On 2025-02-17 at 17:13:23 +0100, Andrey Konovalov wrote: >On Fri, Feb 14, 2025 at 9:21 AM Maciej Wieczor-Retman ><maciej.wieczor-retman@xxxxxxxxx> wrote: >> >> On 2025-02-13 at 17:20:22 +0100, Maciej Wieczor-Retman wrote: >> >On 2025-02-13 at 02:28:08 +0100, Andrey Konovalov wrote: >> >>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(). >> >>> >> > >> >Right, I somehow forgot that obviously the whole LA has to map to 1/16th of the >> >address space and it shold stay contiguous. >> > >> >After rethinking how the mapping worked before and will work after making stuff >> >signed I thought this patch could make use of the overflow? >> > >> >From what I noticed, all the Kconfig values for KASAN_SHADOW_OFFSET should make >> >it so there will be overflow when inputing more and more positive addresses. >> > >> >So maybe we should first find what the most negative and most positive (signed) >> >addresses map to in shadow memory address space. And then when looking for >> >invalid values that aren't the product of kasan_mem_to_shadow() we should check >> > >> > if (addr > kasan_mem_to_shadow(biggest_positive_address) && >> > addr < kasan_mem_to_shadow(smallest_negative_address)) >> > return; >> > >> >Is this correct? >> >> I suppose the original code in the patch does the same thing when you change the >> || into &&: >> >> if (addr < KASAN_SHADOW_OFFSET - max_shadow_size / 2 && >> addr >= KASAN_SHADOW_OFFSET + max_shadow_size / 2) >> return; >> >> kasan_mem_to_shadow(0x7FFFFFFFFFFFFFFF) -> 0x07ff7fffffffffff >> kasan_mem_to_shadow(0x8000000000000000) -> 0xf7ff800000000000 > >I'm a bit lost with these calculations at this point. Please send the >full patch, including the new values for KASAN_SHADOW_OFFSET (do I >understand correctly that you want to change them?). It'll be easier >to look at the code. Sorry, this thread became a little bit confusing. No, I think the KASAN_SHADOW_OFFSET values are fine. I just wanted to embrace the idea of overflow for the purpose of the check in kasan_non_canonical_hook(). But I'll put down my train of thought about the overflow + calculations in the patch message. > >Feel free to send this patch separately from the rest of the series, >so that we can finalize it first. I have the x86 tag-based series basically ready (just need to re-read it) so I think I can send it as whole with this patch and 3 others from this series. -- Kind regards Maciej Wieczór-Retman
