Re: [PATCH v8 01/14] kasan: sw_tags: Use arithmetic shift for shadow computation

[Andrey Ryabinin <ryabinin.a.a@gmail.com>]

On 1/12/26 6:27 PM, Maciej Wieczor-Retman wrote:
  
> diff --git a/mm/kasan/report.c b/mm/kasan/report.c
> index 62c01b4527eb..b5beb1b10bd2 100644
> --- a/mm/kasan/report.c
> +++ b/mm/kasan/report.c
> @@ -642,11 +642,39 @@ void kasan_non_canonical_hook(unsigned long addr)
>  	const char *bug_type;
>  
>  	/*
> -	 * All addresses that came as a result of the memory-to-shadow mapping
> -	 * (even for bogus pointers) must be >= KASAN_SHADOW_OFFSET.
> +	 * For Generic KASAN, kasan_mem_to_shadow() uses the logical right shift
> +	 * and never overflows with the chosen KASAN_SHADOW_OFFSET values (on
> +	 * both x86 and arm64). Thus, the possible shadow addresses (even for
> +	 * bogus pointers) belong to a single contiguous region that is the
> +	 * result of kasan_mem_to_shadow() applied to the whole address space.
>  	 */
> -	if (addr < KASAN_SHADOW_OFFSET)
> -		return;
> +	if (IS_ENABLED(CONFIG_KASAN_GENERIC)) {
> +		if (addr < (unsigned long)kasan_mem_to_shadow((void *)(0ULL)) ||
> +		    addr > (unsigned long)kasan_mem_to_shadow((void *)(~0ULL)))
> +			return;
> +	}
> +
> +	/*
> +	 * For Software Tag-Based KASAN, kasan_mem_to_shadow() uses the
> +	 * arithmetic shift. Normally, this would make checking for a possible
> +	 * shadow address complicated, as the shadow address computation
> +	 * operation would overflow only for some memory addresses. However, due
> +	 * to the chosen KASAN_SHADOW_OFFSET values and the fact the
> +	 * kasan_mem_to_shadow() only operates on pointers with the tag reset,
> +	 * the overflow always happens.
> +	 *
> +	 * For arm64, the top byte of the pointer gets reset to 0xFF. Thus, the
> +	 * possible shadow addresses belong to a region that is the result of
> +	 * kasan_mem_to_shadow() applied to the memory range
> +	 * [0xFF000000000000, 0xFFFFFFFFFFFFFFFF]. Despite the overflow, the
                  ^ Missing couple 00 here

> +	 * resulting possible shadow region is contiguous, as the overflow
> +	 * happens for both 0xFF000000000000 and 0xFFFFFFFFFFFFFFFF.
                                  ^ same as above

> +	 */
> +	if (IS_ENABLED(CONFIG_KASAN_SW_TAGS) && IS_ENABLED(CONFIG_ARM64)) {
> +		if (addr < (unsigned long)kasan_mem_to_shadow((void *)(0xFFULL << 56)) ||

This will not work for inline mode because compiler uses logical shift.
Consider NULL-ptr derefernce. Compiler will calculate shadow address for 0 as:
      (((0x0 | 0xffULL) << 56) >> 4)+0xffff800000000000ULL = 0x0fef8000....0
Which is less than ((0xFF00...00LL) >> 4) +  0xffff800000000000ULL = 0xffff800...0
So we will bail out here.
Perhaps we could do addr |= 0xFFLL to fix this

> +		    addr > (unsigned long)kasan_mem_to_shadow((void *)(~0ULL)))
> +			return;
> +	}
>  
>  	orig_addr = (unsigned long)kasan_shadow_to_mem((void *)addr);
>