Re: cpuctl(formely devcpu) patch test request

On Tue, 5 Aug 2008 14:53:15 +0300
Kostik Belousov <kostikbel_at_gmail.com> mentioned:

> On Tue, Aug 05, 2008 at 02:03:24PM +0400, Stanislav Sedov wrote:
> > On Mon, 16 Jun 2008 14:42:41 -0400
> > Coleman Kane <cokane_at_FreeBSD.org> mentioned:
> > 
> > > 
> > > Is it potentially "unsafe" to use RDMSR?
> > >
> > 
> > Well, it might disclose some sensitive information,
> > as well as create covert channels. E.g. some of the
> > registers contains kernel thread pointers, etc; some
> > of them undocumented. It won't be very wise to give
> > access to the rdmsr feature to all users on a
> > multi-user machine.
> > 
> > Sorry for this taking so long. You messages spotted
> > a bug in my security model for this driver, so I've
> > redone that. Now, the access to the rdmsr and cpuid
> > features will be granted only if the caller has
> > read permissions on the device, and wrmsr/update
> >  - only if he've opened the device for writing.
> > This way you can provide fine-grained control to
> > the driver features.
> > 
> > I've also added the cpucontrol utility which provided
> > userland accesss to the driver, and allows to apply
> > microcode updates.
> > 
> > The latest patch against HEAD is available here:
> > ftp://ftp.SpringDaemons.com/dustheap/cpuctl.4.diff
> > 
> > Thanks!
> 
> --- a/sys/amd64/amd64/support.S
> +++ b/sys/amd64/amd64/support.S
> _at__at_ -765,6 +765,7 _at__at_ ENTRY(wrmsr_safe)
>   */
>    ALIGN_TEXT
>  msr_onfault:
> - movq $0,PCB_ONFAULT(%r8)
> - movl $EFAULT,%eax
> +  movq PCPU(CURPCB),%r8     /* set fault handler */
> +  movq $0,PCB_ONFAULT(%r8)
> +  movq $EFAULT,%rax
>    ret
> 
> movq $EFAULT,%rax is better to be replaced by movl, %eax. Amd64 specifies
> automatic zeroing of the upper-half of the registers on the 32bit operation.
> 

Yeah, it seems that I thought about this initially, but decided
that this was unsafe lately. Thanks for suggestion!

There's a fixed version:
ftp://ftp.SpringDaemons.com/dustheap/cpuctl.5.diff

-- 
Stanislav Sedov
ST4096-RIPE