cpuctl(formely devcpu) patch test request

[Kostik Belousov]

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
@@ -765,6 +765,7 @@ 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.
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