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 <linux/static_key.h>
   7
   8 #include <asm/alternative.h>
   9 #include <asm/alternative-asm.h>
  10 #include <asm/cpufeatures.h>
  11 #include <asm/msr-index.h>
  12
  13 /*
  14  * Fill the CPU return stack buffer.
  15  *
  16  * Each entry in the RSB, if used for a speculative 'ret', contains an
  17  * infinite 'pause; lfence; jmp' loop to capture speculative execution.
  18  *
  19  * This is required in various cases for retpoline and IBRS-based
  20  * mitigations for the Spectre variant 2 vulnerability. Sometimes to
  21  * eliminate potentially bogus entries from the RSB, and sometimes
  22  * purely to ensure that it doesn't get empty, which on some CPUs would
  23  * allow predictions from other (unwanted!) sources to be used.
  24  *
  25  * We define a CPP macro such that it can be used from both .S files and
  26  * inline assembly. It's possible to do a .macro and then include that
  27  * from C via asm(".include <asm/nospec-branch.h>") but let's not go there.
  28  */
  29
  30 #define RSB_CLEAR_LOOPS         32      /* To forcibly overwrite all entries */
  31 #define RSB_FILL_LOOPS          16      /* To avoid underflow */
  32
  33 /*
  34  * Google experimented with loop-unrolling and this turned out to be
  35  * the optimal version — two calls, each with their own speculation
  36  * trap should their return address end up getting used, in a loop.
  37  */
  38 #define __FILL_RETURN_BUFFER(reg, nr, sp)       \
  39         mov     $(nr/2), reg;                   \
  40 771:                                            \
  41         call    772f;                           \
  42 773:    /* speculation trap */                  \
  43         pause;                                  \
  44         lfence;                                 \
  45         jmp     773b;                           \
  46 772:                                            \
  47         call    774f;                           \
  48 775:    /* speculation trap */                  \
  49         pause;                                  \
  50         lfence;                                 \
  51         jmp     775b;                           \
  52 774:                                            \
  53         dec     reg;                            \
  54         jnz     771b;                           \
  55         add     $(BITS_PER_LONG/8) * nr, sp;
  56
  57 #ifdef __ASSEMBLY__
  58
  59 /*
  60  * This should be used immediately before a retpoline alternative.  It tells
  61  * objtool where the retpolines are so that it can make sense of the control
  62  * flow by just reading the original instruction(s) and ignoring the
  63  * alternatives.
  64  */
  65 .macro ANNOTATE_NOSPEC_ALTERNATIVE
  66         .Lannotate_\@:
  67         .pushsection .discard.nospec
  68         .long .Lannotate_\@ - .
  69         .popsection
  70 .endm
  71
  72 /*
  73  * This should be used immediately before an indirect jump/call. It tells
  74  * objtool the subsequent indirect jump/call is vouched safe for retpoline
  75  * builds.
  76  */
  77 .macro ANNOTATE_RETPOLINE_SAFE
  78         .Lannotate_\@:
  79         .pushsection .discard.retpoline_safe
  80         _ASM_PTR .Lannotate_\@
  81         .popsection
  82 .endm
  83
  84 /*
  85  * These are the bare retpoline primitives for indirect jmp and call.
  86  * Do not use these directly; they only exist to make the ALTERNATIVE
  87  * invocation below less ugly.
  88  */
  89 .macro RETPOLINE_JMP reg:req
  90         call    .Ldo_rop_\@
  91 .Lspec_trap_\@:
  92         pause
  93         lfence
  94         jmp     .Lspec_trap_\@
  95 .Ldo_rop_\@:
  96         mov     \reg, (%_ASM_SP)
  97         ret
  98 .endm
  99
 100 /*
 101  * This is a wrapper around RETPOLINE_JMP so the called function in reg
 102  * returns to the instruction after the macro.
 103  */
 104 .macro RETPOLINE_CALL reg:req
 105         jmp     .Ldo_call_\@
 106 .Ldo_retpoline_jmp_\@:
 107         RETPOLINE_JMP \reg
 108 .Ldo_call_\@:
 109         call    .Ldo_retpoline_jmp_\@
 110 .endm
 111
 112 /*
 113  * JMP_NOSPEC and CALL_NOSPEC macros can be used instead of a simple
 114  * indirect jmp/call which may be susceptible to the Spectre variant 2
 115  * attack.
 116  */
 117 .macro JMP_NOSPEC reg:req
 118 #ifdef CONFIG_RETPOLINE
 119         ANNOTATE_NOSPEC_ALTERNATIVE
 120         ALTERNATIVE_2 __stringify(ANNOTATE_RETPOLINE_SAFE; jmp *\reg),  \
 121                 __stringify(RETPOLINE_JMP \reg), X86_FEATURE_RETPOLINE, \
 122                 __stringify(lfence; ANNOTATE_RETPOLINE_SAFE; jmp *\reg), X86_FEATURE_RETPOLINE_AMD
 123 #else
 124         jmp     *\reg
 125 #endif
 126 .endm
 127
 128 .macro CALL_NOSPEC reg:req
 129 #ifdef CONFIG_RETPOLINE
 130         ANNOTATE_NOSPEC_ALTERNATIVE
 131         ALTERNATIVE_2 __stringify(ANNOTATE_RETPOLINE_SAFE; call *\reg), \
 132                 __stringify(RETPOLINE_CALL \reg), X86_FEATURE_RETPOLINE,\
 133                 __stringify(lfence; ANNOTATE_RETPOLINE_SAFE; call *\reg), X86_FEATURE_RETPOLINE_AMD
 134 #else
 135         call    *\reg
 136 #endif
 137 .endm
 138
 139  /*
 140   * A simpler FILL_RETURN_BUFFER macro. Don't make people use the CPP
 141   * monstrosity above, manually.
 142   */
 143 .macro FILL_RETURN_BUFFER reg:req nr:req ftr:req
 144 #ifdef CONFIG_RETPOLINE
 145         ANNOTATE_NOSPEC_ALTERNATIVE
 146         ALTERNATIVE "jmp .Lskip_rsb_\@",                                \
 147                 __stringify(__FILL_RETURN_BUFFER(\reg,\nr,%_ASM_SP))    \
 148                 \ftr
 149 .Lskip_rsb_\@:
 150 #endif
 151 .endm
 152
 153 #else /* __ASSEMBLY__ */
 154
 155 #define ANNOTATE_NOSPEC_ALTERNATIVE                             \
 156         "999:\n\t"                                              \
 157         ".pushsection .discard.nospec\n\t"                      \
 158         ".long 999b - .\n\t"                                    \
 159         ".popsection\n\t"
 160
 161 #define ANNOTATE_RETPOLINE_SAFE                                 \
 162         "999:\n\t"                                              \
 163         ".pushsection .discard.retpoline_safe\n\t"              \
 164         _ASM_PTR " 999b\n\t"                                    \
 165         ".popsection\n\t"
 166
 167 #ifdef CONFIG_RETPOLINE
 168 #ifdef CONFIG_X86_64
 169
 170 /*
 171  * Inline asm uses the %V modifier which is only in newer GCC
 172  * which is ensured when CONFIG_RETPOLINE is defined.
 173  */
 174 # define CALL_NOSPEC                                            \
 175         ANNOTATE_NOSPEC_ALTERNATIVE                             \
 176         ALTERNATIVE_2(                                          \
 177         ANNOTATE_RETPOLINE_SAFE                                 \
 178         "call *%[thunk_target]\n",                              \
 179         "call __x86_indirect_thunk_%V[thunk_target]\n",         \
 180         X86_FEATURE_RETPOLINE,                                  \
 181         "lfence;\n"                                             \
 182         ANNOTATE_RETPOLINE_SAFE                                 \
 183         "call *%[thunk_target]\n",                              \
 184         X86_FEATURE_RETPOLINE_AMD)
 185 # define THUNK_TARGET(addr) [thunk_target] "r" (addr)
 186
 187 #else /* CONFIG_X86_32 */
 188 /*
 189  * For i386 we use the original ret-equivalent retpoline, because
 190  * otherwise we'll run out of registers. We don't care about CET
 191  * here, anyway.
 192  */
 193 # define CALL_NOSPEC                                            \
 194         ANNOTATE_NOSPEC_ALTERNATIVE                             \
 195         ALTERNATIVE_2(                                          \
 196         ANNOTATE_RETPOLINE_SAFE                                 \
 197         "call *%[thunk_target]\n",                              \
 198         "       jmp    904f;\n"                                 \
 199         "       .align 16\n"                                    \
 200         "901:   call   903f;\n"                                 \
 201         "902:   pause;\n"                                       \
 202         "       lfence;\n"                                      \
 203         "       jmp    902b;\n"                                 \
 204         "       .align 16\n"                                    \
 205         "903:   addl   $4, %%esp;\n"                            \
 206         "       pushl  %[thunk_target];\n"                      \
 207         "       ret;\n"                                         \
 208         "       .align 16\n"                                    \
 209         "904:   call   901b;\n",                                \
 210         X86_FEATURE_RETPOLINE,                                  \
 211         "lfence;\n"                                             \
 212         ANNOTATE_RETPOLINE_SAFE                                 \
 213         "call *%[thunk_target]\n",                              \
 214         X86_FEATURE_RETPOLINE_AMD)
 215
 216 # define THUNK_TARGET(addr) [thunk_target] "rm" (addr)
 217 #endif
 218 #else /* No retpoline for C / inline asm */
 219 # define CALL_NOSPEC "call *%[thunk_target]\n"
 220 # define THUNK_TARGET(addr) [thunk_target] "rm" (addr)
 221 #endif
 222
 223 /* The Spectre V2 mitigation variants */
 224 enum spectre_v2_mitigation {
 225         SPECTRE_V2_NONE,
 226         SPECTRE_V2_RETPOLINE_GENERIC,
 227         SPECTRE_V2_RETPOLINE_AMD,
 228         SPECTRE_V2_IBRS_ENHANCED,
 229 };
 230
 231 /* The indirect branch speculation control variants */
 232 enum spectre_v2_user_mitigation {
 233         SPECTRE_V2_USER_NONE,
 234         SPECTRE_V2_USER_STRICT,
 235         SPECTRE_V2_USER_STRICT_PREFERRED,
 236         SPECTRE_V2_USER_PRCTL,
 237         SPECTRE_V2_USER_SECCOMP,
 238 };
 239
 240 /* The Speculative Store Bypass disable variants */
 241 enum ssb_mitigation {
 242         SPEC_STORE_BYPASS_NONE,
 243         SPEC_STORE_BYPASS_DISABLE,
 244         SPEC_STORE_BYPASS_PRCTL,
 245         SPEC_STORE_BYPASS_SECCOMP,
 246 };
 247
 248 extern char __indirect_thunk_start[];
 249 extern char __indirect_thunk_end[];
 250
 251 /*
 252  * On VMEXIT we must ensure that no RSB predictions learned in the guest
 253  * can be followed in the host, by overwriting the RSB completely. Both
 254  * retpoline and IBRS mitigations for Spectre v2 need this; only on future
 255  * CPUs with IBRS_ALL *might* it be avoided.
 256  */
 257 static inline void vmexit_fill_RSB(void)
 258 {
 259 #ifdef CONFIG_RETPOLINE
 260         unsigned long loops;
 261
 262         asm volatile (ANNOTATE_NOSPEC_ALTERNATIVE
 263                       ALTERNATIVE("jmp 910f",
 264                                   __stringify(__FILL_RETURN_BUFFER(%0, RSB_CLEAR_LOOPS, %1)),
 265                                   X86_FEATURE_RETPOLINE)
 266                       "910:"
 267                       : "=r" (loops), ASM_CALL_CONSTRAINT
 268                       : : "memory" );
 269 #endif
 270 }
 271
 272 static __always_inline
 273 void alternative_msr_write(unsigned int msr, u64 val, unsigned int feature)
 274 {
 275         asm volatile(ALTERNATIVE("", "wrmsr", %c[feature])
 276                 : : "c" (msr),
 277                     "a" ((u32)val),
 278                     "d" ((u32)(val >> 32)),
 279                     [feature] "i" (feature)
 280                 : "memory");
 281 }
 282
 283 static inline void indirect_branch_prediction_barrier(void)
 284 {
 285         u64 val = PRED_CMD_IBPB;
 286
 287         alternative_msr_write(MSR_IA32_PRED_CMD, val, X86_FEATURE_USE_IBPB);
 288 }
 289
 290 /* The Intel SPEC CTRL MSR base value cache */
 291 extern u64 x86_spec_ctrl_base;
 292
 293 /*
 294  * With retpoline, we must use IBRS to restrict branch prediction
 295  * before calling into firmware.
 296  *
 297  * (Implemented as CPP macros due to header hell.)
 298  */
 299 #define firmware_restrict_branch_speculation_start()                    \
 300 do {                                                                    \
 301         u64 val = x86_spec_ctrl_base | SPEC_CTRL_IBRS;                  \
 302                                                                         \
 303         preempt_disable();                                              \
 304         alternative_msr_write(MSR_IA32_SPEC_CTRL, val,                  \
 305                               X86_FEATURE_USE_IBRS_FW);                 \
 306 } while (0)
 307
 308 #define firmware_restrict_branch_speculation_end()                      \
 309 do {                                                                    \
 310         u64 val = x86_spec_ctrl_base;                                   \
 311                                                                         \
 312         alternative_msr_write(MSR_IA32_SPEC_CTRL, val,                  \
 313                               X86_FEATURE_USE_IBRS_FW);                 \
 314         preempt_enable();                                               \
 315 } while (0)
 316
 317 DECLARE_STATIC_KEY_FALSE(switch_to_cond_stibp);
 318 DECLARE_STATIC_KEY_FALSE(switch_mm_cond_ibpb);
 319 DECLARE_STATIC_KEY_FALSE(switch_mm_always_ibpb);
 320
 321 #endif /* __ASSEMBLY__ */
 322
 323 /*
 324  * Below is used in the eBPF JIT compiler and emits the byte sequence
 325  * for the following assembly:
 326  *
 327  * With retpolines configured:
 328  *
 329  *    callq do_rop
 330  *  spec_trap:
 331  *    pause
 332  *    lfence
 333  *    jmp spec_trap
 334  *  do_rop:
 335  *    mov %rax,(%rsp)
 336  *    retq
 337  *
 338  * Without retpolines configured:
 339  *
 340  *    jmp *%rax
 341  */
 342 #ifdef CONFIG_RETPOLINE
 343 # define RETPOLINE_RAX_BPF_JIT_SIZE     17
 344 # define RETPOLINE_RAX_BPF_JIT()                                \
 345         EMIT1_off32(0xE8, 7);    /* callq do_rop */             \
 346         /* spec_trap: */                                        \
 347         EMIT2(0xF3, 0x90);       /* pause */                    \
 348         EMIT3(0x0F, 0xAE, 0xE8); /* lfence */                   \
 349         EMIT2(0xEB, 0xF9);       /* jmp spec_trap */            \
 350         /* do_rop: */                                           \
 351         EMIT4(0x48, 0x89, 0x04, 0x24); /* mov %rax,(%rsp) */    \
 352         EMIT1(0xC3);             /* retq */
 353 #else
 354 # define RETPOLINE_RAX_BPF_JIT_SIZE     2
 355 # define RETPOLINE_RAX_BPF_JIT()                                \
 356         EMIT2(0xFF, 0xE0);       /* jmp *%rax */
 357 #endif
 358
 359 #endif /* _ASM_X86_NOSPEC_BRANCH_H_ */