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

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Message-ID: <CA+fCnZeEm+-RzqEXp1FqYJ5Gsm+mUZh5k3nq=92ZuTiqwsaWvA@mail.gmail.com>
Date: Tue, 25 Feb 2025 22:38:06 +0100
From: Andrey Konovalov <andreyknvl@...il.com>
To: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>, Vitaly Buka <vitalybuka@...gle.com>
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Subject: Re: [PATCH v2 01/14] kasan: sw_tags: Use arithmetic shift for shadow computation

On Tue, Feb 25, 2025 at 9:13 PM Maciej Wieczor-Retman
<maciej.wieczor-retman@...el.com> wrote:
>
> >>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?
>
> Sorry, brain freeze, I meant [0x1e00000000000000, 0xffffffffffffffff]

+Vitaly, who implemented [1]

Ah, so when the compiler calculates the shadow memory address on x86,
it does | 0x7E (== 0x3F << 1) [2] for when CompileKernel == true,
because LAM uses bits [62:57], I see.

What value can bit 63 and take for _valid kernel_ pointers (on which
KASAN is intended to operate)? If it is always 1, we could arguably
change the compiler to do | 0xFE for CompileKernel. Which would leave
us with only one region to check: [0xfe00000000000000,
0xffffffffffffffff]. But I don't know whether changing the compiler
makes sense: it technically does as instructed by the LAM spec.
(Vitaly, any thoughts? For context: we are discussing how to check
whether a pointer can be a result of a memory-to-shadow mapping
applied to a potentially invalid pointer in kernel HWASAN.)

With the way the compiler works right now, for the perfectly precise
check, I think we need to check 2 ranges: [0xfe00000000000000,
0xffffffffffffffff] for when bit 63 is set (of a potentially-invalid
pointer to which memory-to-shadow mapping is to be applied) and
[0x7e00000000000000, 0x7fffffffffffffff] for when bit 63 is reset. Bit
56 ranges through [0, 1] in both cases.

However, in these patches, you use only bits [60:57]. The compiler is
not aware of this, so it still sets bits [62:57], and we end up with
the same two ranges. But in the KASAN code, you only set bits [60:57],
and thus we can end up with 8 potential ranges (2 possible values for
each of the top 3 bits), which gets complicated. So checking only one
range that covers all of them seems to be reasonable for simplicity
even though not entirely precise. And yes, [0x1e00000000000000,
0xffffffffffffffff] looks like the what we need.

[1] https://github.com/llvm/llvm-project/commit/cb6099ba43b9262a317083858a29fd31af7efa5c
[2] https://github.com/llvm/llvm-project/blob/llvmorg-20-init/llvm/lib/Transforms/Instrumentation/HWAddressSanitizer.cpp#L1259