: Dan Williams <dan.j.williams@intel.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