On 2025-02-25 at 22:38:06 +0100, Andrey Konovalov wrote: >On Tue, Feb 25, 2025 at 9:13 PM Maciej Wieczor-Retman ><maciej.wieczor-retman@xxxxxxxxx> wrote: >> >> >>Thanks for letting me know about the tag resets, that should make changing the >> >>check in kasan_non_canonical_hook() easier. >> > >> >Ah, but the [0xff00000000000000, 0xffffffffffffffff] won't be true for x86 >> >right? Here the tag reset function only resets bits 60:57. So I presume >> >[0x3e00000000000000, 0xffffffffffffffff] would be the range? >> >> Sorry, brain freeze, I meant [0x1e00000000000000, 0xffffffffffffffff] > >+Vitaly, who implemented [1] > >Ah, so when the compiler calculates the shadow memory address on x86, >it does | 0x7E (== 0x3F << 1) [2] for when CompileKernel == true, >because LAM uses bits [62:57], I see. Thanks for the links, now I see what you meant. > >What value can bit 63 and take for _valid kernel_ pointers (on which >KASAN is intended to operate)? If it is always 1, we could arguably >change the compiler to do | 0xFE for CompileKernel. Which would leave >us with only one region to check: [0xfe00000000000000, >0xffffffffffffffff]. But I don't know whether changing the compiler >makes sense: it technically does as instructed by the LAM spec. >(Vitaly, any thoughts? For context: we are discussing how to check >whether a pointer can be a result of a memory-to-shadow mapping >applied to a potentially invalid pointer in kernel HWASAN.) With LAM, valid pointers need to have bits 63 and 56 equal for 5 level paging and bits 63 and 47 equal for 4 level paging. Both set for kernel addresses and both clear for user addresses. >With the way the compiler works right now, for the perfectly precise >check, I think we need to check 2 ranges: [0xfe00000000000000, >0xffffffffffffffff] for when bit 63 is set (of a potentially-invalid >pointer to which memory-to-shadow mapping is to be applied) and >[0x7e00000000000000, 0x7fffffffffffffff] for when bit 63 is reset. Bit >56 ranges through [0, 1] in both cases. > >However, in these patches, you use only bits [60:57]. The compiler is >not aware of this, so it still sets bits [62:57], and we end up with >the same two ranges. But in the KASAN code, you only set bits [60:57], >and thus we can end up with 8 potential ranges (2 possible values for >each of the top 3 bits), which gets complicated. So checking only one >range that covers all of them seems to be reasonable for simplicity >even though not entirely precise. And yes, [0x1e00000000000000, >0xffffffffffffffff] looks like the what we need. Aren't the 2 ranges you mentioned in the previous paragraph still valid, no matter what bits the __tag_set() function uses? I mean bits 62:57 are still reset by the compiler so bits 62:61 still won't matter. For example addresses 0x1e00000000000000 and 0x3e00000000000000 will resolve to the same thing after the compiler is done with them right? > >[1] https://github.com/llvm/llvm-project/commit/cb6099ba43b9262a317083858a29fd31af7efa5c >[2] https://github.com/llvm/llvm-project/blob/llvmorg-20-init/llvm/lib/Transforms/Instrumentation/HWAddressSanitizer.cpp#L1259 -- Kind regards Maciej Wieczór-Retman
