|
| 1 | +//===--- emupac.cpp - Emulated PAC implementation -------------------------===// |
| 2 | +// |
| 3 | +// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. |
| 4 | +// See https://llvm.org/LICENSE.txt for license information. |
| 5 | +// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception |
| 6 | +// |
| 7 | +//===----------------------------------------------------------------------===// |
| 8 | +// |
| 9 | +// This file implements Emulated PAC using SipHash_1_3 as the IMPDEF hashing |
| 10 | +// scheme. |
| 11 | +// |
| 12 | +//===----------------------------------------------------------------------===// |
| 13 | + |
| 14 | +#include <stdint.h> |
| 15 | + |
| 16 | +#include "siphash/SipHash.h" |
| 17 | + |
| 18 | +// EmuPAC implements runtime emulation of PAC instructions. If the current |
| 19 | +// CPU supports PAC, EmuPAC uses real PAC instructions. Otherwise, it uses the |
| 20 | +// emulation, which is effectively an implementation of PAC with an IMPDEF |
| 21 | +// hashing scheme based on SipHash_1_3. |
| 22 | +// |
| 23 | +// The purpose of the emulation is to allow programs to be built to be portable |
| 24 | +// to machines without PAC support, with some performance loss and increased |
| 25 | +// probability of false positives (due to not being able to portably determine |
| 26 | +// the VA size), while being functionally almost equivalent to running on a |
| 27 | +// machine with PAC support. One example of a use case is if PAC is used in |
| 28 | +// production as a security mitigation, but the testing environment is |
| 29 | +// heterogeneous (i.e. some machines lack PAC support). In this case we would |
| 30 | +// like the testing machines to be able to detect issues resulting |
| 31 | +// from the use of PAC instructions that would affect production by running |
| 32 | +// tests. This can be achieved by building test binaries with EmuPAC and |
| 33 | +// production binaries with real PAC. |
| 34 | +// |
| 35 | +// EmuPAC should not be used in production and is only intended for testing use |
| 36 | +// cases. This is not only because of the performance costs, which will exist |
| 37 | +// even on PAC-supporting machines because of the function call overhead for |
| 38 | +// each sign/auth operation, but because it provides weaker security compared to |
| 39 | +// real PAC: the key is constant and public, which means that we do not mix a |
| 40 | +// global secret. |
| 41 | +// |
| 42 | +// The emulation assumes that the VA size is at most 48 bits. The architecture |
| 43 | +// as of ARMv8.2, which was the last architecture version in which PAC was not |
| 44 | +// mandatory, permitted VA size up to 52 bits via ARMv8.2-LVA, but we are |
| 45 | +// unaware of an ARMv8.2 CPU that implemented ARMv8.2-LVA. |
| 46 | + |
| 47 | +const uint64_t max_va_size = 48; |
| 48 | +const uint64_t pac_mask = ((1ULL << 55) - 1) & ~((1ULL << max_va_size) - 1); |
| 49 | +const uint64_t ttbr1_mask = 1ULL << 55; |
| 50 | + |
| 51 | +// Determine whether PAC is supported without accessing memory. This utilizes |
| 52 | +// the XPACLRI instruction which will copy bit 55 of x30 into at least bit 54 if |
| 53 | +// PAC is supported and acts as a NOP if PAC is not supported. |
| 54 | +static bool pac_supported() { |
| 55 | + register uintptr_t x30 __asm__("x30") = 1ULL << 55; |
| 56 | + __asm__ __volatile__("xpaclri" : "+r"(x30)); |
| 57 | + return x30 & (1ULL << 54); |
| 58 | +} |
| 59 | + |
| 60 | +// This asm snippet is used to force the creation of a frame record when |
| 61 | +// calling the EmuPAC functions. This is important because the EmuPAC functions |
| 62 | +// may crash if an auth failure is detected and may be unwound past using a |
| 63 | +// frame pointer based unwinder. |
| 64 | +#ifdef __GCC_HAVE_DWARF2_CFI_ASM |
| 65 | +#define CFI_INST(inst) inst |
| 66 | +#else |
| 67 | +#define CFI_INST(inst) |
| 68 | +#endif |
| 69 | + |
| 70 | +// clang-format off |
| 71 | +#define FRAME_POINTER_WRAP(sym) \ |
| 72 | + CFI_INST(".cfi_startproc\n") \ |
| 73 | + "stp x29, x30, [sp, #-16]!\n" \ |
| 74 | + CFI_INST(".cfi_def_cfa_offset 16\n") \ |
| 75 | + "mov x29, sp\n" \ |
| 76 | + CFI_INST(".cfi_def_cfa w29, 16\n") \ |
| 77 | + CFI_INST(".cfi_offset w30, -8\n") \ |
| 78 | + CFI_INST(".cfi_offset w29, -16\n") \ |
| 79 | + "bl " #sym "\n" \ |
| 80 | + CFI_INST(".cfi_def_cfa wsp, 16\n") \ |
| 81 | + "ldp x29, x30, [sp], #16\n" \ |
| 82 | + CFI_INST(".cfi_def_cfa_offset 0\n") \ |
| 83 | + CFI_INST(".cfi_restore w30\n") \ |
| 84 | + CFI_INST(".cfi_restore w29\n") \ |
| 85 | + "ret\n" \ |
| 86 | + CFI_INST(".cfi_endproc\n") |
| 87 | +// clang-format on |
| 88 | +
|
| 89 | +// Emulated DA key value. |
| 90 | +static const uint8_t emu_da_key[16] = {0xb5, 0xd4, 0xc9, 0xeb, 0x79, 0x10, |
| 91 | + 0x4a, 0x79, 0x6f, 0xec, 0x8b, 0x1b, |
| 92 | + 0x42, 0x87, 0x81, 0xd4}; |
| 93 | +
|
| 94 | +extern "C" [[gnu::flatten]] uint64_t |
| 95 | +__emupac_pacda_impl(uint64_t ptr, uint64_t disc) { |
| 96 | + if (pac_supported()) { |
| 97 | + __asm__ __volatile__(".arch_extension pauth\npacda %0, %1" |
| 98 | + : "+r"(ptr) |
| 99 | + : "r"(disc)); |
| 100 | + return ptr; |
| 101 | + } |
| 102 | + if (ptr & ttbr1_mask) { |
| 103 | + if ((ptr & pac_mask) != pac_mask) { |
| 104 | + return ptr | pac_mask; |
| 105 | + } |
| 106 | + } else { |
| 107 | + if (ptr & pac_mask) { |
| 108 | + return ptr & ~pac_mask; |
| 109 | + } |
| 110 | + } |
| 111 | + uint64_t hash; |
| 112 | + siphash<1, 3>(reinterpret_cast<uint8_t *>(&ptr), 8, emu_da_key, |
| 113 | + *reinterpret_cast<uint8_t(*)[8]>(&hash)); |
| 114 | + return (ptr & ~pac_mask) | (hash & pac_mask); |
| 115 | +} |
| 116 | +
|
| 117 | +__asm__(".globl __emupac_pacda\n" |
| 118 | + "__emupac_pacda:\n" FRAME_POINTER_WRAP(__emupac_pacda_impl)); |
| 119 | +
|
| 120 | +extern "C" [[gnu::flatten]] uint64_t |
| 121 | +__emupac_autda_impl(uint64_t ptr, uint64_t disc) { |
| 122 | + if (pac_supported()) { |
| 123 | + __asm__ __volatile__(".arch_extension pauth\nautda %0, %1" |
| 124 | + : "+r"(ptr) |
| 125 | + : "r"(disc)); |
| 126 | + return ptr; |
| 127 | + } |
| 128 | + uint64_t ptr_without_pac = |
| 129 | + (ptr & ttbr1_mask) ? (ptr | pac_mask) : (ptr & ~pac_mask); |
| 130 | + uint64_t hash; |
| 131 | + siphash<1, 3>(reinterpret_cast<uint8_t *>(&ptr_without_pac), 8, emu_da_key, |
| 132 | + *reinterpret_cast<uint8_t(*)[8]>(&hash)); |
| 133 | + if (((ptr & ~pac_mask) | (hash & pac_mask)) != ptr) { |
| 134 | + __builtin_trap(); |
| 135 | + } |
| 136 | + return ptr_without_pac; |
| 137 | +} |
| 138 | +
|
| 139 | +__asm__(".globl __emupac_autda\n" |
| 140 | + "__emupac_autda:\n" FRAME_POINTER_WRAP(__emupac_autda_impl)); |
0 commit comments