arch/x86/include/asm/nospec-branch.h

   1 /* SPDX-License-Identifier: GPL-2.0 */
   2
   3 #ifndef _ASM_X86_NOSPEC_BRANCH_H_
   4 #define _ASM_X86_NOSPEC_BRANCH_H_
   5
   6 #include <asm/alternative.h>
   7 #include <asm/alternative-asm.h>
   8 #include <asm/cpufeatures.h>
   9 #include <asm/msr-index.h>
  10
  11 /*
  12  * Fill the CPU return stack buffer.
  13  *
  14  * Each entry in the RSB, if used for a speculative 'ret', contains an
  15  * infinite 'pause; lfence; jmp' loop to capture speculative execution.
  16  *
  17  * This is required in various cases for retpoline and IBRS-based
  18  * mitigations for the Spectre variant 2 vulnerability. Sometimes to
  19  * eliminate potentially bogus entries from the RSB, and sometimes
  20  * purely to ensure that it doesn't get empty, which on some CPUs would
  21  * allow predictions from other (unwanted!) sources to be used.
  22  *
  23  * We define a CPP macro such that it can be used from both .S files and
  24  * inline assembly. It's possible to do a .macro and then include that
  25  * from C via asm(".include <asm/nospec-branch.h>") but let's not go there.
  26  */
  27
  28 #define RSB_CLEAR_LOOPS         32      /* To forcibly overwrite all entries */
  29 #define RSB_FILL_LOOPS          16      /* To avoid underflow */
  30
  31 /*
  32  * Google experimented with loop-unrolling and this turned out to be
  33  * the optimal version — two calls, each with their own speculation
  34  * trap should their return address end up getting used, in a loop.
  35  */
  36 #define __FILL_RETURN_BUFFER(reg, nr, sp)       \
  37         mov     $(nr/2), reg;                   \
  38 771:                                            \
  39         call    772f;                           \
  40 773:    /* speculation trap */                  \
  41         pause;                                  \
  42         lfence;                                 \
  43         jmp     773b;                           \
  44 772:                                            \
  45         call    774f;                           \
  46 775:    /* speculation trap */                  \
  47         pause;                                  \
  48         lfence;                                 \
  49         jmp     775b;                           \
  50 774:                                            \
  51         dec     reg;                            \
  52         jnz     771b;                           \
  53         add     $(BITS_PER_LONG/8) * nr, sp;
  54
  55 #ifdef __ASSEMBLY__
  56
  57 /*
  58  * This should be used immediately before a retpoline alternative.  It tells
  59  * objtool where the retpolines are so that it can make sense of the control
  60  * flow by just reading the original instruction(s) and ignoring the
  61  * alternatives.
  62  */
  63 .macro ANNOTATE_NOSPEC_ALTERNATIVE
  64         .Lannotate_\@:
  65         .pushsection .discard.nospec
  66         .long .Lannotate_\@ - .
  67         .popsection
  68 .endm
  69
  70 /*
  71  * This should be used immediately before an indirect jump/call. It tells
  72  * objtool the subsequent indirect jump/call is vouched safe for retpoline
  73  * builds.
  74  */
  75 .macro ANNOTATE_RETPOLINE_SAFE
  76         .Lannotate_\@:
  77         .pushsection .discard.retpoline_safe
  78         _ASM_PTR .Lannotate_\@
  79         .popsection
  80 .endm
  81
  82 /*
  83  * These are the bare retpoline primitives for indirect jmp and call.
  84  * Do not use these directly; they only exist to make the ALTERNATIVE
  85  * invocation below less ugly.
  86  */
  87 .macro RETPOLINE_JMP reg:req
  88         call    .Ldo_rop_\@
  89 .Lspec_trap_\@:
  90         pause
  91         lfence
  92         jmp     .Lspec_trap_\@
  93 .Ldo_rop_\@:
  94         mov     \reg, (%_ASM_SP)
  95         ret
  96 .endm
  97
  98 /*
  99  * This is a wrapper around RETPOLINE_JMP so the called function in reg
 100  * returns to the instruction after the macro.
 101  */
 102 .macro RETPOLINE_CALL reg:req
 103         jmp     .Ldo_call_\@
 104 .Ldo_retpoline_jmp_\@:
 105         RETPOLINE_JMP \reg
 106 .Ldo_call_\@:
 107         call    .Ldo_retpoline_jmp_\@
 108 .endm
 109
 110 /*
 111  * JMP_NOSPEC and CALL_NOSPEC macros can be used instead of a simple
 112  * indirect jmp/call which may be susceptible to the Spectre variant 2
 113  * attack.
 114  */
 115 .macro JMP_NOSPEC reg:req
 116 #ifdef CONFIG_RETPOLINE
 117         ANNOTATE_NOSPEC_ALTERNATIVE
 118         ALTERNATIVE_2 __stringify(ANNOTATE_RETPOLINE_SAFE; jmp *\reg),  \
 119                 __stringify(RETPOLINE_JMP \reg), X86_FEATURE_RETPOLINE, \
 120                 __stringify(lfence; ANNOTATE_RETPOLINE_SAFE; jmp *\reg), X86_FEATURE_RETPOLINE_AMD
 121 #else
 122         jmp     *\reg
 123 #endif
 124 .endm
 125
 126 .macro CALL_NOSPEC reg:req
 127 #ifdef CONFIG_RETPOLINE
 128         ANNOTATE_NOSPEC_ALTERNATIVE
 129         ALTERNATIVE_2 __stringify(ANNOTATE_RETPOLINE_SAFE; call *\reg), \
 130                 __stringify(RETPOLINE_CALL \reg), X86_FEATURE_RETPOLINE,\
 131                 __stringify(lfence; ANNOTATE_RETPOLINE_SAFE; call *\reg), X86_FEATURE_RETPOLINE_AMD
 132 #else
 133         call    *\reg
 134 #endif
 135 .endm
 136
 137  /*
 138   * A simpler FILL_RETURN_BUFFER macro. Don't make people use the CPP
 139   * monstrosity above, manually.
 140   */
 141 .macro FILL_RETURN_BUFFER reg:req nr:req ftr:req
 142 #ifdef CONFIG_RETPOLINE
 143         ANNOTATE_NOSPEC_ALTERNATIVE
 144         ALTERNATIVE "jmp .Lskip_rsb_\@",                                \
 145                 __stringify(__FILL_RETURN_BUFFER(\reg,\nr,%_ASM_SP))    \
 146                 \ftr
 147 .Lskip_rsb_\@:
 148 #endif
 149 .endm
 150
 151 #else /* __ASSEMBLY__ */
 152
 153 #define ANNOTATE_NOSPEC_ALTERNATIVE                             \
 154         "999:\n\t"                                              \
 155         ".pushsection .discard.nospec\n\t"                      \
 156         ".long 999b - .\n\t"                                    \
 157         ".popsection\n\t"
 158
 159 #define ANNOTATE_RETPOLINE_SAFE                                 \
 160         "999:\n\t"                                              \
 161         ".pushsection .discard.retpoline_safe\n\t"              \
 162         _ASM_PTR " 999b\n\t"                                    \
 163         ".popsection\n\t"
 164
 165 #if defined(CONFIG_X86_64) && defined(RETPOLINE)
 166
 167 /*
 168  * Since the inline asm uses the %V modifier which is only in newer GCC,
 169  * the 64-bit one is dependent on RETPOLINE not CONFIG_RETPOLINE.
 170  */
 171 # define CALL_NOSPEC                                            \
 172         ANNOTATE_NOSPEC_ALTERNATIVE                             \
 173         ALTERNATIVE_2(                                          \
 174         ANNOTATE_RETPOLINE_SAFE                                 \
 175         "call *%[thunk_target]\n",                              \
 176         "call __x86_indirect_thunk_%V[thunk_target]\n",         \
 177         X86_FEATURE_RETPOLINE,                                  \
 178         "lfence;\n"                                             \
 179         ANNOTATE_RETPOLINE_SAFE                                 \
 180         "call *%[thunk_target]\n",                              \
 181         X86_FEATURE_RETPOLINE_AMD)
 182 # define THUNK_TARGET(addr) [thunk_target] "r" (addr)
 183
 184 #elif defined(CONFIG_X86_32) && defined(CONFIG_RETPOLINE)
 185 /*
 186  * For i386 we use the original ret-equivalent retpoline, because
 187  * otherwise we'll run out of registers. We don't care about CET
 188  * here, anyway.
 189  */
 190 # define CALL_NOSPEC                                            \
 191         ANNOTATE_NOSPEC_ALTERNATIVE                             \
 192         ALTERNATIVE_2(                                          \
 193         ANNOTATE_RETPOLINE_SAFE                                 \
 194         "call *%[thunk_target]\n",                              \
 195         "       jmp    904f;\n"                                 \
 196         "       .align 16\n"                                    \
 197         "901:   call   903f;\n"                                 \
 198         "902:   pause;\n"                                       \
 199         "       lfence;\n"                                      \
 200         "       jmp    902b;\n"                                 \
 201         "       .align 16\n"                                    \
 202         "903:   addl   $4, %%esp;\n"                            \
 203         "       pushl  %[thunk_target];\n"                      \
 204         "       ret;\n"                                         \
 205         "       .align 16\n"                                    \
 206         "904:   call   901b;\n",                                \
 207         X86_FEATURE_RETPOLINE,                                  \
 208         "lfence;\n"                                             \
 209         ANNOTATE_RETPOLINE_SAFE                                 \
 210         "call *%[thunk_target]\n",                              \
 211         X86_FEATURE_RETPOLINE_AMD)
 212
 213 # define THUNK_TARGET(addr) [thunk_target] "rm" (addr)
 214 #else /* No retpoline for C / inline asm */
 215 # define CALL_NOSPEC "call *%[thunk_target]\n"
 216 # define THUNK_TARGET(addr) [thunk_target] "rm" (addr)
 217 #endif
 218
 219 /* The Spectre V2 mitigation variants */
 220 enum spectre_v2_mitigation {
 221         SPECTRE_V2_NONE,
 222         SPECTRE_V2_RETPOLINE_MINIMAL,
 223         SPECTRE_V2_RETPOLINE_MINIMAL_AMD,
 224         SPECTRE_V2_RETPOLINE_GENERIC,
 225         SPECTRE_V2_RETPOLINE_AMD,
 226         SPECTRE_V2_IBRS_ENHANCED,
 227 };
 228
 229 /* The Speculative Store Bypass disable variants */
 230 enum ssb_mitigation {
 231         SPEC_STORE_BYPASS_NONE,
 232         SPEC_STORE_BYPASS_DISABLE,
 233         SPEC_STORE_BYPASS_PRCTL,
 234         SPEC_STORE_BYPASS_SECCOMP,
 235 };
 236
 237 extern char __indirect_thunk_start[];
 238 extern char __indirect_thunk_end[];
 239
 240 /*
 241  * On VMEXIT we must ensure that no RSB predictions learned in the guest
 242  * can be followed in the host, by overwriting the RSB completely. Both
 243  * retpoline and IBRS mitigations for Spectre v2 need this; only on future
 244  * CPUs with IBRS_ALL *might* it be avoided.
 245  */
 246 static inline void vmexit_fill_RSB(void)
 247 {
 248 #ifdef CONFIG_RETPOLINE
 249         unsigned long loops;
 250
 251         asm volatile (ANNOTATE_NOSPEC_ALTERNATIVE
 252                       ALTERNATIVE("jmp 910f",
 253                                   __stringify(__FILL_RETURN_BUFFER(%0, RSB_CLEAR_LOOPS, %1)),
 254                                   X86_FEATURE_RETPOLINE)
 255                       "910:"
 256                       : "=r" (loops), ASM_CALL_CONSTRAINT
 257                       : : "memory" );
 258 #endif
 259 }
 260
 261 static __always_inline
 262 void alternative_msr_write(unsigned int msr, u64 val, unsigned int feature)
 263 {
 264         asm volatile(ALTERNATIVE("", "wrmsr", %c[feature])
 265                 : : "c" (msr),
 266                     "a" ((u32)val),
 267                     "d" ((u32)(val >> 32)),
 268                     [feature] "i" (feature)
 269                 : "memory");
 270 }
 271
 272 static inline void indirect_branch_prediction_barrier(void)
 273 {
 274         u64 val = PRED_CMD_IBPB;
 275
 276         alternative_msr_write(MSR_IA32_PRED_CMD, val, X86_FEATURE_USE_IBPB);
 277 }
 278
 279 /* The Intel SPEC CTRL MSR base value cache */
 280 extern u64 x86_spec_ctrl_base;
 281
 282 /*
 283  * With retpoline, we must use IBRS to restrict branch prediction
 284  * before calling into firmware.
 285  *
 286  * (Implemented as CPP macros due to header hell.)
 287  */
 288 #define firmware_restrict_branch_speculation_start()                    \
 289 do {                                                                    \
 290         u64 val = x86_spec_ctrl_base | SPEC_CTRL_IBRS;                  \
 291                                                                         \
 292         preempt_disable();                                              \
 293         alternative_msr_write(MSR_IA32_SPEC_CTRL, val,                  \
 294                               X86_FEATURE_USE_IBRS_FW);                 \
 295 } while (0)
 296
 297 #define firmware_restrict_branch_speculation_end()                      \
 298 do {                                                                    \
 299         u64 val = x86_spec_ctrl_base;                                   \
 300                                                                         \
 301         alternative_msr_write(MSR_IA32_SPEC_CTRL, val,                  \
 302                               X86_FEATURE_USE_IBRS_FW);                 \
 303         preempt_enable();                                               \
 304 } while (0)
 305
 306 #endif /* __ASSEMBLY__ */
 307
 308 /*
 309  * Below is used in the eBPF JIT compiler and emits the byte sequence
 310  * for the following assembly:
 311  *
 312  * With retpolines configured:
 313  *
 314  *    callq do_rop
 315  *  spec_trap:
 316  *    pause
 317  *    lfence
 318  *    jmp spec_trap
 319  *  do_rop:
 320  *    mov %rax,(%rsp) for x86_64
 321  *    mov %edx,(%esp) for x86_32
 322  *    retq
 323  *
 324  * Without retpolines configured:
 325  *
 326  *    jmp *%rax for x86_64
 327  *    jmp *%edx for x86_32
 328  */
 329 #ifdef CONFIG_RETPOLINE
 330 # ifdef CONFIG_X86_64
 331 #  define RETPOLINE_RAX_BPF_JIT_SIZE    17
 332 #  define RETPOLINE_RAX_BPF_JIT()                               \
 333 do {                                                            \
 334         EMIT1_off32(0xE8, 7);    /* callq do_rop */             \
 335         /* spec_trap: */                                        \
 336         EMIT2(0xF3, 0x90);       /* pause */                    \
 337         EMIT3(0x0F, 0xAE, 0xE8); /* lfence */                   \
 338         EMIT2(0xEB, 0xF9);       /* jmp spec_trap */            \
 339         /* do_rop: */                                           \
 340         EMIT4(0x48, 0x89, 0x04, 0x24); /* mov %rax,(%rsp) */    \
 341         EMIT1(0xC3);             /* retq */                     \
 342 } while (0)
 343 # else /* !CONFIG_X86_64 */
 344 #  define RETPOLINE_EDX_BPF_JIT()                               \
 345 do {                                                            \
 346         EMIT1_off32(0xE8, 7);    /* call do_rop */              \
 347         /* spec_trap: */                                        \
 348         EMIT2(0xF3, 0x90);       /* pause */                    \
 349         EMIT3(0x0F, 0xAE, 0xE8); /* lfence */                   \
 350         EMIT2(0xEB, 0xF9);       /* jmp spec_trap */            \
 351         /* do_rop: */                                           \
 352         EMIT3(0x89, 0x14, 0x24); /* mov %edx,(%esp) */          \
 353         EMIT1(0xC3);             /* ret */                      \
 354 } while (0)
 355 # endif
 356 #else /* !CONFIG_RETPOLINE */
 357 # ifdef CONFIG_X86_64
 358 #  define RETPOLINE_RAX_BPF_JIT_SIZE    2
 359 #  define RETPOLINE_RAX_BPF_JIT()                               \
 360         EMIT2(0xFF, 0xE0);       /* jmp *%rax */
 361 # else /* !CONFIG_X86_64 */
 362 #  define RETPOLINE_EDX_BPF_JIT()                               \
 363         EMIT2(0xFF, 0xE2)        /* jmp *%edx */
 364 # endif
 365 #endif
 366
 367 #endif /* _ASM_X86_NOSPEC_BRANCH_H_ */