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

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Message-ID: <lrlnvcxofcnsm5rou3iwbawyfwtz6mx4gn6eflpm4srhjj37kb@pwsozjgdyxfu>
Date: Tue, 11 Feb 2025 09:58:22 +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 2025-02-10 at 23:57:10 +0100, Andrey Konovalov wrote:
>On Mon, Feb 10, 2025 at 4:53 PM Maciej Wieczor-Retman
><maciej.wieczor-retman@...el.com> wrote:
>>
>> On 2025-02-10 at 16:22:41 +0100, Maciej Wieczor-Retman wrote:
>> >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;
>> >
>> >?
>>
>> Sorry, I think this wouldn't work. The tag also needs to be reset. Does this
>> perhaps work for this problem?
>>
>>         if (!addr_in_shadow(kasan_reset_tag((void *)addr)))
>>                 return;
>
>This wouldn't work as well.
>
>addr_in_shadow() checks whether an address belongs to the proper
>shadow memory area. That area is the result of the memory-to-shadow
>mapping applied to the range of proper kernel addresses.
>
>However, what we want to check in this function is whether the given
>address can be the result of the memory-to-shadow mapping for some
>memory address, including userspace addresses, non-canonical
>addresses, etc. So essentially we need to check whether the given
>address belongs to the area that is the result of the memory-to-shadow
>mapping applied to the whole address space, not only to proper kernel
>addresses.k

Ah, okay, I get it. Would the old version

       if (addr < KASAN_SHADOW_OFFSET)
               return;

work if the *addr* had kasan_reset_tag() around it? That would sort of re-unsign
the address only for the purpose of the if().

Also I was thinking about it because x86 even with address masking enabled keeps
bit 63 set, so all kernel addresses will be negative in the signed
kasan_mem_to_shadow(). That's great for simplifying the KASAN_SHADOW_OFFSET but
it differs from the TBI and risc-v ideas where half of addresses are negative,
hald positive. So the temporary re-unsigning could maybe make it simpler for x86
and avoid adding separate cases or alternative kasan_non_canonical_hook()
implementation.

-- 
Kind regards
Maciej Wieczór-Retman