arch/arm64/kernel/module-plts.c

   1 // SPDX-License-Identifier: GPL-2.0-only
   2 /*
   3  * Copyright (C) 2014-2017 Linaro Ltd. <ard.biesheuvel@linaro.org>
   4  */
   5
   6 #include <linux/elf.h>
   7 #include <linux/ftrace.h>
   8 #include <linux/kernel.h>
   9 #include <linux/module.h>
  10 #include <linux/sort.h>
  11
  12 static struct plt_entry __get_adrp_add_pair(u64 dst, u64 pc,
  13                                             enum aarch64_insn_register reg)
  14 {
  15         u32 adrp, add;
  16
  17         adrp = aarch64_insn_gen_adr(pc, dst, reg, AARCH64_INSN_ADR_TYPE_ADRP);
  18         add = aarch64_insn_gen_add_sub_imm(reg, reg, dst % SZ_4K,
  19                                            AARCH64_INSN_VARIANT_64BIT,
  20                                            AARCH64_INSN_ADSB_ADD);
  21
  22         return (struct plt_entry){ cpu_to_le32(adrp), cpu_to_le32(add) };
  23 }
  24
  25 struct plt_entry get_plt_entry(u64 dst, void *pc)
  26 {
  27         struct plt_entry plt;
  28         static u32 br;
  29
  30         if (!br)
  31                 br = aarch64_insn_gen_branch_reg(AARCH64_INSN_REG_16,
  32                                                  AARCH64_INSN_BRANCH_NOLINK);
  33
  34         plt = __get_adrp_add_pair(dst, (u64)pc, AARCH64_INSN_REG_16);
  35         plt.br = cpu_to_le32(br);
  36
  37         return plt;
  38 }
  39
  40 bool plt_entries_equal(const struct plt_entry *a, const struct plt_entry *b)
  41 {
  42         u64 p, q;
  43
  44         /*
  45          * Check whether both entries refer to the same target:
  46          * do the cheapest checks first.
  47          * If the 'add' or 'br' opcodes are different, then the target
  48          * cannot be the same.
  49          */
  50         if (a->add != b->add || a->br != b->br)
  51                 return false;
  52
  53         p = ALIGN_DOWN((u64)a, SZ_4K);
  54         q = ALIGN_DOWN((u64)b, SZ_4K);
  55
  56         /*
  57          * If the 'adrp' opcodes are the same then we just need to check
  58          * that they refer to the same 4k region.
  59          */
  60         if (a->adrp == b->adrp && p == q)
  61                 return true;
  62
  63         return (p + aarch64_insn_adrp_get_offset(le32_to_cpu(a->adrp))) ==
  64                (q + aarch64_insn_adrp_get_offset(le32_to_cpu(b->adrp)));
  65 }
  66
  67 static bool in_init(const struct module *mod, void *loc)
  68 {
  69         return (u64)loc - (u64)mod->init_layout.base < mod->init_layout.size;
  70 }
  71
  72 u64 module_emit_plt_entry(struct module *mod, Elf64_Shdr *sechdrs,
  73                           void *loc, const Elf64_Rela *rela,
  74                           Elf64_Sym *sym)
  75 {
  76         struct mod_plt_sec *pltsec = !in_init(mod, loc) ? &mod->arch.core :
  77                                                           &mod->arch.init;
  78         struct plt_entry *plt = (struct plt_entry *)sechdrs[pltsec->plt_shndx].sh_addr;
  79         int i = pltsec->plt_num_entries;
  80         int j = i - 1;
  81         u64 val = sym->st_value + rela->r_addend;
  82
  83         if (is_forbidden_offset_for_adrp(&plt[i].adrp))
  84                 i++;
  85
  86         plt[i] = get_plt_entry(val, &plt[i]);
  87
  88         /*
  89          * Check if the entry we just created is a duplicate. Given that the
  90          * relocations are sorted, this will be the last entry we allocated.
  91          * (if one exists).
  92          */
  93         if (j >= 0 && plt_entries_equal(plt + i, plt + j))
  94                 return (u64)&plt[j];
  95
  96         pltsec->plt_num_entries += i - j;
  97         if (WARN_ON(pltsec->plt_num_entries > pltsec->plt_max_entries))
  98                 return 0;
  99
 100         return (u64)&plt[i];
 101 }
 102
 103 #ifdef CONFIG_ARM64_ERRATUM_843419
 104 u64 module_emit_veneer_for_adrp(struct module *mod, Elf64_Shdr *sechdrs,
 105                                 void *loc, u64 val)
 106 {
 107         struct mod_plt_sec *pltsec = !in_init(mod, loc) ? &mod->arch.core :
 108                                                           &mod->arch.init;
 109         struct plt_entry *plt = (struct plt_entry *)sechdrs[pltsec->plt_shndx].sh_addr;
 110         int i = pltsec->plt_num_entries++;
 111         u32 br;
 112         int rd;
 113
 114         if (WARN_ON(pltsec->plt_num_entries > pltsec->plt_max_entries))
 115                 return 0;
 116
 117         if (is_forbidden_offset_for_adrp(&plt[i].adrp))
 118                 i = pltsec->plt_num_entries++;
 119
 120         /* get the destination register of the ADRP instruction */
 121         rd = aarch64_insn_decode_register(AARCH64_INSN_REGTYPE_RD,
 122                                           le32_to_cpup((__le32 *)loc));
 123
 124         br = aarch64_insn_gen_branch_imm((u64)&plt[i].br, (u64)loc + 4,
 125                                          AARCH64_INSN_BRANCH_NOLINK);
 126
 127         plt[i] = __get_adrp_add_pair(val, (u64)&plt[i], rd);
 128         plt[i].br = cpu_to_le32(br);
 129
 130         return (u64)&plt[i];
 131 }
 132 #endif
 133
 134 #define cmp_3way(a,b)   ((a) < (b) ? -1 : (a) > (b))
 135
 136 static int cmp_rela(const void *a, const void *b)
 137 {
 138         const Elf64_Rela *x = a, *y = b;
 139         int i;
 140
 141         /* sort by type, symbol index and addend */
 142         i = cmp_3way(ELF64_R_TYPE(x->r_info), ELF64_R_TYPE(y->r_info));
 143         if (i == 0)
 144                 i = cmp_3way(ELF64_R_SYM(x->r_info), ELF64_R_SYM(y->r_info));
 145         if (i == 0)
 146                 i = cmp_3way(x->r_addend, y->r_addend);
 147         return i;
 148 }
 149
 150 static bool duplicate_rel(const Elf64_Rela *rela, int num)
 151 {
 152         /*
 153          * Entries are sorted by type, symbol index and addend. That means
 154          * that, if a duplicate entry exists, it must be in the preceding
 155          * slot.
 156          */
 157         return num > 0 && cmp_rela(rela + num, rela + num - 1) == 0;
 158 }
 159
 160 static unsigned int count_plts(Elf64_Sym *syms, Elf64_Rela *rela, int num,
 161                                Elf64_Word dstidx, Elf_Shdr *dstsec)
 162 {
 163         unsigned int ret = 0;
 164         Elf64_Sym *s;
 165         int i;
 166
 167         for (i = 0; i < num; i++) {
 168                 u64 min_align;
 169
 170                 switch (ELF64_R_TYPE(rela[i].r_info)) {
 171                 case R_AARCH64_JUMP26:
 172                 case R_AARCH64_CALL26:
 173                         if (!IS_ENABLED(CONFIG_RANDOMIZE_BASE))
 174                                 break;
 175
 176                         /*
 177                          * We only have to consider branch targets that resolve
 178                          * to symbols that are defined in a different section.
 179                          * This is not simply a heuristic, it is a fundamental
 180                          * limitation, since there is no guaranteed way to emit
 181                          * PLT entries sufficiently close to the branch if the
 182                          * section size exceeds the range of a branch
 183                          * instruction. So ignore relocations against defined
 184                          * symbols if they live in the same section as the
 185                          * relocation target.
 186                          */
 187                         s = syms + ELF64_R_SYM(rela[i].r_info);
 188                         if (s->st_shndx == dstidx)
 189                                 break;
 190
 191                         /*
 192                          * Jump relocations with non-zero addends against
 193                          * undefined symbols are supported by the ELF spec, but
 194                          * do not occur in practice (e.g., 'jump n bytes past
 195                          * the entry point of undefined function symbol f').
 196                          * So we need to support them, but there is no need to
 197                          * take them into consideration when trying to optimize
 198                          * this code. So let's only check for duplicates when
 199                          * the addend is zero: this allows us to record the PLT
 200                          * entry address in the symbol table itself, rather than
 201                          * having to search the list for duplicates each time we
 202                          * emit one.
 203                          */
 204                         if (rela[i].r_addend != 0 || !duplicate_rel(rela, i))
 205                                 ret++;
 206                         break;
 207                 case R_AARCH64_ADR_PREL_PG_HI21_NC:
 208                 case R_AARCH64_ADR_PREL_PG_HI21:
 209                         if (!IS_ENABLED(CONFIG_ARM64_ERRATUM_843419) ||
 210                             !cpus_have_const_cap(ARM64_WORKAROUND_843419))
 211                                 break;
 212
 213                         /*
 214                          * Determine the minimal safe alignment for this ADRP
 215                          * instruction: the section alignment at which it is
 216                          * guaranteed not to appear at a vulnerable offset.
 217                          *
 218                          * This comes down to finding the least significant zero
 219                          * bit in bits [11:3] of the section offset, and
 220                          * increasing the section's alignment so that the
 221                          * resulting address of this instruction is guaranteed
 222                          * to equal the offset in that particular bit (as well
 223                          * as all less signficant bits). This ensures that the
 224                          * address modulo 4 KB != 0xfff8 or 0xfffc (which would
 225                          * have all ones in bits [11:3])
 226                          */
 227                         min_align = 2ULL << ffz(rela[i].r_offset | 0x7);
 228
 229                         /*
 230                          * Allocate veneer space for each ADRP that may appear
 231                          * at a vulnerable offset nonetheless. At relocation
 232                          * time, some of these will remain unused since some
 233                          * ADRP instructions can be patched to ADR instructions
 234                          * instead.
 235                          */
 236                         if (min_align > SZ_4K)
 237                                 ret++;
 238                         else
 239                                 dstsec->sh_addralign = max(dstsec->sh_addralign,
 240                                                            min_align);
 241                         break;
 242                 }
 243         }
 244
 245         if (IS_ENABLED(CONFIG_ARM64_ERRATUM_843419) &&
 246             cpus_have_const_cap(ARM64_WORKAROUND_843419))
 247                 /*
 248                  * Add some slack so we can skip PLT slots that may trigger
 249                  * the erratum due to the placement of the ADRP instruction.
 250                  */
 251                 ret += DIV_ROUND_UP(ret, (SZ_4K / sizeof(struct plt_entry)));
 252
 253         return ret;
 254 }
 255
 256 static bool branch_rela_needs_plt(Elf64_Sym *syms, Elf64_Rela *rela,
 257                                   Elf64_Word dstidx)
 258 {
 259
 260         Elf64_Sym *s = syms + ELF64_R_SYM(rela->r_info);
 261
 262         if (s->st_shndx == dstidx)
 263                 return false;
 264
 265         return ELF64_R_TYPE(rela->r_info) == R_AARCH64_JUMP26 ||
 266                ELF64_R_TYPE(rela->r_info) == R_AARCH64_CALL26;
 267 }
 268
 269 /* Group branch PLT relas at the front end of the array. */
 270 static int partition_branch_plt_relas(Elf64_Sym *syms, Elf64_Rela *rela,
 271                                       int numrels, Elf64_Word dstidx)
 272 {
 273         int i = 0, j = numrels - 1;
 274
 275         if (!IS_ENABLED(CONFIG_RANDOMIZE_BASE))
 276                 return 0;
 277
 278         while (i < j) {
 279                 if (branch_rela_needs_plt(syms, &rela[i], dstidx))
 280                         i++;
 281                 else if (branch_rela_needs_plt(syms, &rela[j], dstidx))
 282                         swap(rela[i], rela[j]);
 283                 else
 284                         j--;
 285         }
 286
 287         return i;
 288 }
 289
 290 int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
 291                               char *secstrings, struct module *mod)
 292 {
 293         unsigned long core_plts = 0;
 294         unsigned long init_plts = 0;
 295         Elf64_Sym *syms = NULL;
 296         Elf_Shdr *pltsec, *tramp = NULL;
 297         int i;
 298
 299         /*
 300          * Find the empty .plt section so we can expand it to store the PLT
 301          * entries. Record the symtab address as well.
 302          */
 303         for (i = 0; i < ehdr->e_shnum; i++) {
 304                 if (!strcmp(secstrings + sechdrs[i].sh_name, ".plt"))
 305                         mod->arch.core.plt_shndx = i;
 306                 else if (!strcmp(secstrings + sechdrs[i].sh_name, ".init.plt"))
 307                         mod->arch.init.plt_shndx = i;
 308                 else if (!strcmp(secstrings + sechdrs[i].sh_name,
 309                                  ".text.ftrace_trampoline"))
 310                         tramp = sechdrs + i;
 311                 else if (sechdrs[i].sh_type == SHT_SYMTAB)
 312                         syms = (Elf64_Sym *)sechdrs[i].sh_addr;
 313         }
 314
 315         if (!mod->arch.core.plt_shndx || !mod->arch.init.plt_shndx) {
 316                 pr_err("%s: module PLT section(s) missing\n", mod->name);
 317                 return -ENOEXEC;
 318         }
 319         if (!syms) {
 320                 pr_err("%s: module symtab section missing\n", mod->name);
 321                 return -ENOEXEC;
 322         }
 323
 324         for (i = 0; i < ehdr->e_shnum; i++) {
 325                 Elf64_Rela *rels = (void *)ehdr + sechdrs[i].sh_offset;
 326                 int nents, numrels = sechdrs[i].sh_size / sizeof(Elf64_Rela);
 327                 Elf64_Shdr *dstsec = sechdrs + sechdrs[i].sh_info;
 328
 329                 if (sechdrs[i].sh_type != SHT_RELA)
 330                         continue;
 331
 332                 /* ignore relocations that operate on non-exec sections */
 333                 if (!(dstsec->sh_flags & SHF_EXECINSTR))
 334                         continue;
 335
 336                 /*
 337                  * sort branch relocations requiring a PLT by type, symbol index
 338                  * and addend
 339                  */
 340                 nents = partition_branch_plt_relas(syms, rels, numrels,
 341                                                    sechdrs[i].sh_info);
 342                 if (nents)
 343                         sort(rels, nents, sizeof(Elf64_Rela), cmp_rela, NULL);
 344
 345                 if (!str_has_prefix(secstrings + dstsec->sh_name, ".init"))
 346                         core_plts += count_plts(syms, rels, numrels,
 347                                                 sechdrs[i].sh_info, dstsec);
 348                 else
 349                         init_plts += count_plts(syms, rels, numrels,
 350                                                 sechdrs[i].sh_info, dstsec);
 351         }
 352
 353         pltsec = sechdrs + mod->arch.core.plt_shndx;
 354         pltsec->sh_type = SHT_NOBITS;
 355         pltsec->sh_flags = SHF_EXECINSTR | SHF_ALLOC;
 356         pltsec->sh_addralign = L1_CACHE_BYTES;
 357         pltsec->sh_size = (core_plts  + 1) * sizeof(struct plt_entry);
 358         mod->arch.core.plt_num_entries = 0;
 359         mod->arch.core.plt_max_entries = core_plts;
 360
 361         pltsec = sechdrs + mod->arch.init.plt_shndx;
 362         pltsec->sh_type = SHT_NOBITS;
 363         pltsec->sh_flags = SHF_EXECINSTR | SHF_ALLOC;
 364         pltsec->sh_addralign = L1_CACHE_BYTES;
 365         pltsec->sh_size = (init_plts + 1) * sizeof(struct plt_entry);
 366         mod->arch.init.plt_num_entries = 0;
 367         mod->arch.init.plt_max_entries = init_plts;
 368
 369         if (tramp) {
 370                 tramp->sh_type = SHT_NOBITS;
 371                 tramp->sh_flags = SHF_EXECINSTR | SHF_ALLOC;
 372                 tramp->sh_addralign = __alignof__(struct plt_entry);
 373                 tramp->sh_size = NR_FTRACE_PLTS * sizeof(struct plt_entry);
 374         }
 375
 376         return 0;
 377 }