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From:   Nadav Amit <namit@vmware.com>
To:     Jiri Kosina <jikos@kernel.org>
CC:     Peter Zijlstra <peterz@infradead.org>,
        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>
Subject: Re: [GIT PULL] x86/topology changes for v5.3
Thread-Topic: [GIT PULL] x86/topology changes for v5.3
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Date:   Thu, 11 Jul 2019 07:11:19 +0000
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> On Jul 10, 2019, at 7:22 AM, Jiri Kosina <jikos@kernel.org> wrote:
>=20
> On Wed, 10 Jul 2019, Peter Zijlstra wrote:
>=20
>> 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.
>>=20
>> Thanks!
>>=20
>>=20
>> 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 cpuinf=
o_x86 *c)
>> 	cr4_clear_bits(X86_CR4_UMIP);
>> }
>>=20
>> -DEFINE_STATIC_KEY_FALSE_RO(cr_pinning);
>> +DEFINE_STATIC_KEY_FALSE(cr_pinning);
>=20
> Good catch, I guess that is going to fix it.
>=20
> At the same time though, it sort of destroys the original intent of Kees'=
=20
> patch, right? The exploits will just have to call static_key_disable()=20
> prior to calling native_write_cr4() again, and the protection is gone.

Even with DEFINE_STATIC_KEY_FALSE_RO(), I presume you can just call
set_memory_rw(), make the page that holds the key writable, and then call
static_key_disable(), followed by a call to native_write_cr4().=