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From:   Andrey Konovalov <andreyknvl@google.com>
Date:   Wed, 19 Sep 2018 13:54:23 +0200
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Subject: Re: [PATCH v6 14/18] khwasan: add hooks implementation
To:     Dmitry Vyukov <dvyukov@google.com>
Cc:     Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Alexander Potapenko <glider@google.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
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        Andrew Morton <akpm@linux-foundation.org>,
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        Nick Desaulniers <ndesaulniers@google.com>,
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On Wed, Sep 12, 2018 at 8:30 PM, Dmitry Vyukov <dvyukov@google.com> wrote:
> On Wed, Aug 29, 2018 at 1:35 PM, Andrey Konovalov <andreyknvl@google.com> wrote:

>>  void kasan_unpoison_shadow(const void *address, size_t size)
>>  {
>> -       kasan_poison_shadow(address, size, 0);
>> +       u8 tag = get_tag(address);
>> +
>> +       /* Perform shadow offset calculation based on untagged address */
>
> The comment is not super-useful. It would be more useful to say why we
> need to do this.
> Most callers explicitly untag pointer passed to this function, for
> some it's unclear if the pointer contains tag or not.
> For example, __hwasan_tag_memory -- what does it accept? Tagged or untagged?

There are some callers that pass tagged pointers to this functions,
e.g. ksize or kasan_unpoison_object_data. I'll expand the comment.

>
>
>> +       address = reset_tag(address);
>> +
>> +       kasan_poison_shadow(address, size, tag);
>>
>>         if (size & KASAN_SHADOW_MASK) {
>>                 u8 *shadow = (u8 *)kasan_mem_to_shadow(address + size);
>> -               *shadow = size & KASAN_SHADOW_MASK;
>> +
>> +               if (IS_ENABLED(CONFIG_KASAN_HW))
>> +                       *shadow = tag;
>> +               else
>> +                       *shadow = size & KASAN_SHADOW_MASK;
>>         }
>>  }
>
>
> It seems that this function is just different for kasan and khwasan.
> Currently for kasan we have:
>
> kasan_poison_shadow(address, size, tag);
> if (size & KASAN_SHADOW_MASK) {
>         u8 *shadow = (u8 *)kasan_mem_to_shadow(address + size);
>         *shadow = size & KASAN_SHADOW_MASK;
> }
>
> But what we want to say for khwasan is:
>
> kasan_poison_shadow(address, round_up(size, KASAN_SHADOW_SCALE_SIZE),
> get_tag(address));
>
> Not sure if we want to keep a common implementation or just have
> separate implementations...

As per offline discussion leaving as is.


>>  void kasan_free_pages(struct page *page, unsigned int order)
>> @@ -235,6 +248,7 @@ void kasan_cache_create(struct kmem_cache *cache, unsigned int *size,
>>                         slab_flags_t *flags)
>>  {
>>         unsigned int orig_size = *size;
>> +       unsigned int redzone_size = 0;
>
> This variable seems to be always initialized below. We don't general
> initialize local variables in this case.

Will fix in v7.

>>  void check_memory_region(unsigned long addr, size_t size, bool write,
>>                                 unsigned long ret_ip)
>>  {
>> +       u8 tag;
>> +       u8 *shadow_first, *shadow_last, *shadow;
>> +       void *untagged_addr;
>> +
>> +       tag = get_tag((const void *)addr);
>> +
>> +       /* Ignore accesses for pointers tagged with 0xff (native kernel
>
> /* on a separate line

Will fix in v7.

>
>> +        * pointer tag) to suppress false positives caused by kmap.
>> +        *
>> +        * Some kernel code was written to account for archs that don't keep
>> +        * high memory mapped all the time, but rather map and unmap particular
>> +        * pages when needed. Instead of storing a pointer to the kernel memory,
>> +        * this code saves the address of the page structure and offset within
>> +        * that page for later use. Those pages are then mapped and unmapped
>> +        * with kmap/kunmap when necessary and virt_to_page is used to get the
>> +        * virtual address of the page. For arm64 (that keeps the high memory
>> +        * mapped all the time), kmap is turned into a page_address call.
>> +
>> +        * The issue is that with use of the page_address + virt_to_page
>> +        * sequence the top byte value of the original pointer gets lost (gets
>> +        * set to KHWASAN_TAG_KERNEL (0xFF).
>
> Missed closing bracket.

Will fix in v7.