linux-kernel - Re: [PATCH v2 01/14] kasan: sw_tags: Use arithmetic shift for shadow computation

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Message-ID: <uup72ceniis544hgfaojy5omctzf7gs4qlydyv2szkr5hqia32@t6fgaxcaw2oi>
Date: Tue, 25 Feb 2025 18:20:08 +0100
From: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
To: Andrey Konovalov <andreyknvl@...il.com>
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Subject: Re: [PATCH v2 01/14] kasan: sw_tags: Use arithmetic shift for shadow
 computation

On 2025-02-22 at 16:06:02 +0100, Andrey Konovalov wrote:
>On Fri, Feb 21, 2025 at 2:12 PM Maciej Wieczor-Retman
><maciej.wieczor-retman@...el.com> wrote:
>>
>> >Is there any reason we need this change for x86 SW_TAGS besides the
>> >optimization benefits?
>>
>> I wanted to have the shadow memory boundries aligned properly, to not waste page
>> table entries, so the memory map is more straight forward. This patch helps with
>> that, I don't think it would have worked without it.
>
>Ok, I see - let's add this info into the commit message then.

Sure, but if you like the 0xffeffc0000000000 offset I'll just drop this part.

>
>> >However, I just realized that this check is not entirely precise. When
>> >doing the memory-to-shadow mapping, the memory address always has its
>> >top byte set to 0xff: both the inlined compiler code and the outline
>> >KASAN code do this
>>
>> Do you mean that non-canonical addresses passed to kasan_mem_to_shadow() will
>> map to the same space that the canonical version would map to?
>
>No, but non-canonical address are never passed to
>kasan_mem_to_shadow(): KASAN always resets the tag before calling this
>function.
>
>> What does that? Does the compiler do something more than is in
>> kasan_mem_to_shadow() when instrumenting functions?
>
>Same for the compiler, it always untags the pointer first [1].
>
>[1] https://github.com/llvm/llvm-project/blob/llvmorg-20-init/llvm/lib/Transforms/Instrumentation/HWAddressSanitizer.cpp#L922
>
>> >                   Thus, the possible values a shadow address can
>> >take are the result of the memory-to-shadow mapping applied to
>> >[0xff00000000000000, 0xffffffffffffffff], not to the whole address
>> >space. So we can make this check more precise.
>>
>> In case my question above didn't lead to this: what happens to the rest of the
>> values if they get plugged into kasan_mem_to_shadow()?
>
>We will get some invalid addresses. But this should never happen in
>the first place.

Thanks for letting me know about the tag resets, that should make changing the
check in kasan_non_canonical_hook() easier.

-- 
Kind regards
Maciej Wieczór-Retman