linux-kernel - Re: [PATCH 02/15] kasan: Tag checking with dense tag-based mode

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Message-ID: <vc23342eu5igzyobkzro3izsrwfubxlh23kzvzxjdoc7hxr4ug@5aplefpwbhev>
Date: Thu, 6 Feb 2025 15:55:13 +0100
From: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
To: Andrey Konovalov <andreyknvl@...il.com>
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Subject: Re: [PATCH 02/15] kasan: Tag checking with dense tag-based mode

On 2025-02-06 at 00:45:01 +0100, Andrey Konovalov wrote:
>On Tue, Feb 4, 2025 at 6:35 PM Maciej Wieczor-Retman
><maciej.wieczor-retman@...el.com> wrote:
>>
>> In KASAN's tag-based mode (arm64) when a memory access occurs, the tag
>> stored in the top 8 bits of the pointer is compared with tags saved in
>> the region of the shadow memory that maps to memory the pointer points
>> to. If any of the tags in the shadow memory region do not match the one
>> stored in the pointer an error report is generated.
>>
>> With the introduction of the dense mode, tags won't necessarily occupy
>> whole bytes of shadow memory if the previously allocated memory wasn't
>> aligned to 32 bytes - which is the coverage of one shadow byte.
>>
>> Add an alternative implementation of kasan_check_range() that performs
>> special checks on first and last bytes of shadow memory ranges if the
>> originally allocated memory wasn't aligned to 32 bytes.
>>
>> Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
>> ---
>>  include/linux/kasan.h     | 47 +++++++++++++++-------
>>  mm/kasan/Makefile         |  3 ++
>>  mm/kasan/dense.c          | 83 +++++++++++++++++++++++++++++++++++++++
>>  mm/kasan/kasan.h          |  2 +-
>>  mm/kasan/report.c         |  2 +-
>>  mm/kasan/report_sw_tags.c | 12 ++----
>>  mm/kasan/sw_tags.c        |  8 ++++
>>  7 files changed, 133 insertions(+), 24 deletions(-)
>>  create mode 100644 mm/kasan/dense.c
>>
>> diff --git a/include/linux/kasan.h b/include/linux/kasan.h
>> index ea0f5acd875b..5a3e9bec21c2 100644
>> --- a/include/linux/kasan.h
>> +++ b/include/linux/kasan.h
>> @@ -33,6 +33,20 @@ typedef unsigned int __bitwise kasan_vmalloc_flags_t;
>>
>>  #include <linux/pgtable.h>
>>
>> +#ifndef kasan_mem_to_shadow
>> +static inline void *kasan_mem_to_shadow(const void *addr)
>> +{
>> +       void *scaled;
>> +
>> +       if (IS_ENABLED(CONFIG_KASAN_GENERIC))
>> +               scaled = (void *)((unsigned long)addr >> KASAN_SHADOW_SCALE_SHIFT);
>> +       else
>> +               scaled = (void *)((long)addr >> KASAN_SHADOW_SCALE_SHIFT);
>> +
>> +       return KASAN_SHADOW_OFFSET + scaled;
>> +}
>> +#endif
>
>Any reason this is moved up here?

I think it was necessary for something I added, removed and then didn't notice
it's no longer needed. I'll move it back.

-- 
Kind regards
Maciej Wieczór-Retman