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Date:   Wed, 10 Jul 2019 16:26:53 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Jiri Kosina <jikos@kernel.org>
Cc:     Xi Ruoyao <xry111@mengyan1223.wang>,
        Thomas Gleixner <tglx@linutronix.de>,
        Kees Cook <keescook@chromium.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Borislav Petkov <bp@alien8.de>, Len Brown <lenb@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        Tony Luck <tony.luck@intel.com>,
        Bob Moore <robert.moore@intel.com>,
        Erik Schmauss <erik.schmauss@intel.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Nadav Amit <namit@vmware.com>
Subject: Re: [GIT PULL] x86/topology changes for v5.3
Message-ID: <20190710142653.GJ3419@hirez.programming.kicks-ass.net>
References: <alpine.DEB.2.21.1907100039540.1758@nanos.tec.linutronix.de>
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On Wed, Jul 10, 2019 at 04:22:51PM +0200, Jiri Kosina wrote:
> On Wed, 10 Jul 2019, Peter Zijlstra wrote:
> 
> > If we mark the key as RO after init, and then try and modify the key to
> > link module usage sites, things might go bang as described.
> > 
> > Thanks!
> > 
> > 
> > diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> > index 27d7864e7252..5bf7a8354da2 100644
> > --- a/arch/x86/kernel/cpu/common.c
> > +++ b/arch/x86/kernel/cpu/common.c
> > @@ -366,7 +366,7 @@ static __always_inline void setup_umip(struct cpuinfo_x86 *c)
> >  	cr4_clear_bits(X86_CR4_UMIP);
> >  }
> >  
> > -DEFINE_STATIC_KEY_FALSE_RO(cr_pinning);
> > +DEFINE_STATIC_KEY_FALSE(cr_pinning);
> 
> Good catch, I guess that is going to fix it.
> 
> At the same time though, it sort of destroys the original intent of Kees' 
> patch, right? The exploits will just have to call static_key_disable() 
> prior to calling native_write_cr4() again, and the protection is gone.

This is fixable by moving native_write_cr*() out-of-line, such that they
never end up in a module.