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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 >> >> 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.k Ah, okay, I get it. Would the old version if (addr < KASAN_SHADOW_OFFSET) return; work if the *addr* had kasan_reset_tag() around it? That would sort of re-unsign the address only for the purpose of the if(). Also I was thinking about it because x86 even with address masking enabled keeps bit 63 set, so all kernel addresses will be negative in the signed kasan_mem_to_shadow(). That's great for simplifying the KASAN_SHADOW_OFFSET but it differs from the TBI and risc-v ideas where half of addresses are negative, hald positive. So the temporary re-unsigning could maybe make it simpler for x86 and avoid adding separate cases or alternative kasan_non_canonical_hook() implementation. -- Kind regards Maciej Wieczór-Retman