Author: pcc Date: Tue Jul 9 13:22:36 2019 New Revision: 365551 URL: http://llvm.org/viewvc/llvm-project?rev=365551&view=rev Log: hwasan: Improve precision of checks using short granule tags.
A short granule is a granule of size between 1 and `TG-1` bytes. The size of a short granule is stored at the location in shadow memory where the granule's tag is normally stored, while the granule's actual tag is stored in the last byte of the granule. This means that in order to verify that a pointer tag matches a memory tag, HWASAN must check for two possibilities: * the pointer tag is equal to the memory tag in shadow memory, or * the shadow memory tag is actually a short granule size, the value being loaded is in bounds of the granule and the pointer tag is equal to the last byte of the granule. Pointer tags between 1 to `TG-1` are possible and are as likely as any other tag. This means that these tags in memory have two interpretations: the full tag interpretation (where the pointer tag is between 1 and `TG-1` and the last byte of the granule is ordinary data) and the short tag interpretation (where the pointer tag is stored in the granule). When HWASAN detects an error near a memory tag between 1 and `TG-1`, it will show both the memory tag and the last byte of the granule. Currently, it is up to the user to disambiguate the two possibilities. Because this functionality obsoletes the right aligned heap feature of the HWASAN memory allocator (and because we can no longer easily test it), the feature is removed. Also update the documentation to cover both short granule tags and outlined checks. Differential Revision: https://reviews.llvm.org/D63908 Modified: cfe/trunk/docs/HardwareAssistedAddressSanitizerDesign.rst Modified: cfe/trunk/docs/HardwareAssistedAddressSanitizerDesign.rst URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/docs/HardwareAssistedAddressSanitizerDesign.rst?rev=365551&r1=365550&r2=365551&view=diff ============================================================================== --- cfe/trunk/docs/HardwareAssistedAddressSanitizerDesign.rst (original) +++ cfe/trunk/docs/HardwareAssistedAddressSanitizerDesign.rst Tue Jul 9 13:22:36 2019 @@ -38,6 +38,30 @@ Algorithm For a more detailed discussion of this approach see https://arxiv.org/pdf/1802.09517.pdf +Short granules +-------------- + +A short granule is a granule of size between 1 and `TG-1` bytes. The size +of a short granule is stored at the location in shadow memory where the +granule's tag is normally stored, while the granule's actual tag is stored +in the last byte of the granule. This means that in order to verify that a +pointer tag matches a memory tag, HWASAN must check for two possibilities: + +* the pointer tag is equal to the memory tag in shadow memory, or +* the shadow memory tag is actually a short granule size, the value being loaded + is in bounds of the granule and the pointer tag is equal to the last byte of + the granule. + +Pointer tags between 1 to `TG-1` are possible and are as likely as any other +tag. This means that these tags in memory have two interpretations: the full +tag interpretation (where the pointer tag is between 1 and `TG-1` and the +last byte of the granule is ordinary data) and the short tag interpretation +(where the pointer tag is stored in the granule). + +When HWASAN detects an error near a memory tag between 1 and `TG-1`, it +will show both the memory tag and the last byte of the granule. Currently, +it is up to the user to disambiguate the two possibilities. + Instrumentation =============== @@ -46,24 +70,40 @@ Memory Accesses All memory accesses are prefixed with an inline instruction sequence that verifies the tags. Currently, the following sequence is used: - .. code-block:: none // int foo(int *a) { return *a; } - // clang -O2 --target=aarch64-linux -fsanitize=hwaddress -c load.c + // clang -O2 --target=aarch64-linux -fsanitize=hwaddress -fsanitize-recover=hwaddress -c load.c foo: - 0: 08 00 00 90 adrp x8, 0 <__hwasan_shadow> - 4: 08 01 40 f9 ldr x8, [x8] // shadow base (to be resolved by the loader) - 8: 09 dc 44 d3 ubfx x9, x0, #4, #52 // shadow offset - c: 28 69 68 38 ldrb w8, [x9, x8] // load shadow tag - 10: 09 fc 78 d3 lsr x9, x0, #56 // extract address tag - 14: 3f 01 08 6b cmp w9, w8 // compare tags - 18: 61 00 00 54 b.ne 24 // jump on mismatch - 1c: 00 00 40 b9 ldr w0, [x0] // original load - 20: c0 03 5f d6 ret - 24: 40 20 21 d4 brk #0x902 // trap + 0: 90000008 adrp x8, 0 <__hwasan_shadow> + 4: f9400108 ldr x8, [x8] // shadow base (to be resolved by the loader) + 8: d344dc09 ubfx x9, x0, #4, #52 // shadow offset + c: 38696909 ldrb w9, [x8, x9] // load shadow tag + 10: d378fc08 lsr x8, x0, #56 // extract address tag + 14: 6b09011f cmp w8, w9 // compare tags + 18: 54000061 b.ne 24 <foo+0x24> // jump to short tag handler on mismatch + 1c: b9400000 ldr w0, [x0] // original load + 20: d65f03c0 ret + 24: 7100413f cmp w9, #0x10 // is this a short tag? + 28: 54000142 b.cs 50 <foo+0x50> // if not, trap + 2c: 12000c0a and w10, w0, #0xf // find the address's position in the short granule + 30: 11000d4a add w10, w10, #0x3 // adjust to the position of the last byte loaded + 34: 6b09015f cmp w10, w9 // check that position is in bounds + 38: 540000c2 b.cs 50 <foo+0x50> // if not, trap + 3c: 9240dc09 and x9, x0, #0xffffffffffffff + 40: b2400d29 orr x9, x9, #0xf // compute address of last byte of granule + 44: 39400129 ldrb w9, [x9] // load tag from it + 48: 6b09011f cmp w8, w9 // compare with pointer tag + 4c: 54fffe80 b.eq 1c <foo+0x1c> // if so, continue + 50: d4212440 brk #0x922 // otherwise trap + 54: b9400000 ldr w0, [x0] // tail duplicated original load (to handle recovery) + 58: d65f03c0 ret Alternatively, memory accesses are prefixed with a function call. +On AArch64, a function call is used by default in trapping mode. The code size +and performance overhead of the call is reduced by using a custom calling +convention that preserves most registers and is specialized to the register +containing the address and the type and size of the memory access. Heap ---- _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
