i386: Generate lfence with load/indirect branch/ret [CVE-2020-0551]

Add 3 command-line options to generate lfence for load, indirect near
branch and ret to help mitigate:

https://www.intel.com/content/www/us/en/security-center/advisory/intel-sa-00334.html
http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-0551

1. -mlfence-after-load=[no|yes]:
  -mlfence-after-load=yes generates lfence after load instructions.
2. -mlfence-before-indirect-branch=[none|all|memory|register]:
  a. -mlfence-before-indirect-branch=all generates lfence before indirect
  near branches via register and a warning before indirect near branches
  via memory.
  b. -mlfence-before-indirect-branch=memory issue a warning before
  indirect near branches via memory.
  c. -mlfence-before-indirect-branch=register generates lfence before
  indirect near branches via register.
Note that lfence won't be generated before indirect near branches via
register with -mlfence-after-load=yes since lfence will be generated
after loading branch target register.
3. -mlfence-before-ret=[none|or|not]
  a. -mlfence-before-ret=or generates or with lfence before ret.
  b. -mlfence-before-ret=not generates not with lfence before ret.

A warning will be issued and lfence won't be generated before indirect
near branch and ret if the previous item is a prefix or a constant
directive, which may be used to hardcode an instruction, since there
is no clear instruction boundary.

	* config/tc-i386.c (lfence_after_load): New.
	(lfence_before_indirect_branch_kind): New.
	(lfence_before_indirect_branch): New.
	(lfence_before_ret_kind): New.
	(lfence_before_ret): New.
	(last_insn): New.
	(load_insn_p): New.
	(insert_lfence_after): New.
	(insert_lfence_before): New.
	(md_assemble): Call insert_lfence_before and insert_lfence_after.
	Set last_insn.
	(OPTION_MLFENCE_AFTER_LOAD): New.
	(OPTION_MLFENCE_BEFORE_INDIRECT_BRANCH): New.
	(OPTION_MLFENCE_BEFORE_RET): New.
	(md_longopts): Add -mlfence-after-load=,
	-mlfence-before-indirect-branch= and -mlfence-before-ret=.
	(md_parse_option): Handle -mlfence-after-load=,
	-mlfence-before-indirect-branch= and -mlfence-before-ret=.
	(md_show_usage): Display -mlfence-after-load=,
	-mlfence-before-indirect-branch= and -mlfence-before-ret=.
	(i386_cons_align): New.
	* config/tc-i386.h (i386_cons_align): New.
	(md_cons_align): New.
	* doc/c-i386.texi: Document -mlfence-after-load=,
	-mlfence-before-indirect-branch= and -mlfence-before-ret=.

1 files changed, 365 insertions, 1 deletions

diff --git a/gas/config/tc-i386.c b/gas/config/tc-i386.c
index b020f39..09063f7 100644
--- a/gas/config/tc-i386.c
+++ b/gas/config/tc-i386.c
@@ -629,7 +629,29 @@ static int omit_lock_prefix = 0;
    "lock addl $0, (%{re}sp)".  */
 static int avoid_fence = 0;
 
-/* Type of the previous instruction.  */
+/* 1 if lfence should be inserted after every load.  */
+static int lfence_after_load = 0;
+
+/* Non-zero if lfence should be inserted before indirect branch.  */
+static enum lfence_before_indirect_branch_kind
+  {
+    lfence_branch_none = 0,
+    lfence_branch_register,
+    lfence_branch_memory,
+    lfence_branch_all
+  }
+lfence_before_indirect_branch;
+
+/* Non-zero if lfence should be inserted before ret.  */
+static enum lfence_before_ret_kind
+  {
+    lfence_before_ret_none = 0,
+    lfence_before_ret_not,
+    lfence_before_ret_or
+  }
+lfence_before_ret;
+
+/* Types of previous instruction is .byte or prefix.  */
 static struct
   {
     segT seg;
@@ -4311,6 +4333,283 @@ optimize_encoding (void)
     }
 }
 
+/* Return non-zero for load instruction.  */
+
+static int
+load_insn_p (void)
+{
+  unsigned int dest;
+  int any_vex_p = is_any_vex_encoding (&i.tm);
+  unsigned int base_opcode = i.tm.base_opcode | 1;
+
+  if (!any_vex_p)
+    {
+      /* lea  */
+      if (i.tm.base_opcode == 0x8d)
+	return 0;
+
+      /* pop  */
+      if ((i.tm.base_opcode & ~7) == 0x58
+	  || (i.tm.base_opcode == 0x8f && i.tm.extension_opcode == 0))
+	return 1;
+
+      /* movs, cmps, lods, scas.  */
+      if ((i.tm.base_opcode | 0xb) == 0xaf)
+	return 1;
+
+      /* outs */
+      if (base_opcode == 0x6f)
+	return 1;
+    }
+
+  /* No memory operand.  */
+  if (!i.mem_operands)
+    return 0;
+
+  if (any_vex_p)
+    {
+      /* vldmxcsr.  */
+      if (i.tm.base_opcode == 0xae
+	  && i.tm.opcode_modifier.vex
+	  && i.tm.opcode_modifier.vexopcode == VEX0F
+	  && i.tm.extension_opcode == 2)
+	return 1;
+    }
+  else
+    {
+      /* test, not, neg, mul, imul, div, idiv.  */
+      if ((i.tm.base_opcode == 0xf6 || i.tm.base_opcode == 0xf7)
+	  && i.tm.extension_opcode != 1)
+	return 1;
+
+      /* inc, dec.  */
+      if (base_opcode == 0xff && i.tm.extension_opcode <= 1)
+	return 1;
+
+      /* add, or, adc, sbb, and, sub, xor, cmp.  */
+      if (i.tm.base_opcode >= 0x80 && i.tm.base_opcode <= 0x83)
+	return 1;
+
+      /* bt, bts, btr, btc.  */
+      if (i.tm.base_opcode == 0xfba
+	  && (i.tm.extension_opcode >= 4 && i.tm.extension_opcode <= 7))
+	return 1;
+
+      /* rol, ror, rcl, rcr, shl/sal, shr, sar. */
+      if ((base_opcode == 0xc1
+	   || (i.tm.base_opcode >= 0xd0 && i.tm.base_opcode <= 0xd3))
+	  && i.tm.extension_opcode != 6)
+	return 1;
+
+      /* cmpxchg8b, cmpxchg16b, xrstors.  */
+      if (i.tm.base_opcode == 0xfc7
+	  && (i.tm.extension_opcode == 1 || i.tm.extension_opcode == 3))
+	return 1;
+
+      /* fxrstor, ldmxcsr, xrstor.  */
+      if (i.tm.base_opcode == 0xfae
+	  && (i.tm.extension_opcode == 1
+	      || i.tm.extension_opcode == 2
+	      || i.tm.extension_opcode == 5))
+	return 1;
+
+      /* lgdt, lidt, lmsw.  */
+      if (i.tm.base_opcode == 0xf01
+	  && (i.tm.extension_opcode == 2
+	      || i.tm.extension_opcode == 3
+	      || i.tm.extension_opcode == 6))
+	return 1;
+
+      /* vmptrld */
+      if (i.tm.base_opcode == 0xfc7
+	  && i.tm.extension_opcode == 6)
+	return 1;
+
+      /* Check for x87 instructions.  */
+      if (i.tm.base_opcode >= 0xd8 && i.tm.base_opcode <= 0xdf)
+	{
+	  /* Skip fst, fstp, fstenv, fstcw.  */
+	  if (i.tm.base_opcode == 0xd9
+	      && (i.tm.extension_opcode == 2
+		  || i.tm.extension_opcode == 3
+		  || i.tm.extension_opcode == 6
+		  || i.tm.extension_opcode == 7))
+	    return 0;
+
+	  /* Skip fisttp, fist, fistp, fstp.  */
+	  if (i.tm.base_opcode == 0xdb
+	      && (i.tm.extension_opcode == 1
+		  || i.tm.extension_opcode == 2
+		  || i.tm.extension_opcode == 3
+		  || i.tm.extension_opcode == 7))
+	    return 0;
+
+	  /* Skip fisttp, fst, fstp, fsave, fstsw.  */
+	  if (i.tm.base_opcode == 0xdd
+	      && (i.tm.extension_opcode == 1
+		  || i.tm.extension_opcode == 2
+		  || i.tm.extension_opcode == 3
+		  || i.tm.extension_opcode == 6
+		  || i.tm.extension_opcode == 7))
+	    return 0;
+
+	  /* Skip fisttp, fist, fistp, fbstp, fistp.  */
+	  if (i.tm.base_opcode == 0xdf
+	      && (i.tm.extension_opcode == 1
+		  || i.tm.extension_opcode == 2
+		  || i.tm.extension_opcode == 3
+		  || i.tm.extension_opcode == 6
+		  || i.tm.extension_opcode == 7))
+	    return 0;
+
+	  return 1;
+	}
+    }
+
+  dest = i.operands - 1;
+
+  /* Check fake imm8 operand and 3 source operands.  */
+  if ((i.tm.opcode_modifier.immext
+       || i.tm.opcode_modifier.vexsources == VEX3SOURCES)
+      && i.types[dest].bitfield.imm8)
+    dest--;
+
+  /* add, or, adc, sbb, and, sub, xor, cmp, test, xchg, xadd  */
+  if (!any_vex_p
+      && (base_opcode == 0x1
+	  || base_opcode == 0x9
+	  || base_opcode == 0x11
+	  || base_opcode == 0x19
+	  || base_opcode == 0x21
+	  || base_opcode == 0x29
+	  || base_opcode == 0x31
+	  || base_opcode == 0x39
+	  || (i.tm.base_opcode >= 0x84 && i.tm.base_opcode <= 0x87)
+	  || base_opcode == 0xfc1))
+    return 1;
+
+  /* Check for load instruction.  */
+  return (i.types[dest].bitfield.class != ClassNone
+	  || i.types[dest].bitfield.instance == Accum);
+}
+
+/* Output lfence, 0xfaee8, after instruction.  */
+
+static void
+insert_lfence_after (void)
+{
+  if (lfence_after_load && load_insn_p ())
+    {
+      char *p = frag_more (3);
+      *p++ = 0xf;
+      *p++ = 0xae;
+      *p = 0xe8;
+    }
+}
+
+/* Output lfence, 0xfaee8, before instruction.  */
+
+static void
+insert_lfence_before (void)
+{
+  char *p;
+
+  if (is_any_vex_encoding (&i.tm))
+    return;
+
+  if (i.tm.base_opcode == 0xff
+      && (i.tm.extension_opcode == 2 || i.tm.extension_opcode == 4))
+    {
+      /* Insert lfence before indirect branch if needed.  */
+
+      if (lfence_before_indirect_branch == lfence_branch_none)
+	return;
+
+      if (i.operands != 1)
+	abort ();
+
+      if (i.reg_operands == 1)
+	{
+	  /* Indirect branch via register.  Don't insert lfence with
+	     -mlfence-after-load=yes.  */
+	  if (lfence_after_load
+	      || lfence_before_indirect_branch == lfence_branch_memory)
+	    return;
+	}
+      else if (i.mem_operands == 1
+	       && lfence_before_indirect_branch != lfence_branch_register)
+	{
+	  as_warn (_("indirect `%s` with memory operand should be avoided"),
+		   i.tm.name);
+	  return;
+	}
+      else
+	return;
+
+      if (last_insn.kind != last_insn_other
+	  && last_insn.seg == now_seg)
+	{
+	  as_warn_where (last_insn.file, last_insn.line,
+			 _("`%s` skips -mlfence-before-indirect-branch on `%s`"),
+			 last_insn.name, i.tm.name);
+	  return;
+	}
+
+      p = frag_more (3);
+      *p++ = 0xf;
+      *p++ = 0xae;
+      *p = 0xe8;
+      return;
+    }
+
+  /* Output or/not and lfence before ret.  */
+  if (lfence_before_ret != lfence_before_ret_none
+      && (i.tm.base_opcode == 0xc2
+	  || i.tm.base_opcode == 0xc3
+	  || i.tm.base_opcode == 0xca
+	  || i.tm.base_opcode == 0xcb))
+    {
+      if (last_insn.kind != last_insn_other
+	  && last_insn.seg == now_seg)
+	{
+	  as_warn_where (last_insn.file, last_insn.line,
+			 _("`%s` skips -mlfence-before-ret on `%s`"),
+			 last_insn.name, i.tm.name);
+	  return;
+	}
+      if (lfence_before_ret == lfence_before_ret_or)
+	{
+	  /* orl: 0x830c2400.  */
+	  p = frag_more ((flag_code == CODE_64BIT ? 1 : 0) + 4 + 3);
+	  if (flag_code == CODE_64BIT)
+	    *p++ = 0x48;
+	  *p++ = 0x83;
+	  *p++ = 0xc;
+	  *p++ = 0x24;
+	  *p++ = 0x0;
+	}
+      else
+	{
+	  p = frag_more ((flag_code == CODE_64BIT ? 2 : 0) + 6 + 3);
+	  /* notl: 0xf71424.  */
+	  if (flag_code == CODE_64BIT)
+	    *p++ = 0x48;
+	  *p++ = 0xf7;
+	  *p++ = 0x14;
+	  *p++ = 0x24;
+	  /* notl: 0xf71424.  */
+	  if (flag_code == CODE_64BIT)
+	    *p++ = 0x48;
+	  *p++ = 0xf7;
+	  *p++ = 0x14;
+	  *p++ = 0x24;
+	}
+      *p++ = 0xf;
+      *p++ = 0xae;
+      *p = 0xe8;
+    }
+}
+
 /* This is the guts of the machine-dependent assembler.  LINE points to a
    machine dependent instruction.  This function is supposed to emit
    the frags/bytes it assembles to.  */
@@ -4628,9 +4927,13 @@ md_assemble (char *line)
   if (i.rex != 0)
     add_prefix (REX_OPCODE | i.rex);
 
+  insert_lfence_before ();
+
   /* We are ready to output the insn.  */
   output_insn ();
 
+  insert_lfence_after ();
+
   last_insn.seg = now_seg;
 
   if (i.tm.opcode_modifier.isprefix)
@@ -12250,6 +12553,9 @@ const char *md_shortopts = "qnO::";
 #define OPTION_MALIGN_BRANCH_PREFIX_SIZE (OPTION_MD_BASE + 28)
 #define OPTION_MALIGN_BRANCH (OPTION_MD_BASE + 29)
 #define OPTION_MBRANCHES_WITH_32B_BOUNDARIES (OPTION_MD_BASE + 30)
+#define OPTION_MLFENCE_AFTER_LOAD (OPTION_MD_BASE + 31)
+#define OPTION_MLFENCE_BEFORE_INDIRECT_BRANCH (OPTION_MD_BASE + 32)
+#define OPTION_MLFENCE_BEFORE_RET (OPTION_MD_BASE + 33)
 
 struct option md_longopts[] =
 {
@@ -12289,6 +12595,10 @@ struct option md_longopts[] =
   {"malign-branch-prefix-size", required_argument, NULL, OPTION_MALIGN_BRANCH_PREFIX_SIZE},
   {"malign-branch", required_argument, NULL, OPTION_MALIGN_BRANCH},
   {"mbranches-within-32B-boundaries", no_argument, NULL, OPTION_MBRANCHES_WITH_32B_BOUNDARIES},
+  {"mlfence-after-load", required_argument, NULL, OPTION_MLFENCE_AFTER_LOAD},
+  {"mlfence-before-indirect-branch", required_argument, NULL,
+   OPTION_MLFENCE_BEFORE_INDIRECT_BRANCH},
+  {"mlfence-before-ret", required_argument, NULL, OPTION_MLFENCE_BEFORE_RET},
   {"mamd64", no_argument, NULL, OPTION_MAMD64},
   {"mintel64", no_argument, NULL, OPTION_MINTEL64},
   {NULL, no_argument, NULL, 0}
@@ -12668,6 +12978,41 @@ md_parse_option (int c, const char *arg)
         as_fatal (_("invalid -mfence-as-lock-add= option: `%s'"), arg);
       break;
 
+    case OPTION_MLFENCE_AFTER_LOAD:
+      if (strcasecmp (arg, "yes") == 0)
+	lfence_after_load = 1;
+      else if (strcasecmp (arg, "no") == 0)
+	lfence_after_load = 0;
+      else
+        as_fatal (_("invalid -mlfence-after-load= option: `%s'"), arg);
+      break;
+
+    case OPTION_MLFENCE_BEFORE_INDIRECT_BRANCH:
+      if (strcasecmp (arg, "all") == 0)
+	lfence_before_indirect_branch = lfence_branch_all;
+      else if (strcasecmp (arg, "memory") == 0)
+	lfence_before_indirect_branch = lfence_branch_memory;
+      else if (strcasecmp (arg, "register") == 0)
+	lfence_before_indirect_branch = lfence_branch_register;
+      else if (strcasecmp (arg, "none") == 0)
+	lfence_before_indirect_branch = lfence_branch_none;
+      else
+        as_fatal (_("invalid -mlfence-before-indirect-branch= option: `%s'"),
+		  arg);
+      break;
+
+    case OPTION_MLFENCE_BEFORE_RET:
+      if (strcasecmp (arg, "or") == 0)
+	lfence_before_ret = lfence_before_ret_or;
+      else if (strcasecmp (arg, "not") == 0)
+	lfence_before_ret = lfence_before_ret_not;
+      else if (strcasecmp (arg, "none") == 0)
+	lfence_before_ret = lfence_before_ret_none;
+      else
+        as_fatal (_("invalid -mlfence-before-ret= option: `%s'"),
+		  arg);
+      break;
+
     case OPTION_MRELAX_RELOCATIONS:
       if (strcasecmp (arg, "yes") == 0)
         generate_relax_relocations = 1;
@@ -13025,6 +13370,15 @@ md_show_usage (FILE *stream)
   -mbranches-within-32B-boundaries\n\
                           align branches within 32 byte boundary\n"));
   fprintf (stream, _("\
+  -mlfence-after-load=[no|yes] (default: no)\n\
+                          generate lfence after load\n"));
+  fprintf (stream, _("\
+  -mlfence-before-indirect-branch=[none|all|register|memory] (default: none)\n\
+                          generate lfence before indirect near branch\n"));
+  fprintf (stream, _("\
+  -mlfence-before-ret=[none|or|not] (default: none)\n\
+                          generate lfence before ret\n"));
+  fprintf (stream, _("\
   -mamd64                 accept only AMD64 ISA [default]\n"));
   fprintf (stream, _("\
   -mintel64               accept only Intel64 ISA\n"));
@@ -13254,6 +13608,16 @@ i386_cons_align (int ignore ATTRIBUTE_UNUSED)
       last_insn.kind = last_insn_directive;
       last_insn.name = "constant directive";
       last_insn.file = as_where (&last_insn.line);
+      if (lfence_before_ret != lfence_before_ret_none)
+	{
+	  if (lfence_before_indirect_branch != lfence_branch_none)
+	    as_warn (_("constant directive skips -mlfence-before-ret "
+		       "and -mlfence-before-indirect-branch"));
+	  else
+	    as_warn (_("constant directive skips -mlfence-before-ret"));
+	}
+      else if (lfence_before_indirect_branch != lfence_branch_none)
+	as_warn (_("constant directive skips -mlfence-before-indirect-branch"));
     }
 }