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From: Jan Beulich <jbeulich@suse.com>
Subject: x86emul: CMPXCHG8B ignores operand size prefix
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 <jbeulich@suse.com>
--- a/tools/tests/x86_emulator/test_x86_emulator.c
+++ b/tools/tests/x86_emulator/test_x86_emulator.c
@@ -435,6 +435,24 @@ 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
@@ -4775,8 +4775,12 @@ 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 )
+ {
host_and_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,
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