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

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Message-ID: <qnxlqbc4cs7izjilisbjlrup4zyntjyucvfa4s6eegn72wfbkd@czthvwkdvo3v>
Date: Wed, 26 Feb 2025 17:42:18 +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 22:38:06 +0100, Andrey Konovalov wrote:
>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.

Thanks for the links, now I see what you meant.

>
>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 LAM, valid pointers need to have bits 63 and 56 equal for 5 level paging
and bits 63 and 47 equal for 4 level paging. Both set for kernel addresses and
both clear for user addresses.

>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.

Aren't the 2 ranges you mentioned in the previous paragraph still valid, no
matter what bits the __tag_set() function uses? I mean bits 62:57 are still
reset by the compiler so bits 62:61 still won't matter. For example addresses
0x1e00000000000000 and 0x3e00000000000000 will resolve to the same thing after
the compiler is done with them right?

>
>[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

-- 
Kind regards
Maciej Wieczór-Retman