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

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Message-ID: <CA+fCnZd6O0_fc1U-D_i2shcF4Td-6389F3Q=fDkdYYXQupX1NA@mail.gmail.com>
Date: Sat, 22 Feb 2025 16:06:02 +0100
From: Andrey Konovalov <andreyknvl@...il.com>
To: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
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Subject: Re: [PATCH v2 01/14] kasan: sw_tags: Use arithmetic shift for shadow computation

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.

> >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.