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

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Date:   Wed, 14 Sep 2016 18:54:59 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Kyle Huey <me@...ehuey.com>
Cc:     "Robert O'Callahan" <robert@...llahan.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Shuah Khan <shuah@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Borislav Petkov <bp@...e.de>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Len Brown <len.brown@...el.com>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Aravind Gopalakrishnan <Aravind.Gopalakrishnan@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Vladimir Zapolskiy <vladimir_zapolskiy@...tor.com>,
        Andy Lutomirski <luto@...nel.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Dmitry Safonov <dsafonov@...tuozzo.com>,
        Kees Cook <keescook@...omium.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Jiri Slaby <jslaby@...e.cz>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
        <linux-kernel@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH v2 3/3] x86,arch_prctl Add ARCH_[GET|SET]_CPUID for
 controlling the CPUID instruction

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