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

Maciej Wieczor-Retman
2025-02-10T15:25:22Z

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@sifive.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