linux-kernel - Re: [PATCH v2 14/14] x86: Make software tag-based kasan available

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Message-ID: <afc4db6mt3uuimj4lokfeglhqc22u5ckgvunqtiwecjan5vjj2@lvphketnxhhr>
Date: Fri, 21 Feb 2025 15:44:56 +0100
From: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
To: Andrey Konovalov <andreyknvl@...il.com>
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Subject: Re: [PATCH v2 14/14] x86: Make software tag-based kasan available

On 2025-02-20 at 00:31:26 +0100, Andrey Konovalov wrote:
>On Tue, Feb 18, 2025 at 9:20 AM Maciej Wieczor-Retman
><maciej.wieczor-retman@...el.com> wrote:
>>
>> Make CONFIG_KASAN_SW_TAGS available for x86 machines if they have
>> ADDRESS_MASKING enabled (LAM) as that works similarly to Top-Byte Ignore
>> (TBI) that allows the software tag-based mode on arm64 platform.
>>
>> Set scale macro based on KASAN mode: in software tag-based mode 32 bytes
>> of memory map to one shadow byte and 16 in generic mode.
>
>These should be 16 and 8.

Thanks, I thought I got all the dense mode stuff from this series.

>
>>
>> Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
>> ---
>> Changelog v2:
>> - Remove KASAN dense code.
>>
>>  arch/x86/Kconfig                | 6 ++++++
>>  arch/x86/boot/compressed/misc.h | 1 +
>>  arch/x86/include/asm/kasan.h    | 2 +-
>>  arch/x86/kernel/setup.c         | 2 ++
>>  4 files changed, 10 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
>> index f4ef64bf824a..dc48eb5b664f 100644
>> --- a/arch/x86/Kconfig
>> +++ b/arch/x86/Kconfig
>> @@ -195,6 +195,7 @@ config X86
>>         select HAVE_ARCH_JUMP_LABEL_RELATIVE
>>         select HAVE_ARCH_KASAN                  if X86_64
>>         select HAVE_ARCH_KASAN_VMALLOC          if X86_64
>> +       select HAVE_ARCH_KASAN_SW_TAGS          if ADDRESS_MASKING
>>         select HAVE_ARCH_KFENCE
>>         select HAVE_ARCH_KMSAN                  if X86_64
>>         select HAVE_ARCH_KGDB
>> @@ -402,6 +403,11 @@ config KASAN_SHADOW_OFFSET
>>         hex
>>         default 0xdffffc0000000000 if KASAN_GENERIC
>>
>> +config KASAN_SHADOW_SCALE_SHIFT
>> +       int
>> +       default 4 if KASAN_SW_TAGS
>> +       default 3
>
>What's the purpose of this config option? I think we can just change
>the value of the KASAN_SHADOW_SCALE_SHIFT define when KASAN_SW_TAGS is
>enabled.

Well, I was aiming at later adding the "default 5 if KASAN_SW_TAGS_DENSE", and
this way it would look much cleaner than the:

if KASAN_SW_TAGS
	if KASAN_SW_TAGS_DENSE
		KASAN_SHADOW_SCALE_SHIFT = 5
	else
		KASAN_SHADOW_SCALE_SHIFT = 4
else
	KASAN_SHADOW_SCALE_SHIFT = 3

But now that I think of it, it should be possible to overwrite the
KASAN_SHADOW_SCALE_SHIFT from non-arch code if dense mode is enabled.

That's a topic for the next series but I'd imagine all architectures would
normally use the 16 memory bytes / shadow byte and if they'd care for the dense
mode they'd go for 32 memory bytes / shadow byte. Or do you think that's a
faulty assumption?

>
>> +
>>  config HAVE_INTEL_TXT
>>         def_bool y
>>         depends on INTEL_IOMMU && ACPI
>> diff --git a/arch/x86/boot/compressed/misc.h b/arch/x86/boot/compressed/misc.h
>> index dd8d1a85f671..f6a87e9ad200 100644
>> --- a/arch/x86/boot/compressed/misc.h
>> +++ b/arch/x86/boot/compressed/misc.h
>> @@ -13,6 +13,7 @@
>>  #undef CONFIG_PARAVIRT_SPINLOCKS
>>  #undef CONFIG_KASAN
>>  #undef CONFIG_KASAN_GENERIC
>> +#undef CONFIG_KASAN_SW_TAGS
>>
>>  #define __NO_FORTIFY
>>
>> diff --git a/arch/x86/include/asm/kasan.h b/arch/x86/include/asm/kasan.h
>> index 4bfd3641af84..cfc31e4a2f70 100644
>> --- a/arch/x86/include/asm/kasan.h
>> +++ b/arch/x86/include/asm/kasan.h
>> @@ -6,7 +6,7 @@
>>  #include <linux/kasan-tags.h>
>>  #include <linux/types.h>
>>
>> -#define KASAN_SHADOW_SCALE_SHIFT 3
>> +#define KASAN_SHADOW_SCALE_SHIFT CONFIG_KASAN_SHADOW_SCALE_SHIFT
>>
>>  /*
>>   * Compiler uses shadow offset assuming that addresses start
>> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
>> index cebee310e200..768990c573ea 100644
>> --- a/arch/x86/kernel/setup.c
>> +++ b/arch/x86/kernel/setup.c
>> @@ -1124,6 +1124,8 @@ void __init setup_arch(char **cmdline_p)
>>
>>         kasan_init();
>>
>> +       kasan_init_sw_tags();
>> +
>>         /*
>>          * Sync back kernel address range.
>>          *
>> --
>> 2.47.1
>>

-- 
Kind regards
Maciej Wieczór-Retman