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SubjectRe: [PATCH v5 4/4] sparc64: Add support for ADI (Application Data Integrity)
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On 01/25/2017 03:50 PM, Rob Gardner wrote:
> On 01/25/2017 03:20 PM, Khalid Aziz wrote:
>> On 01/25/2017 03:00 PM, Rob Gardner wrote:
>>> On 01/25/2017 12:57 PM, Khalid Aziz wrote:
>>>>
>>>> @@ -157,6 +158,24 @@ int __get_user_pages_fast(unsigned long start,
>>>> int nr_pages, int write,
>>>> pgd_t *pgdp;
>>>> int nr = 0;
>>>> +#ifdef CONFIG_SPARC64
>>>> + if (adi_capable()) {
>>>> + long addr = start;
>>>> +
>>>> + /* If userspace has passed a versioned address, kernel
>>>> + * will not find it in the VMAs since it does not store
>>>> + * the version tags in the list of VMAs. Storing version
>>>> + * tags in list of VMAs is impractical since they can be
>>>> + * changed any time from userspace without dropping into
>>>> + * kernel. Any address search in VMAs will be done with
>>>> + * non-versioned addresses. Ensure the ADI version bits
>>>> + * are dropped here by sign extending the last bit before
>>>> + * ADI bits. IOMMU does not implement version tags.
>>>> + */
>>>> + addr = (addr << (long)adi_nbits()) >> (long)adi_nbits();
>>>
>>>
>>> So you are depending on the sign extension to clear the ADI bits... but
>>> this only happens if there is a zero in that "last bit before ADI bits".
>>> If the last bit is a 1, then the ADI bits will be set instead of
>>> cleared. That seems like an unintended consequence given the comment. I
>>> am aware of the value of adi_nbits() and of the number of valid bits in
>>> a virtual address on the M7 processor, but wouldn't using 'unsigned
>>> long' for everything here guarantee the ADI bits get cleared regardless
>>> of the state of the last non-adi bit?
>>
>> Sign extension is the right thing to do. MMU considers values of 0 and
>> 15 for bits 63-60 to be untagged addresses and expects bit 59 to be
>> sign-extended for untagged virtual addresses. The code I added is
>> explicitly meant to sign-extend, not zero out the top 4 bits.
>
> OK, that wasn't perfectly clear from the comment, which said "version
> bits are dropped".
>
> So sign extending will produce an address that the MMU can use, but will
> it produce an address that will allow a successful search in the page
> tables? ie, was this same sign extending done when first handing out
> that virtual address to the user?
>

Yes to both your questions. When virtual addresses are handed out, the
last implemented virtual address bit is sign-extended. Sign-extending
when dropping version bits preserves that original sign-extension. This
is why MMU considers tag values of 0 as well as 15 to be invalid because
they both represent sign-extension of the last implemented virtual address.

--
Khalid

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