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

Andrey Konovalov
2025-02-10T23:15:37Z

On Mon, Feb 10, 2025 at 4:53 PM Maciej Wieczor-Retman
<maciej.wieczor-retman@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@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.