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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 2025-02-10 at 23:57:10 +0100, Andrey Konovalov wrote: >On Mon, Feb 10, 2025 at 4:53 PM Maciej Wieczor-Retman > 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 >> >> 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. 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? -- Kind regards Maciej Wieczór-Retman