[v12,6/7] x86/arch_prctl: Add ARCH_[GET|SET]_CPUID
diff mbox

Message ID 20161117020610.5302-7-khuey@kylehuey.com
State New
Headers show

Commit Message

Kyle Huey Nov. 17, 2016, 2:06 a.m. UTC
Intel supports faulting on the CPUID instruction beginning with Ivy Bridge.
When enabled, the processor will fault on attempts to execute the CPUID
instruction with CPL>0. Exposing this feature to userspace will allow a
ptracer to trap and emulate the CPUID instruction.

When supported, this feature is controlled by toggling bit 0 of
MSR_MISC_FEATURES_ENABLES. It is documented in detail in Section 2.3.2 of
https://bugzilla.kernel.org/attachment.cgi?id=243991

Implement a new pair of arch_prctls, available on both x86-32 and x86-64.

ARCH_GET_CPUID: Returns the current CPUID faulting state, either
  ARCH_CPUID_ENABLE or ARCH_CPUID_SIGSEGV. arg2 must be 0.

ARCH_SET_CPUID: Set the CPUID faulting state to arg2, which must be either
  ARCH_CPUID_ENABLE or ARCH_CPUID_SIGSEGV. Returns EINVAL if arg2 is
  another value or CPUID faulting is not supported on this system.

The state of the CPUID faulting flag is propagated across forks, but reset
upon exec.

Signed-off-by: Kyle Huey <khuey@kylehuey.com>
---
 arch/x86/include/asm/msr-index.h          |   3 +
 arch/x86/include/asm/processor.h          |   2 +
 arch/x86/include/asm/thread_info.h        |   6 +-
 arch/x86/include/uapi/asm/prctl.h         |   6 +
 arch/x86/kernel/cpu/intel.c               |   7 +
 arch/x86/kernel/process.c                 |  84 ++++++++++
 fs/exec.c                                 |   1 +
 include/linux/thread_info.h               |   4 +
 tools/testing/selftests/x86/Makefile      |   2 +-
 tools/testing/selftests/x86/cpuid-fault.c | 254 ++++++++++++++++++++++++++++++
 10 files changed, 367 insertions(+), 2 deletions(-)
 create mode 100644 tools/testing/selftests/x86/cpuid-fault.c

Comments

Ingo Molnar Nov. 18, 2016, 8:14 a.m. UTC | #1
* Kyle Huey <me@kylehuey.com> wrote:

> Intel supports faulting on the CPUID instruction beginning with Ivy Bridge.
> When enabled, the processor will fault on attempts to execute the CPUID
> instruction with CPL>0. Exposing this feature to userspace will allow a
> ptracer to trap and emulate the CPUID instruction.
> 
> When supported, this feature is controlled by toggling bit 0 of
> MSR_MISC_FEATURES_ENABLES. It is documented in detail in Section 2.3.2 of
> https://bugzilla.kernel.org/attachment.cgi?id=243991
> 
> Implement a new pair of arch_prctls, available on both x86-32 and x86-64.
> 
> ARCH_GET_CPUID: Returns the current CPUID faulting state, either
>   ARCH_CPUID_ENABLE or ARCH_CPUID_SIGSEGV. arg2 must be 0.
> 
> ARCH_SET_CPUID: Set the CPUID faulting state to arg2, which must be either
>   ARCH_CPUID_ENABLE or ARCH_CPUID_SIGSEGV. Returns EINVAL if arg2 is
>   another value or CPUID faulting is not supported on this system.

So the interface is:

> +#define ARCH_GET_CPUID 0x1005
> +#define ARCH_SET_CPUID 0x1006
> +#define ARCH_CPUID_ENABLE 1
> +#define ARCH_CPUID_SIGSEGV 2

Which maps to:

   prctl(ARCH_SET_CPUID, 0); /* -EINVAL */
   prctl(ARCH_SET_CPUID, 1); /* enable CPUID [i.e. make it work without faulting] */
   prctl(ARCH_SET_CPUID, 2); /* disable CPUID [i.e. make it fault] */

   ret = prctl(ARCH_GET_CPUID, 0); /* return current state: 1==on, 2==off */

This is a very broken interface that makes very little sense.

It would be much better to use a more natural interface where 1/0 means on/off and 
where ARCH_GET_CPUID returns the current natural state:

   prctl(ARCH_SET_CPUID, 0); /* disable CPUID [i.e. make it fault] */
   prctl(ARCH_SET_CPUID, 1); /* enable CPUID [i.e. make it work without faulting] */

   ret = prctl(ARCH_GET_CPUID); /* 1==enabled, 0==disabled */

See how natural it is? The use of the ARCH_CPUID_SIGSEGV/ENABLED symbols can be 
avoided altogether. This will cut down on some of the ugliness in the kernel code 
as well - and clean up the argument name as well: instead of naming it 'int arg2' 
it can be named the more natural 'int cpuid_enabled'.

> The state of the CPUID faulting flag is propagated across forks, but reset
> upon exec.

I don't think this is the natural API for propagating settings across exec().
We should reset the flag on exec() only if security considerations require it - 
i.e. like perf events are cleared.

If binaries that assume a working CPUID are exec()-ed then CPUID can be enabled 
explicitly.

Clearing it automatically loses the ability of a pure no-CPUID environment to 
exec() a CPUID-safe binary.

> Signed-off-by: Kyle Huey <khuey@kylehuey.com>
> ---
>  arch/x86/include/asm/msr-index.h          |   3 +
>  arch/x86/include/asm/processor.h          |   2 +
>  arch/x86/include/asm/thread_info.h        |   6 +-
>  arch/x86/include/uapi/asm/prctl.h         |   6 +
>  arch/x86/kernel/cpu/intel.c               |   7 +
>  arch/x86/kernel/process.c                 |  84 ++++++++++
>  fs/exec.c                                 |   1 +
>  include/linux/thread_info.h               |   4 +
>  tools/testing/selftests/x86/Makefile      |   2 +-
>  tools/testing/selftests/x86/cpuid-fault.c | 254 ++++++++++++++++++++++++++++++
>  10 files changed, 367 insertions(+), 2 deletions(-)
>  create mode 100644 tools/testing/selftests/x86/cpuid-fault.c

Please put the self-test into a separate patch.

>  static void init_intel_misc_features_enables(struct cpuinfo_x86 *c)
>  {
>  	u64 msr;
>  
> +	if (rdmsrl_safe(MSR_MISC_FEATURES_ENABLES, &msr))
> +		return;
> +
> +	msr = 0;
> +	wrmsrl(MSR_MISC_FEATURES_ENABLES, msr);
> +	this_cpu_write(msr_misc_features_enables_shadow, msr);
> +
>  	if (!rdmsrl_safe(MSR_PLATFORM_INFO, &msr)) {
>  		if (msr & MSR_PLATFORM_INFO_CPUID_FAULT)
>  			set_cpu_cap(c, X86_FEATURE_CPUID_FAULT);
>  	}
>  }

Sigh, so the Intel MSR index itself is grossly misnamed: MSR_MISC_FEATURES_ENABLES 
- plain reading of 'enables' suggests it's a verb, but in wants to be a noun. A 
better name would be MSR_MISC_FEATURES or so.

So while for the MSR index we want to keep the Intel name, please drop that 
_enables() postfix from the kernel C function names such as this one - and from 
the shadow value name as well.

> +DEFINE_PER_CPU(u64, msr_misc_features_enables_shadow);
> +
> +static void set_cpuid_faulting(bool on)
> +{
> +	u64 msrval;
> +
> +	DEBUG_LOCKS_WARN_ON(!irqs_disabled());
> +
> +	msrval = this_cpu_read(msr_misc_features_enables_shadow);
> +	msrval &= ~MSR_MISC_FEATURES_ENABLES_CPUID_FAULT;
> +	msrval |= (on << MSR_MISC_FEATURES_ENABLES_CPUID_FAULT_BIT);
> +	this_cpu_write(msr_misc_features_enables_shadow, msrval);
> +	wrmsrl(MSR_MISC_FEATURES_ENABLES, msrval);

This gets called from the context switch path and this looks pretty suboptimal, 
especially when combined with the TIF flag check:

>  void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p,
>  		      struct tss_struct *tss)
>  {
>  	struct thread_struct *prev, *next;
>  
>  	prev = &prev_p->thread;
>  	next = &next_p->thread;
>  
> @@ -206,16 +278,21 @@ void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p,
>  
>  		debugctl &= ~DEBUGCTLMSR_BTF;
>  		if (test_tsk_thread_flag(next_p, TIF_BLOCKSTEP))
>  			debugctl |= DEBUGCTLMSR_BTF;
>  
>  		update_debugctlmsr(debugctl);
>  	}
>  
> +	if (test_tsk_thread_flag(prev_p, TIF_NOCPUID) ^
> +	    test_tsk_thread_flag(next_p, TIF_NOCPUID)) {
> +		set_cpuid_faulting(test_tsk_thread_flag(next_p, TIF_NOCPUID));
> +	}
> +

Why not cache the required MSR value in the task struct instead?

That would allow something much more obvious and much faster, like:

	if (prev_p->thread.misc_features_val != next_p->thread.misc_features_val)
		wrmsrl(MSR_MISC_FEATURES_ENABLES, next_p->thread.misc_features_val);

(The TIF flag maintenance is still required to get into __switch_to_xtra().)

It would also be easy to extend without extra overhead, should any other feature 
bit be added to the MSR in the future.

Thanks,

	Ingo
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Thomas Gleixner Nov. 18, 2016, 8:49 a.m. UTC | #2
On Fri, 18 Nov 2016, Ingo Molnar wrote:
> * Kyle Huey <me@kylehuey.com> wrote:
> > +	if (test_tsk_thread_flag(prev_p, TIF_NOCPUID) ^
> > +	    test_tsk_thread_flag(next_p, TIF_NOCPUID)) {
> > +		set_cpuid_faulting(test_tsk_thread_flag(next_p, TIF_NOCPUID));
> > +	}
> > +
> 
> Why not cache the required MSR value in the task struct instead?
> 
> That would allow something much more obvious and much faster, like:
> 
> 	if (prev_p->thread.misc_features_val != next_p->thread.misc_features_val)
> 		wrmsrl(MSR_MISC_FEATURES_ENABLES, next_p->thread.misc_features_val);
> 
> (The TIF flag maintenance is still required to get into __switch_to_xtra().)
> 
> It would also be easy to extend without extra overhead, should any other feature 
> bit be added to the MSR in the future.

I doubt that. There are feature enable bits coming up which are not related
to tasks. So if we have switches enabling/disabling global features, then
we would be forced to chase all threads in order to update all misc_features
thread variables. Surely not what we want to do.

Thanks,

	tglx
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Kyle Huey Nov. 18, 2016, 3:55 p.m. UTC | #3
On Fri, Nov 18, 2016 at 12:14 AM, Ingo Molnar <mingo@kernel.org> wrote:
>
> * Kyle Huey <me@kylehuey.com> wrote:
>
>> Intel supports faulting on the CPUID instruction beginning with Ivy Bridge.
>> When enabled, the processor will fault on attempts to execute the CPUID
>> instruction with CPL>0. Exposing this feature to userspace will allow a
>> ptracer to trap and emulate the CPUID instruction.
>>
>> When supported, this feature is controlled by toggling bit 0 of
>> MSR_MISC_FEATURES_ENABLES. It is documented in detail in Section 2.3.2 of
>> https://bugzilla.kernel.org/attachment.cgi?id=243991
>>
>> Implement a new pair of arch_prctls, available on both x86-32 and x86-64.
>>
>> ARCH_GET_CPUID: Returns the current CPUID faulting state, either
>>   ARCH_CPUID_ENABLE or ARCH_CPUID_SIGSEGV. arg2 must be 0.
>>
>> ARCH_SET_CPUID: Set the CPUID faulting state to arg2, which must be either
>>   ARCH_CPUID_ENABLE or ARCH_CPUID_SIGSEGV. Returns EINVAL if arg2 is
>>   another value or CPUID faulting is not supported on this system.
>
> So the interface is:
>
>> +#define ARCH_GET_CPUID 0x1005
>> +#define ARCH_SET_CPUID 0x1006
>> +#define ARCH_CPUID_ENABLE 1
>> +#define ARCH_CPUID_SIGSEGV 2
>
> Which maps to:
>
>    prctl(ARCH_SET_CPUID, 0); /* -EINVAL */
>    prctl(ARCH_SET_CPUID, 1); /* enable CPUID [i.e. make it work without faulting] */
>    prctl(ARCH_SET_CPUID, 2); /* disable CPUID [i.e. make it fault] */
>
>    ret = prctl(ARCH_GET_CPUID, 0); /* return current state: 1==on, 2==off */

arch_prctl in all cases, but yes.

> This is a very broken interface that makes very little sense.

It's copied from prctl(PR_SET/GET_TSC), for what that's worth.  I'm
happy to change this as long as nobody will complain about the
inconsistency :)

> It would be much better to use a more natural interface where 1/0 means on/off and
> where ARCH_GET_CPUID returns the current natural state:
>
>    prctl(ARCH_SET_CPUID, 0); /* disable CPUID [i.e. make it fault] */
>    prctl(ARCH_SET_CPUID, 1); /* enable CPUID [i.e. make it work without faulting] */
>
>    ret = prctl(ARCH_GET_CPUID); /* 1==enabled, 0==disabled */
>
> See how natural it is? The use of the ARCH_CPUID_SIGSEGV/ENABLED symbols can be
> avoided altogether. This will cut down on some of the ugliness in the kernel code
> as well - and clean up the argument name as well: instead of naming it 'int arg2'
> it can be named the more natural 'int cpuid_enabled'.
>
>> The state of the CPUID faulting flag is propagated across forks, but reset
>> upon exec.
>
> I don't think this is the natural API for propagating settings across exec().
> We should reset the flag on exec() only if security considerations require it -
> i.e. like perf events are cleared.

I had a discussion with Andy Lutomirski about this a couple months
ago. See https://lkml.org/lkml/2016/9/14/968.  So if you want to do
something different here I'd like the two of you to agree before I
change the code :)

> If binaries that assume a working CPUID are exec()-ed then CPUID can be enabled
> explicitly.

glibc's ld.so requires CPUID, so most binaries will.

> Clearing it automatically loses the ability of a pure no-CPUID environment to
> exec() a CPUID-safe binary.

I don't know that this will be particularly useful, given the above.

>> Signed-off-by: Kyle Huey <khuey@kylehuey.com>
>> ---
>>  arch/x86/include/asm/msr-index.h          |   3 +
>>  arch/x86/include/asm/processor.h          |   2 +
>>  arch/x86/include/asm/thread_info.h        |   6 +-
>>  arch/x86/include/uapi/asm/prctl.h         |   6 +
>>  arch/x86/kernel/cpu/intel.c               |   7 +
>>  arch/x86/kernel/process.c                 |  84 ++++++++++
>>  fs/exec.c                                 |   1 +
>>  include/linux/thread_info.h               |   4 +
>>  tools/testing/selftests/x86/Makefile      |   2 +-
>>  tools/testing/selftests/x86/cpuid-fault.c | 254 ++++++++++++++++++++++++++++++
>>  10 files changed, 367 insertions(+), 2 deletions(-)
>>  create mode 100644 tools/testing/selftests/x86/cpuid-fault.c
>
> Please put the self-test into a separate patch.

Ok.

>>  static void init_intel_misc_features_enables(struct cpuinfo_x86 *c)
>>  {
>>       u64 msr;
>>
>> +     if (rdmsrl_safe(MSR_MISC_FEATURES_ENABLES, &msr))
>> +             return;
>> +
>> +     msr = 0;
>> +     wrmsrl(MSR_MISC_FEATURES_ENABLES, msr);
>> +     this_cpu_write(msr_misc_features_enables_shadow, msr);
>> +
>>       if (!rdmsrl_safe(MSR_PLATFORM_INFO, &msr)) {
>>               if (msr & MSR_PLATFORM_INFO_CPUID_FAULT)
>>                       set_cpu_cap(c, X86_FEATURE_CPUID_FAULT);
>>       }
>>  }
>
> Sigh, so the Intel MSR index itself is grossly misnamed: MSR_MISC_FEATURES_ENABLES
> - plain reading of 'enables' suggests it's a verb, but in wants to be a noun. A
> better name would be MSR_MISC_FEATURES or so.
>
> So while for the MSR index we want to keep the Intel name, please drop that
> _enables() postfix from the kernel C function names such as this one - and from
> the shadow value name as well.

Ok.

>> +DEFINE_PER_CPU(u64, msr_misc_features_enables_shadow);
>> +
>> +static void set_cpuid_faulting(bool on)
>> +{
>> +     u64 msrval;
>> +
>> +     DEBUG_LOCKS_WARN_ON(!irqs_disabled());
>> +
>> +     msrval = this_cpu_read(msr_misc_features_enables_shadow);
>> +     msrval &= ~MSR_MISC_FEATURES_ENABLES_CPUID_FAULT;
>> +     msrval |= (on << MSR_MISC_FEATURES_ENABLES_CPUID_FAULT_BIT);
>> +     this_cpu_write(msr_misc_features_enables_shadow, msrval);
>> +     wrmsrl(MSR_MISC_FEATURES_ENABLES, msrval);
>
> This gets called from the context switch path and this looks pretty suboptimal,
> especially when combined with the TIF flag check:
>
>>  void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p,
>>                     struct tss_struct *tss)
>>  {
>>       struct thread_struct *prev, *next;
>>
>>       prev = &prev_p->thread;
>>       next = &next_p->thread;
>>
>> @@ -206,16 +278,21 @@ void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p,
>>
>>               debugctl &= ~DEBUGCTLMSR_BTF;
>>               if (test_tsk_thread_flag(next_p, TIF_BLOCKSTEP))
>>                       debugctl |= DEBUGCTLMSR_BTF;
>>
>>               update_debugctlmsr(debugctl);
>>       }
>>
>> +     if (test_tsk_thread_flag(prev_p, TIF_NOCPUID) ^
>> +         test_tsk_thread_flag(next_p, TIF_NOCPUID)) {
>> +             set_cpuid_faulting(test_tsk_thread_flag(next_p, TIF_NOCPUID));
>> +     }
>> +
>
> Why not cache the required MSR value in the task struct instead?
>
> That would allow something much more obvious and much faster, like:
>
>         if (prev_p->thread.misc_features_val != next_p->thread.misc_features_val)
>                 wrmsrl(MSR_MISC_FEATURES_ENABLES, next_p->thread.misc_features_val);
>
> (The TIF flag maintenance is still required to get into __switch_to_xtra().)
>
> It would also be easy to extend without extra overhead, should any other feature
> bit be added to the MSR in the future.

Thomas covered this one.

- Kyle
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Andy Lutomirski Nov. 18, 2016, 5:32 p.m. UTC | #4
On Nov 18, 2016 12:14 AM, "Ingo Molnar" <mingo@kernel.org> wrote:
>
>
> * Kyle Huey <me@kylehuey.com> wrote:
>
> > Intel supports faulting on the CPUID instruction beginning with Ivy Bridge.
> > When enabled, the processor will fault on attempts to execute the CPUID
> > instruction with CPL>0. Exposing this feature to userspace will allow a
> > ptracer to trap and emulate the CPUID instruction.
> >
> > When supported, this feature is controlled by toggling bit 0 of
> > MSR_MISC_FEATURES_ENABLES. It is documented in detail in Section 2.3.2 of
> > https://bugzilla.kernel.org/attachment.cgi?id=243991
> >
> > Implement a new pair of arch_prctls, available on both x86-32 and x86-64.
> >
> > ARCH_GET_CPUID: Returns the current CPUID faulting state, either
> >   ARCH_CPUID_ENABLE or ARCH_CPUID_SIGSEGV. arg2 must be 0.
> >
> > ARCH_SET_CPUID: Set the CPUID faulting state to arg2, which must be either
> >   ARCH_CPUID_ENABLE or ARCH_CPUID_SIGSEGV. Returns EINVAL if arg2 is
> >   another value or CPUID faulting is not supported on this system.
>
> So the interface is:
>
> > +#define ARCH_GET_CPUID 0x1005
> > +#define ARCH_SET_CPUID 0x1006
> > +#define ARCH_CPUID_ENABLE 1
> > +#define ARCH_CPUID_SIGSEGV 2
>
> Which maps to:
>
>    prctl(ARCH_SET_CPUID, 0); /* -EINVAL */
>    prctl(ARCH_SET_CPUID, 1); /* enable CPUID [i.e. make it work without faulting] */
>    prctl(ARCH_SET_CPUID, 2); /* disable CPUID [i.e. make it fault] */
>
>    ret = prctl(ARCH_GET_CPUID, 0); /* return current state: 1==on, 2==off */
>
> This is a very broken interface that makes very little sense.
>
> It would be much better to use a more natural interface where 1/0 means on/off and
> where ARCH_GET_CPUID returns the current natural state:
>
>    prctl(ARCH_SET_CPUID, 0); /* disable CPUID [i.e. make it fault] */
>    prctl(ARCH_SET_CPUID, 1); /* enable CPUID [i.e. make it work without faulting] */
>
>    ret = prctl(ARCH_GET_CPUID); /* 1==enabled, 0==disabled */
>
> See how natural it is? The use of the ARCH_CPUID_SIGSEGV/ENABLED symbols can be
> avoided altogether. This will cut down on some of the ugliness in the kernel code
> as well - and clean up the argument name as well: instead of naming it 'int arg2'
> it can be named the more natural 'int cpuid_enabled'.
>
> > The state of the CPUID faulting flag is propagated across forks, but reset
> > upon exec.
>
> I don't think this is the natural API for propagating settings across exec().
> We should reset the flag on exec() only if security considerations require it -
> i.e. like perf events are cleared.
>
> If binaries that assume a working CPUID are exec()-ed then CPUID can be enabled
> explicitly.

I disagree.  I'd rather not create more weird state that's carried
across exec.  We already have the iopl screwup IIRC.  I think exec
should stay as close to just working as possible.

Also, if we keep it disabled across exec, we have to come up with a
usable API that respects security considerations.  We could use
no_new_privs or we could auto-clear it on privilege changes.  The
former is IMO overcomplicated and the latter is really ugly especially
when LSMs are involved.

>
> Clearing it automatically loses the ability of a pure no-CPUID environment to
> exec() a CPUID-safe binary.

If we really want this, let's wait until a user appears and add a
"sticky" no-CPUID mode that requires no_new_privs to enable.

--Andy
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Ingo Molnar Nov. 21, 2016, 8:27 a.m. UTC | #5
* Thomas Gleixner <tglx@linutronix.de> wrote:

> On Fri, 18 Nov 2016, Ingo Molnar wrote:
> > * Kyle Huey <me@kylehuey.com> wrote:
> > > +	if (test_tsk_thread_flag(prev_p, TIF_NOCPUID) ^
> > > +	    test_tsk_thread_flag(next_p, TIF_NOCPUID)) {
> > > +		set_cpuid_faulting(test_tsk_thread_flag(next_p, TIF_NOCPUID));
> > > +	}
> > > +
> > 
> > Why not cache the required MSR value in the task struct instead?
> > 
> > That would allow something much more obvious and much faster, like:
> > 
> > 	if (prev_p->thread.misc_features_val != next_p->thread.misc_features_val)
> > 		wrmsrl(MSR_MISC_FEATURES_ENABLES, next_p->thread.misc_features_val);
> > 
> > (The TIF flag maintenance is still required to get into __switch_to_xtra().)
> > 
> > It would also be easy to extend without extra overhead, should any other feature 
> > bit be added to the MSR in the future.
> 
> I doubt that. There are feature enable bits coming up which are not related to 
> tasks.

Any inefficiencies resulting from such features should IMHO be carried by those 
features, not by per task features - but:

> [...] So if we have switches enabling/disabling global features, then we would 
> be forced to chase all threads in order to update all misc_features thread 
> variables. Surely not what we want to do.

What switches would those be? We generally don't twiddle global CPU features post 
bootup - we pick a model on bootup and go with that.

I'd really like to see code (prototype patches are OK - or the person doing it can 
send it to me privately as well if it's not production quality or public yet), or 
some careful description of the features involved.

Thanks,

	Ingo
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Andy Lutomirski Nov. 22, 2016, 5:26 p.m. UTC | #6
On Nov 21, 2016 12:27 AM, "Ingo Molnar" <mingo@kernel.org> wrote:
>
>
> * Thomas Gleixner <tglx@linutronix.de> wrote:
>
> > On Fri, 18 Nov 2016, Ingo Molnar wrote:
> > > * Kyle Huey <me@kylehuey.com> wrote:
> > > > + if (test_tsk_thread_flag(prev_p, TIF_NOCPUID) ^
> > > > +     test_tsk_thread_flag(next_p, TIF_NOCPUID)) {
> > > > +         set_cpuid_faulting(test_tsk_thread_flag(next_p, TIF_NOCPUID));
> > > > + }
> > > > +
> > >
> > > Why not cache the required MSR value in the task struct instead?
> > >
> > > That would allow something much more obvious and much faster, like:
> > >
> > >     if (prev_p->thread.misc_features_val != next_p->thread.misc_features_val)
> > >             wrmsrl(MSR_MISC_FEATURES_ENABLES, next_p->thread.misc_features_val);
> > >
> > > (The TIF flag maintenance is still required to get into __switch_to_xtra().)
> > >
> > > It would also be easy to extend without extra overhead, should any other feature
> > > bit be added to the MSR in the future.
> >
> > I doubt that. There are feature enable bits coming up which are not related to
> > tasks.
>
> Any inefficiencies resulting from such features should IMHO be carried by those
> features, not by per task features - but:
>
> > [...] So if we have switches enabling/disabling global features, then we would
> > be forced to chase all threads in order to update all misc_features thread
> > variables. Surely not what we want to do.
>
> What switches would those be? We generally don't twiddle global CPU features post
> bootup - we pick a model on bootup and go with that.

I don't see what problem we're trying to solve here.  If we end up
with a mix of global (and changeable!) features and per-task features,
we can just do:

wrmsrl(MSR_MISC_FEATURES_ENABLES, global_misc_features_val |
next_p->thread.misc_features_val);

This is *still* way faster than rdmsr.
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Patch
diff mbox

diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
index 3ac0acf..b138b8f 100644
--- a/arch/x86/include/asm/msr-index.h
+++ b/arch/x86/include/asm/msr-index.h
@@ -49,16 +49,19 @@ 
 #define NHM_C1_AUTO_DEMOTE		(1UL << 26)
 #define ATM_LNC_C6_AUTO_DEMOTE		(1UL << 25)
 #define SNB_C1_AUTO_UNDEMOTE		(1UL << 27)
 #define SNB_C3_AUTO_UNDEMOTE		(1UL << 28)
 
 #define MSR_MTRRcap			0x000000fe
 #define MSR_IA32_BBL_CR_CTL		0x00000119
 #define MSR_IA32_BBL_CR_CTL3		0x0000011e
+#define MSR_MISC_FEATURES_ENABLES	0x00000140
+#define MSR_MISC_FEATURES_ENABLES_CPUID_FAULT_BIT	0
+#define MSR_MISC_FEATURES_ENABLES_CPUID_FAULT		BIT_ULL(MSR_MISC_FEATURES_ENABLES_CPUID_FAULT_BIT)
 
 #define MSR_IA32_SYSENTER_CS		0x00000174
 #define MSR_IA32_SYSENTER_ESP		0x00000175
 #define MSR_IA32_SYSENTER_EIP		0x00000176
 
 #define MSR_IA32_MCG_CAP		0x00000179
 #define MSR_IA32_MCG_STATUS		0x0000017a
 #define MSR_IA32_MCG_CTL		0x0000017b
diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 984a7bf..4c1088c 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -803,16 +803,18 @@  extern void start_thread(struct pt_regs *regs, unsigned long new_ip,
 
 /* Get/set a process' ability to use the timestamp counter instruction */
 #define GET_TSC_CTL(adr)	get_tsc_mode((adr))
 #define SET_TSC_CTL(val)	set_tsc_mode((val))
 
 extern int get_tsc_mode(unsigned long adr);
 extern int set_tsc_mode(unsigned int val);
 
+DECLARE_PER_CPU(u64, msr_misc_features_enables_shadow);
+
 /* Register/unregister a process' MPX related resource */
 #define MPX_ENABLE_MANAGEMENT()	mpx_enable_management()
 #define MPX_DISABLE_MANAGEMENT()	mpx_disable_management()
 
 #ifdef CONFIG_X86_INTEL_MPX
 extern int mpx_enable_management(void);
 extern int mpx_disable_management(void);
 #else
diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h
index ad6f5eb0..9fc44b9 100644
--- a/arch/x86/include/asm/thread_info.h
+++ b/arch/x86/include/asm/thread_info.h
@@ -82,16 +82,17 @@  struct thread_info {
 #define TIF_SIGPENDING		2	/* signal pending */
 #define TIF_NEED_RESCHED	3	/* rescheduling necessary */
 #define TIF_SINGLESTEP		4	/* reenable singlestep on user return*/
 #define TIF_SYSCALL_EMU		6	/* syscall emulation active */
 #define TIF_SYSCALL_AUDIT	7	/* syscall auditing active */
 #define TIF_SECCOMP		8	/* secure computing */
 #define TIF_USER_RETURN_NOTIFY	11	/* notify kernel of userspace return */
 #define TIF_UPROBE		12	/* breakpointed or singlestepping */
+#define TIF_NOCPUID		15	/* CPUID is not accessible in userland */
 #define TIF_NOTSC		16	/* TSC is not accessible in userland */
 #define TIF_IA32		17	/* IA32 compatibility process */
 #define TIF_NOHZ		19	/* in adaptive nohz mode */
 #define TIF_MEMDIE		20	/* is terminating due to OOM killer */
 #define TIF_POLLING_NRFLAG	21	/* idle is polling for TIF_NEED_RESCHED */
 #define TIF_IO_BITMAP		22	/* uses I/O bitmap */
 #define TIF_FORCED_TF		24	/* true if TF in eflags artificially */
 #define TIF_BLOCKSTEP		25	/* set when we want DEBUGCTLMSR_BTF */
@@ -105,16 +106,17 @@  struct thread_info {
 #define _TIF_SIGPENDING		(1 << TIF_SIGPENDING)
 #define _TIF_SINGLESTEP		(1 << TIF_SINGLESTEP)
 #define _TIF_NEED_RESCHED	(1 << TIF_NEED_RESCHED)
 #define _TIF_SYSCALL_EMU	(1 << TIF_SYSCALL_EMU)
 #define _TIF_SYSCALL_AUDIT	(1 << TIF_SYSCALL_AUDIT)
 #define _TIF_SECCOMP		(1 << TIF_SECCOMP)
 #define _TIF_USER_RETURN_NOTIFY	(1 << TIF_USER_RETURN_NOTIFY)
 #define _TIF_UPROBE		(1 << TIF_UPROBE)
+#define _TIF_NOCPUID		(1 << TIF_NOCPUID)
 #define _TIF_NOTSC		(1 << TIF_NOTSC)
 #define _TIF_IA32		(1 << TIF_IA32)
 #define _TIF_NOHZ		(1 << TIF_NOHZ)
 #define _TIF_POLLING_NRFLAG	(1 << TIF_POLLING_NRFLAG)
 #define _TIF_IO_BITMAP		(1 << TIF_IO_BITMAP)
 #define _TIF_FORCED_TF		(1 << TIF_FORCED_TF)
 #define _TIF_BLOCKSTEP		(1 << TIF_BLOCKSTEP)
 #define _TIF_LAZY_MMU_UPDATES	(1 << TIF_LAZY_MMU_UPDATES)
@@ -133,17 +135,17 @@  struct thread_info {
 
 /* work to do on any return to user space */
 #define _TIF_ALLWORK_MASK						\
 	((0x0000FFFF & ~_TIF_SECCOMP) | _TIF_SYSCALL_TRACEPOINT |	\
 	_TIF_NOHZ)
 
 /* flags to check in __switch_to() */
 #define _TIF_WORK_CTXSW							\
-	(_TIF_IO_BITMAP|_TIF_NOTSC|_TIF_BLOCKSTEP)
+	(_TIF_IO_BITMAP|_TIF_NOCPUID|_TIF_NOTSC|_TIF_BLOCKSTEP)
 
 #define _TIF_WORK_CTXSW_PREV (_TIF_WORK_CTXSW|_TIF_USER_RETURN_NOTIFY)
 #define _TIF_WORK_CTXSW_NEXT (_TIF_WORK_CTXSW)
 
 #define STACK_WARN		(THREAD_SIZE/8)
 
 /*
  * macros/functions for gaining access to the thread information structure
@@ -234,11 +236,13 @@  static inline int arch_within_stack_frames(const void * const stack,
  * EFLAGS values that other (fast) syscall return instructions
  * are not able to restore properly.
  */
 #define force_iret() set_thread_flag(TIF_NOTIFY_RESUME)
 
 extern void arch_task_cache_init(void);
 extern int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src);
 extern void arch_release_task_struct(struct task_struct *tsk);
+extern void arch_setup_new_exec(void);
+#define arch_setup_new_exec arch_setup_new_exec
 #endif	/* !__ASSEMBLY__ */
 
 #endif /* _ASM_X86_THREAD_INFO_H */
diff --git a/arch/x86/include/uapi/asm/prctl.h b/arch/x86/include/uapi/asm/prctl.h
index ae135de..0f6389c 100644
--- a/arch/x86/include/uapi/asm/prctl.h
+++ b/arch/x86/include/uapi/asm/prctl.h
@@ -1,15 +1,21 @@ 
 #ifndef _ASM_X86_PRCTL_H
 #define _ASM_X86_PRCTL_H
 
 #define ARCH_SET_GS 0x1001
 #define ARCH_SET_FS 0x1002
 #define ARCH_GET_FS 0x1003
 #define ARCH_GET_GS 0x1004
 
+/* Get/set the process' ability to use the CPUID instruction */
+#define ARCH_GET_CPUID 0x1005
+#define ARCH_SET_CPUID 0x1006
+# define ARCH_CPUID_ENABLE		1	/* allow the use of the CPUID instruction */
+# define ARCH_CPUID_SIGSEGV		2	/* throw a SIGSEGV instead of reading the CPUID */
+
 #ifdef CONFIG_CHECKPOINT_RESTORE
 # define ARCH_MAP_VDSO_X32	0x2001
 # define ARCH_MAP_VDSO_32	0x2002
 # define ARCH_MAP_VDSO_64	0x2003
 #endif
 
 #endif /* _ASM_X86_PRCTL_H */
diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index 19b56b5..a26b484 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -451,16 +451,23 @@  static void intel_bsp_resume(struct cpuinfo_x86 *c)
 	 */
 	init_intel_energy_perf(c);
 }
 
 static void init_intel_misc_features_enables(struct cpuinfo_x86 *c)
 {
 	u64 msr;
 
+	if (rdmsrl_safe(MSR_MISC_FEATURES_ENABLES, &msr))
+		return;
+
+	msr = 0;
+	wrmsrl(MSR_MISC_FEATURES_ENABLES, msr);
+	this_cpu_write(msr_misc_features_enables_shadow, msr);
+
 	if (!rdmsrl_safe(MSR_PLATFORM_INFO, &msr)) {
 		if (msr & MSR_PLATFORM_INFO_CPUID_FAULT)
 			set_cpu_cap(c, X86_FEATURE_CPUID_FAULT);
 	}
 }
 
 static void init_intel(struct cpuinfo_x86 *c)
 {
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index d0126b2..e3ad28a 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -28,16 +28,17 @@ 
 #include <asm/mwait.h>
 #include <asm/fpu/internal.h>
 #include <asm/debugreg.h>
 #include <asm/nmi.h>
 #include <asm/tlbflush.h>
 #include <asm/mce.h>
 #include <asm/vm86.h>
 #include <asm/switch_to.h>
+#include <asm/prctl.h>
 
 /*
  * per-CPU TSS segments. Threads are completely 'soft' on Linux,
  * no more per-task TSS's. The TSS size is kept cacheline-aligned
  * so they are allowed to end up in the .data..cacheline_aligned
  * section. Since TSS's are completely CPU-local, we want them
  * on exact cacheline boundaries, to eliminate cacheline ping-pong.
  */
@@ -187,16 +188,87 @@  int set_tsc_mode(unsigned int val)
 	else if (val == PR_TSC_ENABLE)
 		enable_TSC();
 	else
 		return -EINVAL;
 
 	return 0;
 }
 
+DEFINE_PER_CPU(u64, msr_misc_features_enables_shadow);
+
+static void set_cpuid_faulting(bool on)
+{
+	u64 msrval;
+
+	DEBUG_LOCKS_WARN_ON(!irqs_disabled());
+
+	msrval = this_cpu_read(msr_misc_features_enables_shadow);
+	msrval &= ~MSR_MISC_FEATURES_ENABLES_CPUID_FAULT;
+	msrval |= (on << MSR_MISC_FEATURES_ENABLES_CPUID_FAULT_BIT);
+	this_cpu_write(msr_misc_features_enables_shadow, msrval);
+	wrmsrl(MSR_MISC_FEATURES_ENABLES, msrval);
+}
+
+static void disable_cpuid(void)
+{
+	preempt_disable();
+	if (!test_and_set_thread_flag(TIF_NOCPUID)) {
+		/*
+		 * Must flip the CPU state synchronously with
+		 * TIF_NOCPUID in the current running context.
+		 */
+		set_cpuid_faulting(true);
+	}
+	preempt_enable();
+}
+
+static void enable_cpuid(void)
+{
+	preempt_disable();
+	if (test_and_clear_thread_flag(TIF_NOCPUID)) {
+		/*
+		 * Must flip the CPU state synchronously with
+		 * TIF_NOCPUID in the current running context.
+		 */
+		set_cpuid_faulting(false);
+	}
+	preempt_enable();
+}
+
+static int get_cpuid_mode(void)
+{
+	return test_thread_flag(TIF_NOCPUID) ? ARCH_CPUID_SIGSEGV : ARCH_CPUID_ENABLE;
+}
+
+static int set_cpuid_mode(struct task_struct *task, unsigned long val)
+{
+	if (!static_cpu_has(X86_FEATURE_CPUID_FAULT))
+		return -ENODEV;
+
+	if (val == ARCH_CPUID_ENABLE)
+		enable_cpuid();
+	else if (val == ARCH_CPUID_SIGSEGV)
+		disable_cpuid();
+	else
+		return -EINVAL;
+
+	return 0;
+}
+
+/*
+ * Called immediately after a successful exec.
+ */
+void arch_setup_new_exec(void)
+{
+	/* If cpuid was previously disabled for this task, re-enable it. */
+	if (test_thread_flag(TIF_NOCPUID))
+		enable_cpuid();
+}
+
 void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p,
 		      struct tss_struct *tss)
 {
 	struct thread_struct *prev, *next;
 
 	prev = &prev_p->thread;
 	next = &next_p->thread;
 
@@ -206,16 +278,21 @@  void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p,
 
 		debugctl &= ~DEBUGCTLMSR_BTF;
 		if (test_tsk_thread_flag(next_p, TIF_BLOCKSTEP))
 			debugctl |= DEBUGCTLMSR_BTF;
 
 		update_debugctlmsr(debugctl);
 	}
 
+	if (test_tsk_thread_flag(prev_p, TIF_NOCPUID) ^
+	    test_tsk_thread_flag(next_p, TIF_NOCPUID)) {
+		set_cpuid_faulting(test_tsk_thread_flag(next_p, TIF_NOCPUID));
+	}
+
 	if (test_tsk_thread_flag(prev_p, TIF_NOTSC) ^
 	    test_tsk_thread_flag(next_p, TIF_NOTSC)) {
 		/* prev and next are different */
 		if (test_tsk_thread_flag(next_p, TIF_NOTSC))
 			hard_disable_TSC();
 		else
 			hard_enable_TSC();
 	}
@@ -582,10 +659,17 @@  unsigned long get_wchan(struct task_struct *p)
 
 out:
 	put_task_stack(p);
 	return ret;
 }
 
 long do_arch_prctl_common(struct task_struct *task, int code, unsigned long arg2)
 {
+	switch (code) {
+	case ARCH_GET_CPUID:
+		return arg2 ? -EINVAL : get_cpuid_mode();
+	case ARCH_SET_CPUID:
+		return set_cpuid_mode(task, arg2);
+	}
+
 	return -EINVAL;
 }
diff --git a/fs/exec.c b/fs/exec.c
index 4e497b9..3e6872c 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1287,16 +1287,17 @@  void setup_new_exec(struct linux_binprm * bprm)
 	/* This is the point of no return */
 	current->sas_ss_sp = current->sas_ss_size = 0;
 
 	if (uid_eq(current_euid(), current_uid()) && gid_eq(current_egid(), current_gid()))
 		set_dumpable(current->mm, SUID_DUMP_USER);
 	else
 		set_dumpable(current->mm, suid_dumpable);
 
+	arch_setup_new_exec();
 	perf_event_exec();
 	__set_task_comm(current, kbasename(bprm->filename), true);
 
 	/* Set the new mm task size. We have to do that late because it may
 	 * depend on TIF_32BIT which is only updated in flush_thread() on
 	 * some architectures like powerpc
 	 */
 	current->mm->task_size = TASK_SIZE;
diff --git a/include/linux/thread_info.h b/include/linux/thread_info.h
index 2873baf..d46f50d 100644
--- a/include/linux/thread_info.h
+++ b/include/linux/thread_info.h
@@ -129,11 +129,15 @@  static __always_inline void check_object_size(const void *ptr, unsigned long n,
 		__check_object_size(ptr, n, to_user);
 }
 #else
 static inline void check_object_size(const void *ptr, unsigned long n,
 				     bool to_user)
 { }
 #endif /* CONFIG_HARDENED_USERCOPY */
 
+#ifndef arch_setup_new_exec
+static inline void arch_setup_new_exec(void) {}
+#endif
+
 #endif	/* __KERNEL__ */
 
 #endif /* _LINUX_THREAD_INFO_H */
diff --git a/tools/testing/selftests/x86/Makefile b/tools/testing/selftests/x86/Makefile
index a89f80a..3744f28 100644
--- a/tools/testing/selftests/x86/Makefile
+++ b/tools/testing/selftests/x86/Makefile
@@ -1,17 +1,17 @@ 
 all:
 
 include ../lib.mk
 
 .PHONY: all all_32 all_64 warn_32bit_failure clean
 
 TARGETS_C_BOTHBITS := single_step_syscall sysret_ss_attrs syscall_nt ptrace_syscall test_mremap_vdso \
 			check_initial_reg_state sigreturn ldt_gdt iopl \
-			protection_keys
+			protection_keys cpuid-fault
 TARGETS_C_32BIT_ONLY := entry_from_vm86 syscall_arg_fault test_syscall_vdso unwind_vdso \
 			test_FCMOV test_FCOMI test_FISTTP \
 			vdso_restorer
 TARGETS_C_64BIT_ONLY := fsgsbase
 
 TARGETS_C_32BIT_ALL := $(TARGETS_C_BOTHBITS) $(TARGETS_C_32BIT_ONLY)
 TARGETS_C_64BIT_ALL := $(TARGETS_C_BOTHBITS) $(TARGETS_C_64BIT_ONLY)
 BINARIES_32 := $(TARGETS_C_32BIT_ALL:%=%_32)
diff --git a/tools/testing/selftests/x86/cpuid-fault.c b/tools/testing/selftests/x86/cpuid-fault.c
new file mode 100644
index 0000000..b42135a
--- /dev/null
+++ b/tools/testing/selftests/x86/cpuid-fault.c
@@ -0,0 +1,254 @@ 
+
+/*
+ * Tests for arch_prctl(ARCH_GET_CPUID, ...) / arch_prctl(ARCH_SET_CPUID, ...)
+ *
+ * Basic test to test behaviour of ARCH_GET_CPUID and ARCH_SET_CPUID
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <signal.h>
+#include <inttypes.h>
+#include <cpuid.h>
+#include <err.h>
+#include <errno.h>
+#include <sys/wait.h>
+
+#include <sys/prctl.h>
+#include <linux/prctl.h>
+
+/*
+#define ARCH_GET_CPUID 0x1005
+#define ARCH_SET_CPUID 0x1006
+#define ARCH_CPUID_ENABLE 1
+#define ARCH_CPUID_SIGSEGV 2
+#ifdef __x86_64__
+#define SYS_arch_prctl 158
+#else
+#define SYS_arch_prctl 385
+#endif
+*/
+
+const char *cpuid_names[] = {
+	[0] = "[not set]",
+	[ARCH_CPUID_ENABLE] = "ARCH_CPUID_ENABLE",
+	[ARCH_CPUID_SIGSEGV] = "ARCH_CPUID_SIGSEGV",
+};
+
+int arch_prctl(int code, unsigned long arg2)
+{
+	return syscall(SYS_arch_prctl, code, arg2);
+}
+
+int cpuid(unsigned int *eax, unsigned int *ebx, unsigned int *ecx,
+	  unsigned int *edx)
+{
+	return __get_cpuid(0, eax, ebx, ecx, edx);
+}
+
+int do_child_exec_test(int eax, int ebx, int ecx, int edx)
+{
+	int cpuid_val = 0, child = 0, status = 0;
+
+	printf("arch_prctl(ARCH_GET_CPUID); ");
+
+	cpuid_val = arch_prctl(ARCH_GET_CPUID, 0);
+	if (cpuid_val < 0)
+		errx(1, "ARCH_GET_CPUID fails now, but not before?");
+
+	printf("cpuid_val == %s\n", cpuid_names[cpuid_val]);
+	if (cpuid_val != ARCH_CPUID_SIGSEGV)
+		errx(1, "How did cpuid get re-enabled on fork?");
+
+	child = fork();
+	if (child == 0) {
+		cpuid_val = arch_prctl(ARCH_GET_CPUID, 0);
+		if (cpuid_val < 0)
+			errx(1, "ARCH_GET_CPUID fails now, but not before?");
+
+		printf("cpuid_val == %s\n", cpuid_names[cpuid_val]);
+		if (cpuid_val != ARCH_CPUID_SIGSEGV)
+			errx(1, "How did cpuid get re-enabled on fork?");
+
+		printf("exec\n");
+		execl("/proc/self/exe", "cpuid-fault", "-early-return", NULL);
+	}
+
+	if (child != waitpid(child, &status, 0))
+		errx(1, "waitpid failed!?");
+
+	if (WEXITSTATUS(status) != 0)
+		errx(1, "Execed child exited abnormally");
+
+	return 0;
+}
+
+int child_received_signal;
+
+void child_sigsegv_cb(int sig)
+{
+	int cpuid_val = 0;
+
+	child_received_signal = 1;
+	printf("[ SIG_SEGV ]\n");
+	printf("arch_prctl(ARCH_GET_CPUID); ");
+
+	cpuid_val = arch_prctl(ARCH_GET_CPUID, 0);
+	if (cpuid_val < 0)
+		errx(1, "ARCH_GET_CPUID fails now, but not before?");
+
+	printf("cpuid_val == %s\n", cpuid_names[cpuid_val]);
+	printf("arch_prctl(ARCH_SET_CPUID, ARCH_CPUID_ENABLE)\n");
+	if (arch_prctl(ARCH_SET_CPUID, ARCH_CPUID_ENABLE) != 0)
+		exit(errno);
+
+	printf("cpuid() == ");
+}
+
+int do_child_test(void)
+{
+	unsigned int eax = 0, ebx = 0, ecx = 0, edx = 0;
+
+	signal(SIGSEGV, child_sigsegv_cb);
+
+	/* the child starts out with cpuid disabled, the signal handler
+	 * attempts to enable and retry
+	 */
+	printf("cpuid() == ");
+	cpuid(&eax, &ebx, &ecx, &edx);
+	printf("{%x, %x, %x, %x}\n", eax, ebx, ecx, edx);
+	return child_received_signal ? 0 : 42;
+}
+
+int signal_count;
+
+void sigsegv_cb(int sig)
+{
+	int cpuid_val = 0;
+
+	signal_count++;
+	printf("[ SIG_SEGV ]\n");
+	printf("arch_prctl(ARCH_GET_CPUID); ");
+
+	cpuid_val = arch_prctl(ARCH_GET_CPUID, 0);
+	if (cpuid_val < 0)
+		errx(1, "ARCH_GET_CPUID fails now, but not before?");
+
+	printf("cpuid_val == %s\n", cpuid_names[cpuid_val]);
+	printf("arch_prctl(ARC_SET_CPUID, ARCH_CPUID_ENABLE)\n");
+	if (arch_prctl(ARCH_SET_CPUID, ARCH_CPUID_ENABLE) != 0)
+		errx(1, "ARCH_SET_CPUID failed!");
+
+	printf("cpuid() == ");
+}
+
+int main(int argc, char **argv)
+{
+	int cpuid_val = 0, child = 0, status = 0;
+	unsigned int eax = 0, ebx = 0, ecx = 0, edx = 0;
+
+	signal(SIGSEGV, sigsegv_cb);
+	setvbuf(stdout, NULL, _IONBF, 0);
+
+	cpuid(&eax, &ebx, &ecx, &edx);
+	printf("cpuid() == {%x, %x, %x, %x}\n", eax, ebx, ecx, edx);
+	printf("arch_prctl(ARCH_GET_CPUID); ");
+
+	cpuid_val = arch_prctl(ARCH_GET_CPUID, 0);
+	if (cpuid_val < 0) {
+		if (errno == EINVAL) {
+			printf("ARCH_GET_CPUID is unsupported on this kernel.\n");
+			fflush(stdout);
+			exit(0); /* no ARCH_GET_CPUID on this system */
+		} else if (errno == ENODEV) {
+			printf("ARCH_GET_CPUID is unsupported on this hardware.\n");
+			fflush(stdout);
+			exit(0); /* no ARCH_GET_CPUID on this system */
+		} else {
+			errx(errno, "ARCH_GET_CPUID failed unexpectedly!");
+		}
+	}
+
+	printf("cpuid_val == %s\n", cpuid_names[cpuid_val]);
+	cpuid(&eax, &ebx, &ecx, &edx);
+	printf("cpuid() == {%x, %x, %x, %x}\n", eax, ebx, ecx, edx);
+	printf("arch_prctl(ARCH_SET_CPUID, ARCH_CPUID_ENABLE)\n");
+
+	if (arch_prctl(ARCH_SET_CPUID, ARCH_CPUID_ENABLE) != 0) {
+		if (errno == EINVAL) {
+			printf("ARCH_SET_CPUID is unsupported on this kernel.");
+			exit(0); /* no ARCH_SET_CPUID on this system */
+		} else if (errno == ENODEV) {
+			printf("ARCH_SET_CPUID is unsupported on this hardware.");
+			exit(0); /* no ARCH_SET_CPUID on this system */
+		} else {
+			errx(errno, "ARCH_SET_CPUID failed unexpectedly!");
+		}
+	}
+
+
+	cpuid(&eax, &ebx, &ecx, &edx);
+	printf("cpuid() == {%x, %x, %x, %x}\n", eax, ebx, ecx, edx);
+	printf("arch_prctl(ARCH_SET_CPUID, ARCH_CPUID_SIGSEGV)\n");
+	fflush(stdout);
+
+	if (arch_prctl(ARCH_SET_CPUID, ARCH_CPUID_SIGSEGV) == -1)
+		errx(1, "ARCH_SET_CPUID failed!");
+
+	printf("cpuid() == ");
+	eax = ebx = ecx = edx = 0;
+	cpuid(&eax, &ebx, &ecx, &edx);
+	printf("{%x, %x, %x, %x}\n", eax, ebx, ecx, edx);
+	printf("arch_prctl(ARCH_SET_CPUID, ARCH_CPUID_SIGSEGV)\n");
+
+	if (signal_count != 1)
+		errx(1, "cpuid didn't fault!");
+
+	if (arch_prctl(ARCH_SET_CPUID, ARCH_CPUID_SIGSEGV) == -1)
+		errx(1, "ARCH_SET_CPUID failed!");
+
+	if (argc > 1)
+		exit(0); /* Don't run the whole test again if we were execed */
+
+	printf("do_child_test\n");
+	child = fork();
+	if (child == 0)
+		return do_child_test();
+
+	if (child != waitpid(child, &status, 0))
+		errx(1, "waitpid failed!?");
+
+	if (WEXITSTATUS(status) != 0)
+		errx(1, "Child exited abnormally!");
+
+	/* The child enabling cpuid should not have affected us */
+	printf("cpuid() == ");
+	eax = ebx = ecx = edx = 0;
+	cpuid(&eax, &ebx, &ecx, &edx);
+	printf("{%x, %x, %x, %x}\n", eax, ebx, ecx, edx);
+	printf("arch_prctl(ARCH_SET_CPUID, ARCH_CPUID_SIGSEGV)\n");
+
+	if (signal_count != 2)
+		errx(1, "cpuid didn't fault!");
+
+	if (arch_prctl(ARCH_SET_CPUID, ARCH_CPUID_SIGSEGV) == -1)
+		errx(1, "ARCH_SET_CPUID failed!");
+
+	/* Our ARCH_CPUID_SIGSEGV should not propagate through exec */
+	printf("do_child_exec_test\n");
+	fflush(stdout);
+
+	child = fork();
+	if (child == 0)
+		return do_child_exec_test(eax, ebx, ecx, edx);
+
+	if (child != waitpid(child, &status, 0))
+		errx(1, "waitpid failed!?");
+
+	if (WEXITSTATUS(status) != 0)
+		errx(1, "Child exited abnormally!");
+
+	printf("All tests passed!\n");
+	exit(EXIT_SUCCESS);
+}