linux-kernel - [PATCH] x86/paravirt: Guard against invalid cpu # in pv_vcpu_is

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Date:   Mon, 25 Mar 2019 11:57:06 -0400
From:   Waiman Long <longman@...hat.com>
To:     Juergen Gross <jgross@...e.com>,
        Alok Kataria <akataria@...are.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will.deacon@....com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Waiman Long <longman@...hat.com>
Subject: [PATCH] x86/paravirt: Guard against invalid cpu # in pv_vcpu_is_preempted()

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@...hat.com>
---
 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