arch-x86: Fixed M5InternalError when decoding certain bytes

0F 38 is the two bytes prefixes to decode a three-byte opcode. To prevent errors, the two_bytes_opcode decoder will complain if it tries to decode 38 as the opcode, because it is a prefix. The decoder, will treat 38 as a prefix, preventing it to end in the two_byte_opcode decoder. However, using the VEX prefix is possible to reach this forbidden state. The set of bytes C4 01 01 38 00 will trigger the mentioned M5InternalError. The previous instruction is not valid, but it could be decoded from an speculative path. In its place, a UD2 instructtion should be emitted if the VEX prefix is present. Change-Id: I6b7c4b3593dd8e6e8ac99aaf306b8feeb7784b56 Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/49990 Reviewed-by: Gabe Black <[email protected]> Maintainer: Gabe Black <[email protected]> Tested-by: kokoro <[email protected]>
ease-lab · Oct 7, 2021 · 2b46872 · 2b46872
1 parent 2b69ff2
commit 2b46872
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Showing 2 changed files with 12 additions and 8 deletions.
diff --git a/src/arch/x86/isa/bitfields.isa b/src/arch/x86/isa/bitfields.isa
@@ -86,5 +86,6 @@ def bitfield MODE mode;
 def bitfield MODE_MODE mode.mode;
 def bitfield MODE_SUBMODE mode.submode;
 
+def bitfield VEX_PRESENT vex.present;
 def bitfield VEX_V vex.v;
 def bitfield VEX_L vex.l;
diff --git a/src/arch/x86/isa/decoder/two_byte_opcodes.isa b/src/arch/x86/isa/decoder/two_byte_opcodes.isa
@@ -354,14 +354,17 @@
             0x6: Inst::UD2();
             0x7: getsec();
         }
-        0x07: decode OPCODE_OP_BOTTOM3 {
-            0x0: M5InternalError::error(
-                {{"Three byte opcode shouldn't be handled by "
-                  "two_byte_opcodes.isa!"}});
-            0x2: M5InternalError::error(
-                {{"Three byte opcode shouldn't be handled by "
-                  "two_byte_opcodes.isa!"}});
-            default: UD2();
+        0x07: decode VEX_PRESENT {
+            0x0: decode OPCODE_OP_BOTTOM3 {
+                0x0: M5InternalError::error(
+                    {{"Three byte opcode shouldn't be handled by "
+                      "two_byte_opcodes.isa!"}});
+                0x2: M5InternalError::error(
+                    {{"Three byte opcode shouldn't be handled by "
+                      "two_byte_opcodes.isa!"}});
+                default: UD2();
+            }
+            0x1: UD2();
         }
         format Inst {
             0x08: decode OPCODE_OP_BOTTOM3 {