Subject: x86emul: CMPXCHG8B ignores operand size prefix
From: Jan Beulich <[email-masked]>

Otherwise besides mis-handling the instruction, the comparison failure
case would result in uninitialized stack data being handed back to the
guest in rDX:rAX (32 bits leaked for 32-bit guests, 96 bits for 64-bit
ones).

This is XSA-200.

Signed-off-by: Jan Beulich <[email-masked]>

--- a/tools/tests/x86_emulator/test_x86_emulator.c
+++ b/tools/tests/x86_emulator/test_x86_emulator.c
@@ -429,6 +429,24 @@ int main(int argc, char **argv)
         goto fail;
     printf("okay\n");
 
+    printf("%-40s", "Testing cmpxchg8b (%edi) [opsize]...");
+    instr[0] = 0x66; instr[1] = 0x0f; instr[2] = 0xc7; instr[3] = 0x0f;
+    res[0]      = 0x12345678;
+    res[1]      = 0x87654321;
+    regs.eflags = 0x200;
+    regs.eip    = (unsigned long)&instr[0];
+    regs.edi    = (unsigned long)res;
+    rc = x86_emulate(&ctxt, &emulops);
+    if ( (rc != X86EMUL_OKAY) ||
+         (res[0] != 0x12345678) ||
+         (res[1] != 0x87654321) ||
+         (regs.eax != 0x12345678) ||
+         (regs.edx != 0x87654321) ||
+         ((regs.eflags&0x240) != 0x200) ||
+         (regs.eip != (unsigned long)&instr[4]) )
+        goto fail;
+    printf("okay\n");
+
     printf("%-40s", "Testing movsxbd (%%eax),%%ecx...");
     instr[0] = 0x0f; instr[1] = 0xbe; instr[2] = 0x08;
     regs.eflags = 0x200;
--- a/xen/arch/x86/x86_emulate/x86_emulate.c
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -4739,8 +4739,12 @@ x86_emulate(
         generate_exception_if((modrm_reg & 7) != 1, EXC_UD, -1);
         generate_exception_if(ea.type != OP_MEM, EXC_UD, -1);
         if ( op_bytes == 8 )
+        {
             vcpu_must_have_cx16();
-        op_bytes *= 2;
+            op_bytes = 16;
+        }
+        else
+            op_bytes = 8;
 
         /* Get actual old value. */
         if ( (rc = ops->read(ea.mem.seg, ea.mem.off, old, op_bytes,