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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <2592f303-05f5-4646-b59f-38cb7549834e@gmail.com>
Date: Thu, 15 Jan 2026 23:42:02 +0100
From: Andrey Ryabinin <ryabinin.a.a@...il.com>
To: Maciej Wieczor-Retman <m.wieczorretman@...me>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Jonathan Corbet <corbet@....net>, Alexander Potapenko <glider@...gle.com>,
 Andrey Konovalov <andreyknvl@...il.com>, Dmitry Vyukov <dvyukov@...gle.com>,
 Vincenzo Frascino <vincenzo.frascino@....com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Jan Kiszka <jan.kiszka@...mens.com>, Kieran Bingham <kbingham@...nel.org>,
 Nathan Chancellor <nathan@...nel.org>,
 Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
 Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>
Cc: Samuel Holland <samuel.holland@...ive.com>,
 Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>,
 linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, kasan-dev@...glegroups.com,
 linux-mm@...ck.org, llvm@...ts.linux.dev
Subject: Re: [PATCH v8 01/14] kasan: sw_tags: Use arithmetic shift for shadow
 computation



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);
>