1 // SPDX-License-Identifier: GPL-2.0-or-later 2 /* 3 * Copyright (C) 2015 Josh Poimboeuf <jpoimboe@redhat.com> 4 */ 5 6 /* 7 * This file reads all the special sections which have alternate instructions 8 * which can be patched in or redirected to at runtime. 9 */ 10 11 #include <stdlib.h> 12 #include <string.h> 13 14 #include "builtin.h" 15 #include "special.h" 16 #include "warn.h" 17 18 #define EX_ENTRY_SIZE 12 19 #define EX_ORIG_OFFSET 0 20 #define EX_NEW_OFFSET 4 21 22 #define JUMP_ENTRY_SIZE 16 23 #define JUMP_ORIG_OFFSET 0 24 #define JUMP_NEW_OFFSET 4 25 26 #define ALT_ENTRY_SIZE 13 27 #define ALT_ORIG_OFFSET 0 28 #define ALT_NEW_OFFSET 4 29 #define ALT_FEATURE_OFFSET 8 30 #define ALT_ORIG_LEN_OFFSET 10 31 #define ALT_NEW_LEN_OFFSET 11 32 33 #define X86_FEATURE_POPCNT (4*32+23) 34 #define X86_FEATURE_SMAP (9*32+20) 35 36 struct special_entry { 37 const char *sec; 38 bool group, jump_or_nop; 39 unsigned char size, orig, new; 40 unsigned char orig_len, new_len; /* group only */ 41 unsigned char feature; /* ALTERNATIVE macro CPU feature */ 42 }; 43 44 struct special_entry entries[] = { 45 { 46 .sec = ".altinstructions", 47 .group = true, 48 .size = ALT_ENTRY_SIZE, 49 .orig = ALT_ORIG_OFFSET, 50 .orig_len = ALT_ORIG_LEN_OFFSET, 51 .new = ALT_NEW_OFFSET, 52 .new_len = ALT_NEW_LEN_OFFSET, 53 .feature = ALT_FEATURE_OFFSET, 54 }, 55 { 56 .sec = "__jump_table", 57 .jump_or_nop = true, 58 .size = JUMP_ENTRY_SIZE, 59 .orig = JUMP_ORIG_OFFSET, 60 .new = JUMP_NEW_OFFSET, 61 }, 62 { 63 .sec = "__ex_table", 64 .size = EX_ENTRY_SIZE, 65 .orig = EX_ORIG_OFFSET, 66 .new = EX_NEW_OFFSET, 67 }, 68 {}, 69 }; 70 71 static int get_alt_entry(struct elf *elf, struct special_entry *entry, 72 struct section *sec, int idx, 73 struct special_alt *alt) 74 { 75 struct rela *orig_rela, *new_rela; 76 unsigned long offset; 77 78 offset = idx * entry->size; 79 80 alt->group = entry->group; 81 alt->jump_or_nop = entry->jump_or_nop; 82 83 if (alt->group) { 84 alt->orig_len = *(unsigned char *)(sec->data->d_buf + offset + 85 entry->orig_len); 86 alt->new_len = *(unsigned char *)(sec->data->d_buf + offset + 87 entry->new_len); 88 } 89 90 if (entry->feature) { 91 unsigned short feature; 92 93 feature = *(unsigned short *)(sec->data->d_buf + offset + 94 entry->feature); 95 96 /* 97 * It has been requested that we don't validate the !POPCNT 98 * feature path which is a "very very small percentage of 99 * machines". 100 */ 101 if (feature == X86_FEATURE_POPCNT) 102 alt->skip_orig = true; 103 104 /* 105 * If UACCESS validation is enabled; force that alternative; 106 * otherwise force it the other way. 107 * 108 * What we want to avoid is having both the original and the 109 * alternative code flow at the same time, in that case we can 110 * find paths that see the STAC but take the NOP instead of 111 * CLAC and the other way around. 112 */ 113 if (feature == X86_FEATURE_SMAP) { 114 if (uaccess) 115 alt->skip_orig = true; 116 else 117 alt->skip_alt = true; 118 } 119 } 120 121 orig_rela = find_rela_by_dest(elf, sec, offset + entry->orig); 122 if (!orig_rela) { 123 WARN_FUNC("can't find orig rela", sec, offset + entry->orig); 124 return -1; 125 } 126 if (orig_rela->sym->type != STT_SECTION) { 127 WARN_FUNC("don't know how to handle non-section rela symbol %s", 128 sec, offset + entry->orig, orig_rela->sym->name); 129 return -1; 130 } 131 132 alt->orig_sec = orig_rela->sym->sec; 133 alt->orig_off = orig_rela->addend; 134 135 if (!entry->group || alt->new_len) { 136 new_rela = find_rela_by_dest(elf, sec, offset + entry->new); 137 if (!new_rela) { 138 WARN_FUNC("can't find new rela", 139 sec, offset + entry->new); 140 return -1; 141 } 142 143 alt->new_sec = new_rela->sym->sec; 144 alt->new_off = (unsigned int)new_rela->addend; 145 146 /* _ASM_EXTABLE_EX hack */ 147 if (alt->new_off >= 0x7ffffff0) 148 alt->new_off -= 0x7ffffff0; 149 } 150 151 return 0; 152 } 153 154 /* 155 * Read all the special sections and create a list of special_alt structs which 156 * describe all the alternate instructions which can be patched in or 157 * redirected to at runtime. 158 */ 159 int special_get_alts(struct elf *elf, struct list_head *alts) 160 { 161 struct special_entry *entry; 162 struct section *sec; 163 unsigned int nr_entries; 164 struct special_alt *alt; 165 int idx, ret; 166 167 INIT_LIST_HEAD(alts); 168 169 for (entry = entries; entry->sec; entry++) { 170 sec = find_section_by_name(elf, entry->sec); 171 if (!sec) 172 continue; 173 174 if (sec->len % entry->size != 0) { 175 WARN("%s size not a multiple of %d", 176 sec->name, entry->size); 177 return -1; 178 } 179 180 nr_entries = sec->len / entry->size; 181 182 for (idx = 0; idx < nr_entries; idx++) { 183 alt = malloc(sizeof(*alt)); 184 if (!alt) { 185 WARN("malloc failed"); 186 return -1; 187 } 188 memset(alt, 0, sizeof(*alt)); 189 190 ret = get_alt_entry(elf, entry, sec, idx, alt); 191 if (ret) 192 return ret; 193 194 list_add_tail(&alt->list, alts); 195 } 196 } 197 198 return 0; 199 } 200