Re: [PATCH v2 3/3] x86,arch_prctl Add ARCH_[GET|SET]_CPUID for controlling the CPUID instruction

[Kyle Huey]

On Wed, Sep 14, 2016 at 6:54 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Wed, Sep 14, 2016 at 6:47 PM, Kyle Huey <me@xxxxxxxxxxxx> wrote:
>> On Wed, Sep 14, 2016 at 6:29 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>>> On Wed, Sep 14, 2016 at 2:01 PM, Kyle Huey <me@xxxxxxxxxxxx> wrote:
>
>>>> +
>>>> +int set_cpuid_mode(struct task_struct *task, unsigned long val)
>>>> +{
>>>> +       /* Only disable/enable_cpuid() if it is supported on this hardware. */
>>>> +       bool cpuid_fault_supported = static_cpu_has(X86_FEATURE_CPUID_FAULT);
>>>> +
>>>> +       if (val == ARCH_CPUID_ENABLE && cpuid_fault_supported) {
>>>> +               if (task_no_new_privs(task) && test_thread_flag(TIF_NOCPUID))
>>>> +                       return -EACCES;
>>>
>>> This check seems confused.  If this flag were preserved on execve,
>>> it's the SIGSEGV mode that would need the check.
>>
>> Not sure I follow this one.  no_new_privs should block transitions
>> from SIGSEGV to ENABLE, right?  That's what this check does.
>
> It's the other way around entirely: if you make a change to your
> process context such that a subseqently execve()'d setuid program
> might malfunction, you've just done something dangerous.  This is only
> okay, at least in newly-supported instances, if you are either
> privileged or if you have no_new_privs set.  Having privilege makes it
> okay: unprivileged programs can't use it to subvert setuid programs.
> no_new_privs makes it safe as well: if no_new_privs is set, you can't
> gain privilege via execve(), so there's no attack surface.  So, if you
> have execve() keep ARCH_CPUID_SIGSEGV set, then setting it that way in
> the first place should require privilege or no_new_privs.
>
> I personally favor resetting to ARCH_CPUID_ENABLE on execve() and not
> worrying about no_new_privs.
>
> Does that make sense?

Yes, ok.  Robert and I agree that resetting does make the most sense.
Using this usefully requires a ptrace supervisor (to catch the traps),
which can easily inject a call to arch_prctl to reenable
ARCH_CPUID_SIGSEGV when desired.

- Kyle