[patch added to the 3.12 stable tree] x86, kvm: Clear paravirt_enabled on KVM guests for espfix32's benefit

[Jiri Slaby <jslaby@xxxxxxx>]

From: Andy Lutomirski <luto@xxxxxxxxxxxxxx>

This patch has been added to the 3.12 stable tree. If you have any
objections, please let us know.

===============

commit 29fa6825463c97e5157284db80107d1bfac5d77b upstream.

paravirt_enabled has the following effects:

 - Disables the F00F bug workaround warning.  There is no F00F bug
   workaround any more because Linux's standard IDT handling already
   works around the F00F bug, but the warning still exists.  This
   is only cosmetic, and, in any event, there is no such thing as
   KVM on a CPU with the F00F bug.

 - Disables 32-bit APM BIOS detection.  On a KVM paravirt system,
   there should be no APM BIOS anyway.

 - Disables tboot.  I think that the tboot code should check the
   CPUID hypervisor bit directly if it matters.

 - paravirt_enabled disables espfix32.  espfix32 should *not* be
   disabled under KVM paravirt.

The last point is the purpose of this patch.  It fixes a leak of the
high 16 bits of the kernel stack address on 32-bit KVM paravirt
guests.  Fixes CVE-2014-8134.

Suggested-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Signed-off-by: Jiri Slaby <jslaby@xxxxxxx>
---
 arch/x86/kernel/kvm.c      | 9 ++++++++-
 arch/x86/kernel/kvmclock.c | 1 -
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index f022c54a79a4..e72593338df6 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -280,7 +280,14 @@ do_async_page_fault(struct pt_regs *regs, unsigned long error_code)
 static void __init paravirt_ops_setup(void)
 {
 	pv_info.name = "KVM";
-	pv_info.paravirt_enabled = 1;
+
+	/*
+	 * KVM isn't paravirt in the sense of paravirt_enabled.  A KVM
+	 * guest kernel works like a bare metal kernel with additional
+	 * features, and paravirt_enabled is about features that are
+	 * missing.
+	 */
+	pv_info.paravirt_enabled = 0;
 
 	if (kvm_para_has_feature(KVM_FEATURE_NOP_IO_DELAY))
 		pv_cpu_ops.io_delay = kvm_io_delay;
diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
index 1570e0741344..23457e5f0f4f 100644
--- a/arch/x86/kernel/kvmclock.c
+++ b/arch/x86/kernel/kvmclock.c
@@ -262,7 +262,6 @@ void __init kvmclock_init(void)
 #endif
 	kvm_get_preset_lpj();
 	clocksource_register_hz(&kvm_clock, NSEC_PER_SEC);
-	pv_info.paravirt_enabled = 1;
 	pv_info.name = "KVM";
 
 	if (kvm_para_has_feature(KVM_FEATURE_CLOCKSOURCE_STABLE_BIT))
-- 
2.2.1

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