On Thu, Feb 13, 2025 at 2:21 AM Andrey Konovalov <andreyknvl@xxxxxxxxx> wrote: > > On Tue, Feb 11, 2025 at 7:07 PM Maciej Wieczor-Retman > <maciej.wieczor-retman@xxxxxxxxx> 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
