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

[Maciej Wieczor-Retman]

On 2025-02-10 at 23:57:10 +0100, Andrey Konovalov wrote:
>On Mon, Feb 10, 2025 at 4:53 PM Maciej Wieczor-Retman
><maciej.wieczor-retman at intel.com> wrote:
>>
>> On 2025-02-10 at 16:22:41 +0100, Maciej Wieczor-Retman wrote:
>> >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 at 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;
>> >
>> >?
>>
>> Sorry, I think this wouldn't work. The tag also needs to be reset. Does this
>> perhaps work for this problem?
>>
>>         if (!addr_in_shadow(kasan_reset_tag((void *)addr)))
>>                 return;
>
>This wouldn't work as well.
>
>addr_in_shadow() checks whether an address belongs to the proper
>shadow memory area. That area is the result of the memory-to-shadow
>mapping applied to the range of proper kernel addresses.
>
>However, what we want to check in this function is whether the given
>address can be the result of the memory-to-shadow mapping for some
>memory address, including userspace addresses, non-canonical
>addresses, etc. So essentially we need to check whether the given
>address belongs to the area that is the result of the memory-to-shadow
>mapping applied to the whole address space, not only to proper kernel
>addresses.

I did some experiments with multiple addresses passed through
kasan_mem_to_shadow(). And it seems like we can get almost any address out when
we consider any random bogus pointers.

I used the KASAN_SHADOW_OFFSET from your example above. Userspace addresses seem
to map to the range [KASAN_SHADOW_OFFSET - 0xffff8fffffffffff]. Then going
through non-canonical addresses until 0x0007ffffffffffff we reach the end of
kernel LA and we loop around. Then the addresses seem to go from 0 until we
again start reaching the kernel space and then it maps into the proper shadow
memory.

It gave me the same results when using the previous version of
kasan_mem_to_shadow() so I'm wondering whether I'm doing this experiment
incorrectly or if there aren't any addresses we can rule out here?

-- 
Kind regards
Maciej Wieczór-Retman