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
- Compile this code
void main() {
asm("mov rax, 0x17b3693f77fb6e9");
asm("mov rbx, 0x8f635a775ad3b9b4");
asm("mov rcx, 0xb717b75da9983018");
asm("bextr eax, ebx, ecx");
}
- Execute and compare the result with the CPU.
- CPU
- RAX = 0x5a
- QEMU
- RAX = 0x635a775a
- CPU
Additional information
This bug is discovered by research conducted by KAIST SoftSec.