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

   1 /* SPDX-License-Identifier: GPL-2.0 */
   2
   3 #ifndef __NOSPEC_BRANCH_H__
   4 #define __NOSPEC_BRANCH_H__
   5
   6 #include <asm/alternative.h>
   7 #include <asm/alternative-asm.h>
   8 #include <asm/cpufeatures.h>
   9
  10 /*
  11  * Fill the CPU return stack buffer.
  12  *
  13  * Each entry in the RSB, if used for a speculative 'ret', contains an
  14  * infinite 'pause; lfence; jmp' loop to capture speculative execution.
  15  *
  16  * This is required in various cases for retpoline and IBRS-based
  17  * mitigations for the Spectre variant 2 vulnerability. Sometimes to
  18  * eliminate potentially bogus entries from the RSB, and sometimes
  19  * purely to ensure that it doesn't get empty, which on some CPUs would
  20  * allow predictions from other (unwanted!) sources to be used.
  21  *
  22  * We define a CPP macro such that it can be used from both .S files and
  23  * inline assembly. It's possible to do a .macro and then include that
  24  * from C via asm(".include <asm/nospec-branch.h>") but let's not go there.
  25  */
  26
  27 #define RSB_CLEAR_LOOPS         32      /* To forcibly overwrite all entries */
  28 #define RSB_FILL_LOOPS          16      /* To avoid underflow */
  29
  30 /*
  31  * Google experimented with loop-unrolling and this turned out to be
  32  * the optimal version — two calls, each with their own speculation
  33  * trap should their return address end up getting used, in a loop.
  34  */
  35 #define __FILL_RETURN_BUFFER(reg, nr, sp)       \
  36         mov     $(nr/2), reg;                   \
  37 771:                                            \
  38         call    772f;                           \
  39 773:    /* speculation trap */                  \
  40         pause;                                  \
  41         lfence;                                 \
  42         jmp     773b;                           \
  43 772:                                            \
  44         call    774f;                           \
  45 775:    /* speculation trap */                  \
  46         pause;                                  \
  47         lfence;                                 \
  48         jmp     775b;                           \
  49 774:                                            \
  50         dec     reg;                            \
  51         jnz     771b;                           \
  52         add     $(BITS_PER_LONG/8) * nr, sp;
  53
  54 #ifdef __ASSEMBLY__
  55
  56 /*
  57  * This should be used immediately before a retpoline alternative.  It tells
  58  * objtool where the retpolines are so that it can make sense of the control
  59  * flow by just reading the original instruction(s) and ignoring the
  60  * alternatives.
  61  */
  62 .macro ANNOTATE_NOSPEC_ALTERNATIVE
  63         .Lannotate_\@:
  64         .pushsection .discard.nospec
  65         .long .Lannotate_\@ - .
  66         .popsection
  67 .endm
  68
  69 /*
  70  * These are the bare retpoline primitives for indirect jmp and call.
  71  * Do not use these directly; they only exist to make the ALTERNATIVE
  72  * invocation below less ugly.
  73  */
  74 .macro RETPOLINE_JMP reg:req
  75         call    .Ldo_rop_\@
  76 .Lspec_trap_\@:
  77         pause
  78         lfence
  79         jmp     .Lspec_trap_\@
  80 .Ldo_rop_\@:
  81         mov     \reg, (%_ASM_SP)
  82         ret
  83 .endm
  84
  85 /*
  86  * This is a wrapper around RETPOLINE_JMP so the called function in reg
  87  * returns to the instruction after the macro.
  88  */
  89 .macro RETPOLINE_CALL reg:req
  90         jmp     .Ldo_call_\@
  91 .Ldo_retpoline_jmp_\@:
  92         RETPOLINE_JMP \reg
  93 .Ldo_call_\@:
  94         call    .Ldo_retpoline_jmp_\@
  95 .endm
  96
  97 /*
  98  * JMP_NOSPEC and CALL_NOSPEC macros can be used instead of a simple
  99  * indirect jmp/call which may be susceptible to the Spectre variant 2
 100  * attack.
 101  */
 102 .macro JMP_NOSPEC reg:req
 103 #ifdef CONFIG_RETPOLINE
 104         ANNOTATE_NOSPEC_ALTERNATIVE
 105         ALTERNATIVE_2 __stringify(jmp *\reg),                           \
 106                 __stringify(RETPOLINE_JMP \reg), X86_FEATURE_RETPOLINE, \
 107                 __stringify(lfence; jmp *\reg), X86_FEATURE_RETPOLINE_AMD
 108 #else
 109         jmp     *\reg
 110 #endif
 111 .endm
 112
 113 .macro CALL_NOSPEC reg:req
 114 #ifdef CONFIG_RETPOLINE
 115         ANNOTATE_NOSPEC_ALTERNATIVE
 116         ALTERNATIVE_2 __stringify(call *\reg),                          \
 117                 __stringify(RETPOLINE_CALL \reg), X86_FEATURE_RETPOLINE,\
 118                 __stringify(lfence; call *\reg), X86_FEATURE_RETPOLINE_AMD
 119 #else
 120         call    *\reg
 121 #endif
 122 .endm
 123
 124  /*
 125   * A simpler FILL_RETURN_BUFFER macro. Don't make people use the CPP
 126   * monstrosity above, manually.
 127   */
 128 .macro FILL_RETURN_BUFFER reg:req nr:req ftr:req
 129 #ifdef CONFIG_RETPOLINE
 130         ANNOTATE_NOSPEC_ALTERNATIVE
 131         ALTERNATIVE "jmp .Lskip_rsb_\@",                                \
 132                 __stringify(__FILL_RETURN_BUFFER(\reg,\nr,%_ASM_SP))    \
 133                 \ftr
 134 .Lskip_rsb_\@:
 135 #endif
 136 .endm
 137
 138 #else /* __ASSEMBLY__ */
 139
 140 #define ANNOTATE_NOSPEC_ALTERNATIVE                             \
 141         "999:\n\t"                                              \
 142         ".pushsection .discard.nospec\n\t"                      \
 143         ".long 999b - .\n\t"                                    \
 144         ".popsection\n\t"
 145
 146 #if defined(CONFIG_X86_64) && defined(RETPOLINE)
 147
 148 /*
 149  * Since the inline asm uses the %V modifier which is only in newer GCC,
 150  * the 64-bit one is dependent on RETPOLINE not CONFIG_RETPOLINE.
 151  */
 152 # define CALL_NOSPEC                                            \
 153         ANNOTATE_NOSPEC_ALTERNATIVE                             \
 154         ALTERNATIVE(                                            \
 155         "call *%[thunk_target]\n",                              \
 156         "call __x86_indirect_thunk_%V[thunk_target]\n",         \
 157         X86_FEATURE_RETPOLINE)
 158 # define THUNK_TARGET(addr) [thunk_target] "r" (addr)
 159
 160 #elif defined(CONFIG_X86_32) && defined(CONFIG_RETPOLINE)
 161 /*
 162  * For i386 we use the original ret-equivalent retpoline, because
 163  * otherwise we'll run out of registers. We don't care about CET
 164  * here, anyway.
 165  */
 166 # define CALL_NOSPEC ALTERNATIVE("call *%[thunk_target]\n",     \
 167         "       jmp    904f;\n"                                 \
 168         "       .align 16\n"                                    \
 169         "901:   call   903f;\n"                                 \
 170         "902:   pause;\n"                                       \
 171         "       lfence;\n"                                      \
 172         "       jmp    902b;\n"                                 \
 173         "       .align 16\n"                                    \
 174         "903:   addl   $4, %%esp;\n"                            \
 175         "       pushl  %[thunk_target];\n"                      \
 176         "       ret;\n"                                         \
 177         "       .align 16\n"                                    \
 178         "904:   call   901b;\n",                                \
 179         X86_FEATURE_RETPOLINE)
 180
 181 # define THUNK_TARGET(addr) [thunk_target] "rm" (addr)
 182 #else /* No retpoline for C / inline asm */
 183 # define CALL_NOSPEC "call *%[thunk_target]\n"
 184 # define THUNK_TARGET(addr) [thunk_target] "rm" (addr)
 185 #endif
 186
 187 /* The Spectre V2 mitigation variants */
 188 enum spectre_v2_mitigation {
 189         SPECTRE_V2_NONE,
 190         SPECTRE_V2_RETPOLINE_MINIMAL,
 191         SPECTRE_V2_RETPOLINE_MINIMAL_AMD,
 192         SPECTRE_V2_RETPOLINE_GENERIC,
 193         SPECTRE_V2_RETPOLINE_AMD,
 194         SPECTRE_V2_IBRS,
 195 };
 196
 197 extern char __indirect_thunk_start[];
 198 extern char __indirect_thunk_end[];
 199
 200 /*
 201  * On VMEXIT we must ensure that no RSB predictions learned in the guest
 202  * can be followed in the host, by overwriting the RSB completely. Both
 203  * retpoline and IBRS mitigations for Spectre v2 need this; only on future
 204  * CPUs with IBRS_ATT *might* it be avoided.
 205  */
 206 static inline void vmexit_fill_RSB(void)
 207 {
 208 #ifdef CONFIG_RETPOLINE
 209         unsigned long loops;
 210
 211         asm volatile (ANNOTATE_NOSPEC_ALTERNATIVE
 212                       ALTERNATIVE("jmp 910f",
 213                                   __stringify(__FILL_RETURN_BUFFER(%0, RSB_CLEAR_LOOPS, %1)),
 214                                   X86_FEATURE_RETPOLINE)
 215                       "910:"
 216                       : "=r" (loops), ASM_CALL_CONSTRAINT
 217                       : : "memory" );
 218 #endif
 219 }
 220
 221 #endif /* __ASSEMBLY__ */
 222 #endif /* __NOSPEC_BRANCH_H__ */