linux-kernel - Re: [PATCH v5 4/4] sparc64: Add support for ADI (Application Data Integrity)

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Date:   Wed, 25 Jan 2017 15:20:33 -0700
From:   Khalid Aziz <khalid.aziz@...cle.com>
To:     Rob Gardner <rob.gardner@...cle.com>, davem@...emloft.net,
        corbet@....net
Cc:     viro@...iv.linux.org.uk, nitin.m.gupta@...cle.com,
        mike.kravetz@...cle.com, akpm@...ux-foundation.org,
        mingo@...nel.org, kirill.shutemov@...ux.intel.com,
        adam.buchbinder@...il.com, hughd@...gle.com, minchan@...nel.org,
        keescook@...omium.org, chris.hyser@...cle.com,
        atish.patra@...cle.com, cmetcalf@...lanox.com, atomlin@...hat.com,
        jslaby@...e.cz, joe@...ches.com, paul.gortmaker@...driver.com,
        mhocko@...e.com, lstoakes@...il.com, jack@...e.cz,
        dave.hansen@...ux.intel.com, vbabka@...e.cz,
        dan.j.williams@...el.com, iamjoonsoo.kim@....com,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        sparclinux@...r.kernel.org, linux-mm@...ck.org,
        Khalid Aziz <khalid@...ehiking.org>
Subject: Re: [PATCH v5 4/4] sparc64: Add support for ADI (Application Data
 Integrity)

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.

--
Khalid