linux-kernel - Re: [PATCH v2 1/9] kasan: sw_tags: Use arithmetic shift for shadow computation

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Message-ID: <e7t5yzfw3dq5stp5xjy5yclcx6ikne4vwz7d6w2ukfw2b7gr6t@oomoynf3b2jl>
Date: Mon, 10 Feb 2025 16:22:41 +0100
From: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
To: Andrey Konovalov <andreyknvl@...il.com>
CC: Samuel Holland <samuel.holland@...ive.com>, Palmer Dabbelt
	<palmer@...belt.com>, <linux-riscv@...ts.infradead.org>, Andrey Ryabinin
	<ryabinin.a.a@...il.com>, Alexander Potapenko <glider@...gle.com>, "Dmitry
 Vyukov" <dvyukov@...gle.com>, Vincenzo Frascino <vincenzo.frascino@....com>,
	<kasan-dev@...glegroups.com>, <llvm@...ts.linux.dev>, Catalin Marinas
	<catalin.marinas@....com>, <linux-kernel@...r.kernel.org>,
	<linux-mm@...ck.org>, Alexandre Ghiti <alexghiti@...osinc.com>, Will Deacon
	<will@...nel.org>, Evgenii Stepanov <eugenis@...gle.com>, Andrew Morton
	<akpm@...ux-foundation.org>, <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2 1/9] kasan: sw_tags: Use arithmetic shift for shadow
 computation

On 2024-10-23 at 20:41:57 +0200, Andrey Konovalov wrote:
>On Tue, Oct 22, 2024 at 3:59 AM Samuel Holland
><samuel.holland@...ive.com> wrote:
...
>> +        * Software Tag-Based KASAN, the displacement is signed, so
>> +        * KASAN_SHADOW_OFFSET is the center of the range.
>>          */
>> -       if (addr < KASAN_SHADOW_OFFSET)
>> -               return;
>> +       if (IS_ENABLED(CONFIG_KASAN_GENERIC)) {
>> +               if (addr < KASAN_SHADOW_OFFSET ||
>> +                   addr >= KASAN_SHADOW_OFFSET + max_shadow_size)
>> +                       return;
>> +       } else {
>> +               if (addr < KASAN_SHADOW_OFFSET - max_shadow_size / 2 ||
>> +                   addr >= KASAN_SHADOW_OFFSET + max_shadow_size / 2)
>> +                       return;
>
>Hm, I might be wrong, but I think this check does not work.
>
>Let's say we have non-canonical address 0x4242424242424242 and number
>of VA bits is 48.
>
>Then:
>
>KASAN_SHADOW_OFFSET == 0xffff800000000000
>kasan_mem_to_shadow(0x4242424242424242) == 0x0423a42424242424
>max_shadow_size == 0x1000000000000000
>KASAN_SHADOW_OFFSET - max_shadow_size / 2 == 0xf7ff800000000000
>KASAN_SHADOW_OFFSET + max_shadow_size / 2 == 0x07ff800000000000 (overflows)
>
>0x0423a42424242424 is < than 0xf7ff800000000000, so the function will
>wrongly return.

As I understand this check aims to figure out if the address landed in shadow
space and if it didn't we can return.

Can't this above snippet be a simple:

	if (!addr_in_shadow(addr))
		return;

?

-- 
Kind regards
Maciej Wieczór-Retman