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 #include <linux/objtool.h>
   8 #include <linux/linkage.h>
   9
  10 #include <asm/alternative.h>
  11 #include <asm/cpufeatures.h>
  12 #include <asm/msr-index.h>
  13 #include <asm/unwind_hints.h>
  14
  15 #define RETPOLINE_THUNK_SIZE    32
  16
  17 /*
  18  * Fill the CPU return stack buffer.
  19  *
  20  * Each entry in the RSB, if used for a speculative 'ret', contains an
  21  * infinite 'pause; lfence; jmp' loop to capture speculative execution.
  22  *
  23  * This is required in various cases for retpoline and IBRS-based
  24  * mitigations for the Spectre variant 2 vulnerability. Sometimes to
  25  * eliminate potentially bogus entries from the RSB, and sometimes
  26  * purely to ensure that it doesn't get empty, which on some CPUs would
  27  * allow predictions from other (unwanted!) sources to be used.
  28  *
  29  * We define a CPP macro such that it can be used from both .S files and
  30  * inline assembly. It's possible to do a .macro and then include that
  31  * from C via asm(".include <asm/nospec-branch.h>") but let's not go there.
  32  */
  33
  34 #define RSB_CLEAR_LOOPS         32      /* To forcibly overwrite all entries */
  35
  36 /*
  37  * Google experimented with loop-unrolling and this turned out to be
  38  * the optimal version - two calls, each with their own speculation
  39  * trap should their return address end up getting used, in a loop.
  40  */
  41 #define __FILL_RETURN_BUFFER(reg, nr, sp)       \
  42         mov     $(nr/2), reg;                   \
  43 771:                                            \
  44         ANNOTATE_INTRA_FUNCTION_CALL;           \
  45         call    772f;                           \
  46 773:    /* speculation trap */                  \
  47         UNWIND_HINT_EMPTY;                      \
  48         pause;                                  \
  49         lfence;                                 \
  50         jmp     773b;                           \
  51 772:                                            \
  52         ANNOTATE_INTRA_FUNCTION_CALL;           \
  53         call    774f;                           \
  54 775:    /* speculation trap */                  \
  55         UNWIND_HINT_EMPTY;                      \
  56         pause;                                  \
  57         lfence;                                 \
  58         jmp     775b;                           \
  59 774:                                            \
  60         add     $(BITS_PER_LONG/8) * 2, sp;     \
  61         dec     reg;                            \
  62         jnz     771b;
  63
  64 #ifdef __ASSEMBLY__
  65
  66 /*
  67  * This should be used immediately before an indirect jump/call. It tells
  68  * objtool the subsequent indirect jump/call is vouched safe for retpoline
  69  * builds.
  70  */
  71 .macro ANNOTATE_RETPOLINE_SAFE
  72         .Lannotate_\@:
  73         .pushsection .discard.retpoline_safe
  74         _ASM_PTR .Lannotate_\@
  75         .popsection
  76 .endm
  77
  78 /*
  79  * (ab)use RETPOLINE_SAFE on RET to annotate away 'bare' RET instructions
  80  * vs RETBleed validation.
  81  */
  82 #define ANNOTATE_UNRET_SAFE ANNOTATE_RETPOLINE_SAFE
  83
  84 /*
  85  * Abuse ANNOTATE_RETPOLINE_SAFE on a NOP to indicate UNRET_END, should
  86  * eventually turn into it's own annotation.
  87  */
  88 .macro ANNOTATE_UNRET_END
  89 #ifdef CONFIG_DEBUG_ENTRY
  90         ANNOTATE_RETPOLINE_SAFE
  91         nop
  92 #endif
  93 .endm
  94
  95 /*
  96  * JMP_NOSPEC and CALL_NOSPEC macros can be used instead of a simple
  97  * indirect jmp/call which may be susceptible to the Spectre variant 2
  98  * attack.
  99  */
 100 .macro JMP_NOSPEC reg:req
 101 #ifdef CONFIG_RETPOLINE
 102         ALTERNATIVE_2 __stringify(ANNOTATE_RETPOLINE_SAFE; jmp *%\reg), \
 103                       __stringify(jmp __x86_indirect_thunk_\reg), X86_FEATURE_RETPOLINE, \
 104                       __stringify(lfence; ANNOTATE_RETPOLINE_SAFE; jmp *%\reg), X86_FEATURE_RETPOLINE_LFENCE
 105 #else
 106         jmp     *%\reg
 107 #endif
 108 .endm
 109
 110 .macro CALL_NOSPEC reg:req
 111 #ifdef CONFIG_RETPOLINE
 112         ALTERNATIVE_2 __stringify(ANNOTATE_RETPOLINE_SAFE; call *%\reg), \
 113                       __stringify(call __x86_indirect_thunk_\reg), X86_FEATURE_RETPOLINE, \
 114                       __stringify(lfence; ANNOTATE_RETPOLINE_SAFE; call *%\reg), X86_FEATURE_RETPOLINE_LFENCE
 115 #else
 116         call    *%\reg
 117 #endif
 118 .endm
 119
 120  /*
 121   * A simpler FILL_RETURN_BUFFER macro. Don't make people use the CPP
 122   * monstrosity above, manually.
 123   */
 124 .macro FILL_RETURN_BUFFER reg:req nr:req ftr:req
 125         ALTERNATIVE "jmp .Lskip_rsb_\@", "", \ftr
 126         __FILL_RETURN_BUFFER(\reg,\nr,%_ASM_SP)
 127 .Lskip_rsb_\@:
 128 .endm
 129
 130 /*
 131  * Mitigate RETBleed for AMD/Hygon Zen uarch. Requires KERNEL CR3 because the
 132  * return thunk isn't mapped into the userspace tables (then again, AMD
 133  * typically has NO_MELTDOWN).
 134  *
 135  * While zen_untrain_ret() doesn't clobber anything but requires stack,
 136  * entry_ibpb() will clobber AX, CX, DX.
 137  *
 138  * As such, this must be placed after every *SWITCH_TO_KERNEL_CR3 at a point
 139  * where we have a stack but before any RET instruction.
 140  */
 141 .macro UNTRAIN_RET
 142 #ifdef CONFIG_RETPOLINE
 143         ANNOTATE_UNRET_END
 144         ALTERNATIVE_2 "",                                               \
 145                       "call zen_untrain_ret", X86_FEATURE_UNRET,        \
 146                       "call entry_ibpb", X86_FEATURE_ENTRY_IBPB
 147 #endif
 148 .endm
 149
 150 #else /* __ASSEMBLY__ */
 151
 152 #define ANNOTATE_RETPOLINE_SAFE                                 \
 153         "999:\n\t"                                              \
 154         ".pushsection .discard.retpoline_safe\n\t"              \
 155         _ASM_PTR " 999b\n\t"                                    \
 156         ".popsection\n\t"
 157
 158 typedef u8 retpoline_thunk_t[RETPOLINE_THUNK_SIZE];
 159 extern retpoline_thunk_t __x86_indirect_thunk_array[];
 160
 161 extern void __x86_return_thunk(void);
 162 extern void zen_untrain_ret(void);
 163 extern void entry_ibpb(void);
 164
 165 #ifdef CONFIG_RETPOLINE
 166
 167 #define GEN(reg) \
 168         extern retpoline_thunk_t __x86_indirect_thunk_ ## reg;
 169 #include <asm/GEN-for-each-reg.h>
 170 #undef GEN
 171
 172 #ifdef CONFIG_X86_64
 173
 174 /*
 175  * Inline asm uses the %V modifier which is only in newer GCC
 176  * which is ensured when CONFIG_RETPOLINE is defined.
 177  */
 178 # define CALL_NOSPEC                                            \
 179         ALTERNATIVE_2(                                          \
 180         ANNOTATE_RETPOLINE_SAFE                                 \
 181         "call *%[thunk_target]\n",                              \
 182         "call __x86_indirect_thunk_%V[thunk_target]\n",         \
 183         X86_FEATURE_RETPOLINE,                                  \
 184         "lfence;\n"                                             \
 185         ANNOTATE_RETPOLINE_SAFE                                 \
 186         "call *%[thunk_target]\n",                              \
 187         X86_FEATURE_RETPOLINE_LFENCE)
 188
 189 # define THUNK_TARGET(addr) [thunk_target] "r" (addr)
 190
 191 #else /* CONFIG_X86_32 */
 192 /*
 193  * For i386 we use the original ret-equivalent retpoline, because
 194  * otherwise we'll run out of registers. We don't care about CET
 195  * here, anyway.
 196  */
 197 # define CALL_NOSPEC                                            \
 198         ALTERNATIVE_2(                                          \
 199         ANNOTATE_RETPOLINE_SAFE                                 \
 200         "call *%[thunk_target]\n",                              \
 201         "       jmp    904f;\n"                                 \
 202         "       .align 16\n"                                    \
 203         "901:   call   903f;\n"                                 \
 204         "902:   pause;\n"                                       \
 205         "       lfence;\n"                                      \
 206         "       jmp    902b;\n"                                 \
 207         "       .align 16\n"                                    \
 208         "903:   lea    4(%%esp), %%esp;\n"                      \
 209         "       pushl  %[thunk_target];\n"                      \
 210         "       ret;\n"                                         \
 211         "       .align 16\n"                                    \
 212         "904:   call   901b;\n",                                \
 213         X86_FEATURE_RETPOLINE,                                  \
 214         "lfence;\n"                                             \
 215         ANNOTATE_RETPOLINE_SAFE                                 \
 216         "call *%[thunk_target]\n",                              \
 217         X86_FEATURE_RETPOLINE_LFENCE)
 218
 219 # define THUNK_TARGET(addr) [thunk_target] "rm" (addr)
 220 #endif
 221 #else /* No retpoline for C / inline asm */
 222 # define CALL_NOSPEC "call *%[thunk_target]\n"
 223 # define THUNK_TARGET(addr) [thunk_target] "rm" (addr)
 224 #endif
 225
 226 /* The Spectre V2 mitigation variants */
 227 enum spectre_v2_mitigation {
 228         SPECTRE_V2_NONE,
 229         SPECTRE_V2_RETPOLINE,
 230         SPECTRE_V2_LFENCE,
 231         SPECTRE_V2_EIBRS,
 232         SPECTRE_V2_EIBRS_RETPOLINE,
 233         SPECTRE_V2_EIBRS_LFENCE,
 234         SPECTRE_V2_IBRS,
 235 };
 236
 237 /* The indirect branch speculation control variants */
 238 enum spectre_v2_user_mitigation {
 239         SPECTRE_V2_USER_NONE,
 240         SPECTRE_V2_USER_STRICT,
 241         SPECTRE_V2_USER_STRICT_PREFERRED,
 242         SPECTRE_V2_USER_PRCTL,
 243         SPECTRE_V2_USER_SECCOMP,
 244 };
 245
 246 /* The Speculative Store Bypass disable variants */
 247 enum ssb_mitigation {
 248         SPEC_STORE_BYPASS_NONE,
 249         SPEC_STORE_BYPASS_DISABLE,
 250         SPEC_STORE_BYPASS_PRCTL,
 251         SPEC_STORE_BYPASS_SECCOMP,
 252 };
 253
 254 extern char __indirect_thunk_start[];
 255 extern char __indirect_thunk_end[];
 256
 257 static __always_inline
 258 void alternative_msr_write(unsigned int msr, u64 val, unsigned int feature)
 259 {
 260         asm volatile(ALTERNATIVE("", "wrmsr", %c[feature])
 261                 : : "c" (msr),
 262                     "a" ((u32)val),
 263                     "d" ((u32)(val >> 32)),
 264                     [feature] "i" (feature)
 265                 : "memory");
 266 }
 267
 268 static inline void indirect_branch_prediction_barrier(void)
 269 {
 270         u64 val = PRED_CMD_IBPB;
 271
 272         alternative_msr_write(MSR_IA32_PRED_CMD, val, X86_FEATURE_USE_IBPB);
 273 }
 274
 275 /* The Intel SPEC CTRL MSR base value cache */
 276 extern u64 x86_spec_ctrl_base;
 277 extern void write_spec_ctrl_current(u64 val, bool force);
 278 extern u64 spec_ctrl_current(void);
 279
 280 /*
 281  * With retpoline, we must use IBRS to restrict branch prediction
 282  * before calling into firmware.
 283  *
 284  * (Implemented as CPP macros due to header hell.)
 285  */
 286 #define firmware_restrict_branch_speculation_start()                    \
 287 do {                                                                    \
 288         preempt_disable();                                              \
 289         alternative_msr_write(MSR_IA32_SPEC_CTRL,                       \
 290                               spec_ctrl_current() | SPEC_CTRL_IBRS,     \
 291                               X86_FEATURE_USE_IBRS_FW);                 \
 292 } while (0)
 293
 294 #define firmware_restrict_branch_speculation_end()                      \
 295 do {                                                                    \
 296         alternative_msr_write(MSR_IA32_SPEC_CTRL,                       \
 297                               spec_ctrl_current(),                      \
 298                               X86_FEATURE_USE_IBRS_FW);                 \
 299         preempt_enable();                                               \
 300 } while (0)
 301
 302 DECLARE_STATIC_KEY_FALSE(switch_to_cond_stibp);
 303 DECLARE_STATIC_KEY_FALSE(switch_mm_cond_ibpb);
 304 DECLARE_STATIC_KEY_FALSE(switch_mm_always_ibpb);
 305
 306 DECLARE_STATIC_KEY_FALSE(mds_user_clear);
 307 DECLARE_STATIC_KEY_FALSE(mds_idle_clear);
 308
 309 DECLARE_STATIC_KEY_FALSE(switch_mm_cond_l1d_flush);
 310
 311 DECLARE_STATIC_KEY_FALSE(mmio_stale_data_clear);
 312
 313 #include <asm/segment.h>
 314
 315 /**
 316  * mds_clear_cpu_buffers - Mitigation for MDS and TAA vulnerability
 317  *
 318  * This uses the otherwise unused and obsolete VERW instruction in
 319  * combination with microcode which triggers a CPU buffer flush when the
 320  * instruction is executed.
 321  */
 322 static __always_inline void mds_clear_cpu_buffers(void)
 323 {
 324         static const u16 ds = __KERNEL_DS;
 325
 326         /*
 327          * Has to be the memory-operand variant because only that
 328          * guarantees the CPU buffer flush functionality according to
 329          * documentation. The register-operand variant does not.
 330          * Works with any segment selector, but a valid writable
 331          * data segment is the fastest variant.
 332          *
 333          * "cc" clobber is required because VERW modifies ZF.
 334          */
 335         asm volatile("verw %[ds]" : : [ds] "m" (ds) : "cc");
 336 }
 337
 338 /**
 339  * mds_user_clear_cpu_buffers - Mitigation for MDS and TAA vulnerability
 340  *
 341  * Clear CPU buffers if the corresponding static key is enabled
 342  */
 343 static __always_inline void mds_user_clear_cpu_buffers(void)
 344 {
 345         if (static_branch_likely(&mds_user_clear))
 346                 mds_clear_cpu_buffers();
 347 }
 348
 349 /**
 350  * mds_idle_clear_cpu_buffers - Mitigation for MDS vulnerability
 351  *
 352  * Clear CPU buffers if the corresponding static key is enabled
 353  */
 354 static inline void mds_idle_clear_cpu_buffers(void)
 355 {
 356         if (static_branch_likely(&mds_idle_clear))
 357                 mds_clear_cpu_buffers();
 358 }
 359
 360 #endif /* __ASSEMBLY__ */
 361
 362 #endif /* _ASM_X86_NOSPEC_BRANCH_H_ */