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

From: Kyle Huey <me@kylehuey.com>
To: Andy Lutomirski <luto@amacapital.net>
Cc: "Robert O'Callahan" <robert@ocallahan.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
	<x86@kernel.org>, Shuah Khan <shuah@kernel.org>,
	"Peter Zijlstra (Intel)" <peterz@infradead.org>,
	Borislav Petkov <bp@suse.de>,
	"Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
	Len Brown <len.brown@intel.com>,
	Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
	Aravind Gopalakrishnan <Aravind.Gopalakrishnan@amd.com>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Vladimir Zapolskiy <vladimir_zapolskiy@mentor.com>,
	Andy Lutomirski <luto@kernel.org>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Dmitry Safonov <dsafonov@virtuozzo.com>,
	Kees Cook <keescook@chromium.org>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	Andrey Ryabinin <aryabinin@virtuozzo.com>,
	Jiri Slaby <jslaby@suse.cz>,
	Paul Gortmaker <paul.gortmaker@windriver.com>,
	Denys Vlasenko <dvlasenk@redhat.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
	<linux-kernel@vger.kernel.org>,
	"open list:KERNEL SELFTEST FRAMEWORK" 
	<linux-kselftest@vger.kernel.org>
Subject: Re: [PATCH v2 3/3] x86,arch_prctl Add ARCH_[GET|SET]_CPUID for controlling the CPUID instruction
Date: Wed, 14 Sep 2016 19:19:49 -0700	[thread overview]
Message-ID: <CAP045AqaB=+QKOnm7iOXimF0JxRfvHfBGhRoyRd1u2D19EMDqA@mail.gmail.com> (raw)
In-Reply-To: <CALCETrWqYrotMSKqvsug-fgbO1-4X6q0FQpsiTrPkWSzu56NAA@mail.gmail.com>

On Wed, Sep 14, 2016 at 6:54 PM, Andy Lutomirski <luto@amacapital.net> wrote:
> On Wed, Sep 14, 2016 at 6:47 PM, Kyle Huey <me@kylehuey.com> wrote:
>> On Wed, Sep 14, 2016 at 6:29 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>>> On Wed, Sep 14, 2016 at 2:01 PM, Kyle Huey <me@kylehuey.com> 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