linux-kernel - Re: [PATCH] x86: Fix /proc/mtrr with base/size more than 44bits

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Date:	Thu, 13 Jun 2013 15:11:20 -0700
From:	Yinghai Lu <yinghai@...nel.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH] x86: Fix /proc/mtrr with base/size more than 44bits

On Thu, Jun 13, 2013 at 1:47 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 06/13/2013 11:53 AM, Yinghai Lu wrote:
>>
>> -     if (base & size_or_mask || size & size_or_mask) {
>> +     if (base >> (boot_cpu_data.x86_phys_bits - PAGE_SHIFT) ||
>> +         base > (base + size) ||
>> +         (base + size - 1) >> (boot_cpu_data.x86_phys_bits - PAGE_SHIFT)) {
>>               pr_warning("mtrr: base or size exceeds the MTRR width\n");
>>               return -EINVAL;
>>       }
>
> Most of this patch looks good as far as being a minimal patch, but I'm
> really confused about this bit.  Could you explain the reason for why
> the original doesn't work?  (To be fair: I am not even sure the original
> does anything useful so it could just be a "this is just too broken to
> live" kind of thing.)

all because I update size_of_mask for old cpus that does not have
cpuid 80000008.
by make high 32bits to be all 1s. otherwise size = -mask trick will not work.

then check those range size_or_mask using, found that is not right.

as base and size could be all small, but base + size -1 could be big enough.
then the original will not detect that is out of boundary.

also we could even use x86_phys_bits directly.

>
> The first and third clause of the test can be simplified, however:
>
>         (base | (base + size - 1)) >> (boot_cpu_data.x86_phys_bits - PAGE_SHIFT)
>
> ... although it would be cleaner to put boot_cpu_data.x86_phys_bits -
> PAGE_SHIFT into a variable.

Yes.

also we can drop base > (base + size) checking, as
base and size are already shifted with PAGE_SHIFT to pfn.
so base+size can not be wrapped.

>
> A lot of the mask_hi/mask_lo stuff should just get removed by using
> rdmsrl/wrmsrl, but that is not stable material obviously.

yes.

will send updated version shortly.

Thanks

Yinghai
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