Re: [v3 PATCH 00/10] x86: Enable User-Mode Instruction Prevention

Ricardo Neri
2017-01-26T05:51:16Z

Hi Peter,
On Wed, 2017-01-25 at 12:34 -0800, H. Peter Anvin wrote:
> On 01/25/17 12:23, Ricardo Neri wrote:
> > * SMSW returns the value with which the CR0 register is programmed in
> > head_32/64.S at boot time. This is, the following bits are enabed:
> > CR0.0 for Protection Enable, CR.1 for Monitor Coprocessor, CR.4 for
> > Extension Type, which will always be 1 in recent processors with UMIP;
> > CR.5 for Numeric Error, CR0.16 for Write Protect, CR0.18 for Alignment
> > Mask. Additionally, in x86_64, CR0.31 for Paging is set.
>
> SMSW only returns CR0[15:0], so the reference here to CR0[31:16] seems odd.

I checked again the latest version (from Dec 2016) of the Intel Software
Development Manual. The documentation for SMSW states the following:

SMSW r16 operand size 16, store CR0[15:0] in r16
SMSW r32 operand size 32, zero-extend CR0[31:0], and store in r32
SMSW r64 operand size 64, zero-extend CR0[63:0], and store in r64

When the operand is a memory location, yes, it only returns CR0[15:0]

Also, in the tests that I ran I wrote the result of SMSW to a 64-bit
register. I get 0x80050033. It seems to me that it does write as many
bits as the register operand can hold.

Am I missing something?

Thanks and BR,
Ricardo
>
> -hpa
>
>