[PATCH] x86/paravirt: Guard against invalid cpu # in pv_vcpu_is_preempted()

[Waiman Long]

It was found that passing an invalid cpu number to pv_vcpu_is_preempted()
might panic the kernel in a VM guest. For example,

[    2.531077] Oops: 0000 [#1] SMP PTI
  :
[    2.532545] Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2011
[    2.533321] RIP: 0010:__raw_callee_save___kvm_vcpu_is_preempted+0x0/0x20

To guard against this kind of kernel panic, check is added to
pv_vcpu_is_preempted() to make sure that no invalid cpu number will
be used.

Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
 arch/x86/include/asm/paravirt.h | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
index c25c38a05c1c..4cfb465dcde4 100644
--- a/arch/x86/include/asm/paravirt.h
+++ b/arch/x86/include/asm/paravirt.h
@@ -671,6 +671,12 @@ static __always_inline void pv_kick(int cpu)
 
 static __always_inline bool pv_vcpu_is_preempted(long cpu)
 {
+	/*
+	 * Guard against invalid cpu number or the kernel might panic.
+	 */
+	if (WARN_ON_ONCE((unsigned long)cpu >= nr_cpu_ids))
+		return false;
+
 	return PVOP_CALLEE1(bool, lock.vcpu_is_preempted, cpu);
 }
 
-- 
2.18.1