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; 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         jmp     773b;                           \
  42 772:                                            \
  43         call    774f;                           \
  44 775:    /* speculation trap */                  \
  45         pause;                                  \
  46         jmp     775b;                           \
  47 774:                                            \
  48         dec     reg;                            \
  49         jnz     771b;                           \
  50         add     $(BITS_PER_LONG/8) * nr, sp;
  51
  52 #ifdef __ASSEMBLY__
  53
  54 /*
  55  * This should be used immediately before a retpoline alternative.  It tells
  56  * objtool where the retpolines are so that it can make sense of the control
  57  * flow by just reading the original instruction(s) and ignoring the
  58  * alternatives.
  59  */
  60 .macro ANNOTATE_NOSPEC_ALTERNATIVE
  61         .Lannotate_\@:
  62         .pushsection .discard.nospec
  63         .long .Lannotate_\@ - .
  64         .popsection
  65 .endm
  66
  67 /*
  68  * These are the bare retpoline primitives for indirect jmp and call.
  69  * Do not use these directly; they only exist to make the ALTERNATIVE
  70  * invocation below less ugly.
  71  */
  72 .macro RETPOLINE_JMP reg:req
  73         call    .Ldo_rop_\@
  74 .Lspec_trap_\@:
  75         pause
  76         jmp     .Lspec_trap_\@
  77 .Ldo_rop_\@:
  78         mov     \reg, (%_ASM_SP)
  79         ret
  80 .endm
  81
  82 /*
  83  * This is a wrapper around RETPOLINE_JMP so the called function in reg
  84  * returns to the instruction after the macro.
  85  */
  86 .macro RETPOLINE_CALL reg:req
  87         jmp     .Ldo_call_\@
  88 .Ldo_retpoline_jmp_\@:
  89         RETPOLINE_JMP \reg
  90 .Ldo_call_\@:
  91         call    .Ldo_retpoline_jmp_\@
  92 .endm
  93
  94 /*
  95  * JMP_NOSPEC and CALL_NOSPEC macros can be used instead of a simple
  96  * indirect jmp/call which may be susceptible to the Spectre variant 2
  97  * attack.
  98  */
  99 .macro JMP_NOSPEC reg:req
 100 #ifdef CONFIG_RETPOLINE
 101         ANNOTATE_NOSPEC_ALTERNATIVE
 102         ALTERNATIVE_2 __stringify(jmp *\reg),                           \
 103                 __stringify(RETPOLINE_JMP \reg), X86_FEATURE_RETPOLINE, \
 104                 __stringify(lfence; jmp *\reg), X86_FEATURE_RETPOLINE_AMD
 105 #else
 106         jmp     *\reg
 107 #endif
 108 .endm
 109
 110 .macro CALL_NOSPEC reg:req
 111 #ifdef CONFIG_RETPOLINE
 112         ANNOTATE_NOSPEC_ALTERNATIVE
 113         ALTERNATIVE_2 __stringify(call *\reg),                          \
 114                 __stringify(RETPOLINE_CALL \reg), X86_FEATURE_RETPOLINE,\
 115                 __stringify(lfence; call *\reg), X86_FEATURE_RETPOLINE_AMD
 116 #else
 117         call    *\reg
 118 #endif
 119 .endm
 120
 121  /*
 122   * A simpler FILL_RETURN_BUFFER macro. Don't make people use the CPP
 123   * monstrosity above, manually.
 124   */
 125 .macro FILL_RETURN_BUFFER reg:req nr:req ftr:req
 126 #ifdef CONFIG_RETPOLINE
 127         ANNOTATE_NOSPEC_ALTERNATIVE
 128         ALTERNATIVE "jmp .Lskip_rsb_\@",                                \
 129                 __stringify(__FILL_RETURN_BUFFER(\reg,\nr,%_ASM_SP))    \
 130                 \ftr
 131 .Lskip_rsb_\@:
 132 #endif
 133 .endm
 134
 135 #else /* __ASSEMBLY__ */
 136
 137 #define ANNOTATE_NOSPEC_ALTERNATIVE                             \
 138         "999:\n\t"                                              \
 139         ".pushsection .discard.nospec\n\t"                      \
 140         ".long 999b - .\n\t"                                    \
 141         ".popsection\n\t"
 142
 143 #if defined(CONFIG_X86_64) && defined(RETPOLINE)
 144
 145 /*
 146  * Since the inline asm uses the %V modifier which is only in newer GCC,
 147  * the 64-bit one is dependent on RETPOLINE not CONFIG_RETPOLINE.
 148  */
 149 # define CALL_NOSPEC                                            \
 150         ANNOTATE_NOSPEC_ALTERNATIVE                             \
 151         ALTERNATIVE(                                            \
 152         "call *%[thunk_target]\n",                              \
 153         "call __x86_indirect_thunk_%V[thunk_target]\n",         \
 154         X86_FEATURE_RETPOLINE)
 155 # define THUNK_TARGET(addr) [thunk_target] "r" (addr)
 156
 157 #elif defined(CONFIG_X86_32) && defined(CONFIG_RETPOLINE)
 158 /*
 159  * For i386 we use the original ret-equivalent retpoline, because
 160  * otherwise we'll run out of registers. We don't care about CET
 161  * here, anyway.
 162  */
 163 # define CALL_NOSPEC ALTERNATIVE("call *%[thunk_target]\n",     \
 164         "       jmp    904f;\n"                                 \
 165         "       .align 16\n"                                    \
 166         "901:   call   903f;\n"                                 \
 167         "902:   pause;\n"                                       \
 168         "       jmp    902b;\n"                                 \
 169         "       .align 16\n"                                    \
 170         "903:   addl   $4, %%esp;\n"                            \
 171         "       pushl  %[thunk_target];\n"                      \
 172         "       ret;\n"                                         \
 173         "       .align 16\n"                                    \
 174         "904:   call   901b;\n",                                \
 175         X86_FEATURE_RETPOLINE)
 176
 177 # define THUNK_TARGET(addr) [thunk_target] "rm" (addr)
 178 #else /* No retpoline for C / inline asm */
 179 # define CALL_NOSPEC "call *%[thunk_target]\n"
 180 # define THUNK_TARGET(addr) [thunk_target] "rm" (addr)
 181 #endif
 182
 183 /* The Spectre V2 mitigation variants */
 184 enum spectre_v2_mitigation {
 185         SPECTRE_V2_NONE,
 186         SPECTRE_V2_RETPOLINE_MINIMAL,
 187         SPECTRE_V2_RETPOLINE_MINIMAL_AMD,
 188         SPECTRE_V2_RETPOLINE_GENERIC,
 189         SPECTRE_V2_RETPOLINE_AMD,
 190         SPECTRE_V2_IBRS,
 191 };
 192
 193 /*
 194  * On VMEXIT we must ensure that no RSB predictions learned in the guest
 195  * can be followed in the host, by overwriting the RSB completely. Both
 196  * retpoline and IBRS mitigations for Spectre v2 need this; only on future
 197  * CPUs with IBRS_ATT *might* it be avoided.
 198  */
 199 static inline void vmexit_fill_RSB(void)
 200 {
 201 #ifdef CONFIG_RETPOLINE
 202         unsigned long loops = RSB_CLEAR_LOOPS / 2;
 203
 204         asm volatile (ANNOTATE_NOSPEC_ALTERNATIVE
 205                       ALTERNATIVE("jmp 910f",
 206                                   __stringify(__FILL_RETURN_BUFFER(%0, RSB_CLEAR_LOOPS, %1)),
 207                                   X86_FEATURE_RETPOLINE)
 208                       "910:"
 209                       : "=&r" (loops), ASM_CALL_CONSTRAINT
 210                       : "r" (loops) : "memory" );
 211 #endif
 212 }
 213 #endif /* __ASSEMBLY__ */
 214 #endif /* __NOSPEC_BRANCH_H__ */