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
47static const uint64_t max_va_size = 48;
48static const uint64_t pac_mask =
49 ((1ULL << 55) - 1) & ~((1ULL << max_va_size) - 1);
50static const uint64_t ttbr1_mask = 1ULL << 55;
51
52// Determine whether PAC is supported without accessing memory. This utilizes
53// the XPACLRI instruction which will copy bit 55 of x30 into at least bit 54 if
54// PAC is supported and acts as a NOP if PAC is not supported.
55static bool pac_supported() {
56 register uintptr_t x30 __asm__("x30") = 1ULL << 55;
57 __asm__ __volatile__("xpaclri" : "+r"(x30));
58 return x30 & (1ULL << 54);
59}
60
61#ifdef __GCC_HAVE_DWARF2_CFI_ASM
62#define CFI_INST(inst) inst
63#else
64#define CFI_INST(inst)
65#endif
66
67#ifdef __APPLE__
68#define ASM_SYMBOL(symbol) "_" #symbol
69#else
70#define ASM_SYMBOL(symbol) #symbol
71#endif
72
73// This asm snippet is used to force the creation of a frame record when
74// calling the EmuPAC functions. This is important because the EmuPAC functions
75// may crash if an auth failure is detected and may be unwound past using a
76// frame pointer based unwinder.
77// clang-format off
78#define FRAME_POINTER_WRAP(sym) \
79 CFI_INST(".cfi_startproc\n") \
80 "stp x29, x30, [sp, #-16]!\n" \
81 CFI_INST(".cfi_def_cfa_offset 16\n") \
82 "mov x29, sp\n" \
83 CFI_INST(".cfi_def_cfa w29, 16\n") \
84 CFI_INST(".cfi_offset w30, -8\n") \
85 CFI_INST(".cfi_offset w29, -16\n") \
86 "bl " ASM_SYMBOL(sym) "\n" \
87 CFI_INST(".cfi_def_cfa wsp, 16\n") \
88 "ldp x29, x30, [sp], #16\n" \
89 CFI_INST(".cfi_def_cfa_offset 0\n") \
90 CFI_INST(".cfi_restore w30\n") \
91 CFI_INST(".cfi_restore w29\n") \
92 "ret\n" \
93 CFI_INST(".cfi_endproc\n")
94// clang-format on
95
96// Emulated DA key value.
97static const uint8_t emu_da_key[16] = {0xb5, 0xd4, 0xc9, 0xeb, 0x79, 0x10,
98 0x4a, 0x79, 0x6f, 0xec, 0x8b, 0x1b,
99 0x42, 0x87, 0x81, 0xd4};
100
101extern "C" [[gnu::flatten]] uint64_t __emupac_pacda_impl(uint64_t ptr,
102 uint64_t disc) {
103 if (pac_supported()) {
104 __asm__ __volatile__(".arch_extension pauth\npacda %0, %1"
105 : "+r"(ptr)
106 : "r"(disc));
107 return ptr;
108 }
109 if (ptr & ttbr1_mask) {
110 if ((ptr & pac_mask) != pac_mask) {
111 return ptr | pac_mask;
112 }
113 } else {
114 if (ptr & pac_mask) {
115 return ptr & ~pac_mask;
116 }
117 }
118 uint64_t hash;
119 siphash<1, 3>(reinterpret_cast<uint8_t *>(&ptr), 8, emu_da_key,
120 *reinterpret_cast<uint8_t (*)[8]>(&hash));
121 return (ptr & ~pac_mask) | (hash & pac_mask);
122}
123
124// clang-format off
125__asm__(
126 ".globl " ASM_SYMBOL(__emupac_pacda) "\n"
127 ASM_SYMBOL(__emupac_pacda) ":\n"
128 FRAME_POINTER_WRAP(__emupac_pacda_impl)
129);
130// clang-format on
131
132extern "C" [[gnu::flatten]] uint64_t __emupac_autda_impl(uint64_t ptr,
133 uint64_t disc) {
134 if (pac_supported()) {
135 __asm__ __volatile__(".arch_extension pauth\nautda %0, %1"
136 : "+r"(ptr)
137 : "r"(disc));
138 return ptr;
139 }
140 uint64_t ptr_without_pac =
141 (ptr & ttbr1_mask) ? (ptr | pac_mask) : (ptr & ~pac_mask);
142 uint64_t hash;
143 siphash<1, 3>(reinterpret_cast<uint8_t *>(&ptr_without_pac), 8, emu_da_key,
144 *reinterpret_cast<uint8_t (*)[8]>(&hash));
145 if (((ptr & ~pac_mask) | (hash & pac_mask)) != ptr) {
146 __builtin_trap();
147 }
148 return ptr_without_pac;
149}
150
151// clang-format off
152__asm__(
153 ".globl " ASM_SYMBOL(__emupac_autda) "\n"
154 ASM_SYMBOL(__emupac_autda) ":\n"
155 FRAME_POINTER_WRAP(__emupac_autda_impl)
156);
157// clang-format on
158

source code of compiler-rt/lib/builtins/aarch64/emupac.cpp