x86 BEXTR semantic bug

Host environment

• Operating system: Windows 10 20H2
• OS/kernel version: WSL2 Ubuntu 20.04.4 LTS (GNU/Linux 5.10.102.1-microsoft-standard-WSL2 x86_64)
• Architecture: x86
• QEMU flavor: qemu-x86_64
• QEMU version: 7.1.90 (v7.2.0-rc0)
• QEMU command line: qemu-x86_64 -cpu max a.out

Emulated/Virtualized environment

• Operating system: None
• OS/kernel version: None
• Architecture: x86

Description of problem

The result of instruction BEXTR is different with from the CPU. The value of destination register is different. I think QEMU does not consider the operand size limit.

Steps to reproduce

1. Compile this code

void main() {
    asm("mov rax, 0x17b3693f77fb6e9");
    asm("mov rbx, 0x8f635a775ad3b9b4");
    asm("mov rcx, 0xb717b75da9983018");
    asm("bextr eax, ebx, ecx");
}

2. Execute and compare the result with the CPU.
   • CPU
     • RAX = 0x5a
   • QEMU
     • RAX = 0x635a775a

Additional information

This bug is discovered by research conducted by KAIST SoftSec.