Re: [v3 PATCH 07/10] x86: Add emulation code for UMIP instructions

H. Peter Anvin
2017-01-25T20:38:55Z

On 01/25/17 12:23, Ricardo Neri wrote:
> + case UMIP_SMSW:
> + dummy_value = CR0_STATE;

Unless the user space process is running in 64-bit mode this value
should be & 0xffff. I'm not sure if we should even support fixing up
UMIP instructions in 64-bit mode.

Also, please put an explicit /* fall through */ here.

> + /*
> + * These two instructions return a 16-bit value. We return
> + * all zeros. This is equivalent to a null descriptor for
> + * str and sldt.
> + */
> + case UMIP_SLDT:
> + case UMIP_STR:
> + /* if operand is a register, it is zero-extended*/
> + if (X86_MODRM_MOD(insn->modrm.value) == 3) {
> + memset(data, 0, insn->opnd_bytes);
> + *data_size = insn->opnd_bytes;
> + /* if not, only the two least significant bytes are copied */
> + } else {
> + *data_size = 2;
> + }
> + memcpy(data, &dummy_value, sizeof(dummy_value));
> + break;
> + default:
> + return -EINVAL;
> + }
> + return 0;

> +bool fixup_umip_exception(struct pt_regs *regs)
> +{
> + struct insn insn;
> + unsigned char buf[MAX_INSN_SIZE];
> + /* 10 bytes is the maximum size of the result of UMIP instructions */
> + unsigned char dummy_data[10] = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0};
> +#ifdef CONFIG_X86_64
> + int x86_64 = user_64bit_mode(regs);
> +#else
> + int x86_64 = 0;
> +#endif

Again, could we simply do:

if (user_64bit_mode(regs))
return false;

or are there known users of these instructions *in 64-bit mode*?

-hpa