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

Andrey Konovalov
2025-02-13T01:30:00Z

On Thu, Feb 13, 2025 at 2:21 AM Andrey Konovalov <andreyknvl@gmail.com> wrote:
>
> On Tue, Feb 11, 2025 at 7:07 PM Maciej Wieczor-Retman
> <maciej.wieczor-retman@intel.com> wrote:
> >
> > 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?
>
> By the definition of the shadow mapping, if we apply that mapping to
> the whole 64-bit address space, the result will only contain 1/8th
> (1/16th for SW/HW_TAGS) of that space.
>
> For example, with the current upstream value of KASAN_SHADOW_OFFSET on
> x86 and arm64, the value of the top 3 bits (4 for SW/HW_TAGS) of any
> shadow address are always the same: KASAN_SHADOW_OFFSET's value is
> such that the shadow address calculation never overflows. Addresses
> that have a different value for those top 3 bits are the once we can
> rule out.

Eh, scratch that, the 3rd bit from the top changes, as
KASAN_SHADOW_OFFSET is not a that-well-aligned value, the overall size
of the mapping holds.

> The KASAN_SHADOW_OFFSET value from my example does rely on the
> overflow (arguably, this makes things more confusing [1]). But still,
> the possible values of shadow addresses should only cover 1/16th of
> the address space.
>
> So whether the address belongs to that 1/8th (1/16th) of the address
> space is what we want to check in kasan_non_canonical_hook().
>
> The current upstream version of kasan_non_canonical_hook() actually
> does a simplified check by only checking for the lower bound (e.g. for
> x86, there's also an upper bound: KASAN_SHADOW_OFFSET +
> (0xffffffffffffffff >> 3) == 0xfffffbffffffffff), so we could improve
> it.
>
> [1] https://bugzilla.kernel.org/show_bug.cgi?id=218043