arch/x86/kernel/static_call.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 #include <linux/static_call.h>
   3 #include <linux/memory.h>
   4 #include <linux/bug.h>
   5 #include <asm/text-patching.h>
   6
   7 enum insn_type {
   8         CALL = 0, /* site call */
   9         NOP = 1,  /* site cond-call */
  10         JMP = 2,  /* tramp / site tail-call */
  11         RET = 3,  /* tramp / site cond-tail-call */
  12 };
  13
  14 /*
  15  * ud1 %esp, %ecx - a 3 byte #UD that is unique to trampolines, chosen such
  16  * that there is no false-positive trampoline identification while also being a
  17  * speculation stop.
  18  */
  19 static const u8 tramp_ud[] = { 0x0f, 0xb9, 0xcc };
  20
  21 /*
  22  * cs cs cs xorl %eax, %eax - a single 5 byte instruction that clears %[er]ax
  23  */
  24 static const u8 xor5rax[] = { 0x2e, 0x2e, 0x2e, 0x31, 0xc0 };
  25
  26 static const u8 retinsn[] = { RET_INSN_OPCODE, 0xcc, 0xcc, 0xcc, 0xcc };
  27
  28 static void __ref __static_call_transform(void *insn, enum insn_type type, void *func)
  29 {
  30         const void *emulate = NULL;
  31         int size = CALL_INSN_SIZE;
  32         const void *code;
  33
  34         switch (type) {
  35         case CALL:
  36                 code = text_gen_insn(CALL_INSN_OPCODE, insn, func);
  37                 if (func == &__static_call_return0) {
  38                         emulate = code;
  39                         code = &xor5rax;
  40                 }
  41
  42                 break;
  43
  44         case NOP:
  45                 code = x86_nops[5];
  46                 break;
  47
  48         case JMP:
  49                 code = text_gen_insn(JMP32_INSN_OPCODE, insn, func);
  50                 break;
  51
  52         case RET:
  53                 if (cpu_feature_enabled(X86_FEATURE_RETHUNK))
  54                         code = text_gen_insn(JMP32_INSN_OPCODE, insn, &__x86_return_thunk);
  55                 else
  56                         code = &retinsn;
  57                 break;
  58         }
  59
  60         if (memcmp(insn, code, size) == 0)
  61                 return;
  62
  63         if (unlikely(system_state == SYSTEM_BOOTING))
  64                 return text_poke_early(insn, code, size);
  65
  66         text_poke_bp(insn, code, size, emulate);
  67 }
  68
  69 static void __static_call_validate(void *insn, bool tail)
  70 {
  71         u8 opcode = *(u8 *)insn;
  72
  73         if (tail) {
  74                 if (opcode == JMP32_INSN_OPCODE ||
  75                     opcode == RET_INSN_OPCODE)
  76                         return;
  77         } else {
  78                 if (opcode == CALL_INSN_OPCODE ||
  79                     !memcmp(insn, x86_nops[5], 5) ||
  80                     !memcmp(insn, xor5rax, 5))
  81                         return;
  82         }
  83
  84         /*
  85          * If we ever trigger this, our text is corrupt, we'll probably not live long.
  86          */
  87         WARN_ONCE(1, "unexpected static_call insn opcode 0x%x at %pS\n", opcode, insn);
  88 }
  89
  90 static inline enum insn_type __sc_insn(bool null, bool tail)
  91 {
  92         /*
  93          * Encode the following table without branches:
  94          *
  95          *      tail    null    insn
  96          *      -----+-------+------
  97          *        0  |   0   |  CALL
  98          *        0  |   1   |  NOP
  99          *        1  |   0   |  JMP
 100          *        1  |   1   |  RET
 101          */
 102         return 2*tail + null;
 103 }
 104
 105 void arch_static_call_transform(void *site, void *tramp, void *func, bool tail)
 106 {
 107         mutex_lock(&text_mutex);
 108
 109         if (tramp) {
 110                 __static_call_validate(tramp, true);
 111                 __static_call_transform(tramp, __sc_insn(!func, true), func);
 112         }
 113
 114         if (IS_ENABLED(CONFIG_HAVE_STATIC_CALL_INLINE) && site) {
 115                 __static_call_validate(site, tail);
 116                 __static_call_transform(site, __sc_insn(!func, tail), func);
 117         }
 118
 119         mutex_unlock(&text_mutex);
 120 }
 121 EXPORT_SYMBOL_GPL(arch_static_call_transform);
 122
 123 #ifdef CONFIG_RETPOLINE
 124 /*
 125  * This is called by apply_returns() to fix up static call trampolines,
 126  * specifically ARCH_DEFINE_STATIC_CALL_NULL_TRAMP which is recorded as
 127  * having a return trampoline.
 128  *
 129  * The problem is that static_call() is available before determining
 130  * X86_FEATURE_RETHUNK and, by implication, running alternatives.
 131  *
 132  * This means that __static_call_transform() above can have overwritten the
 133  * return trampoline and we now need to fix things up to be consistent.
 134  */
 135 bool __static_call_fixup(void *tramp, u8 op, void *dest)
 136 {
 137         if (memcmp(tramp+5, tramp_ud, 3)) {
 138                 /* Not a trampoline site, not our problem. */
 139                 return false;
 140         }
 141
 142         if (op == RET_INSN_OPCODE || dest == &__x86_return_thunk)
 143                 __static_call_transform(tramp, RET, NULL);
 144
 145         return true;
 146 }
 147 #endif