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

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Message-ID: <gisttijkccu6pynsdhvv3lpyxx7bxpvqbni43ybsa5axujr7qj@7feqy5fy2kgt>
Date: Tue, 25 Feb 2025 20:12:40 +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 2025-02-25 at 18:20:08 +0100, Maciej Wieczor-Retman wrote:
>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.

Ah, but the [0xff00000000000000, 0xffffffffffffffff] won't be true for x86
right? Here the tag reset function only resets bits 60:57. So I presume
[0x3e00000000000000, 0xffffffffffffffff] would be the range?

-- 
Kind regards
Maciej Wieczór-Retman