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From:   Wanpeng Li <kernellwp@gmail.com>
To:     linux-kernel@vger.kernel.org, kvm@vger.kernel.org
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        =?UTF-8?q?Radim=20Kr=C4=8Dm=C3=A1=C5=99?= <rkrcmar@redhat.com>,
        stable@vger.kernel.org
Subject: [PATCH 3/3] KVM: Fix leak vCPU's VMCS value into other pCPU
Date:   Wed, 31 Jul 2019 19:27:18 +0800
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From: Wanpeng Li <wanpengli@tencent.com>

After commit d73eb57b80b (KVM: Boost vCPUs that are delivering interrupts), a 
five years ago bug is exposed. Running ebizzy benchmark in three 80 vCPUs VMs 
on one 80 pCPUs Skylake server, a lot of rcu_sched stall warning splatting 
in the VMs after stress testing:

 INFO: rcu_sched detected stalls on CPUs/tasks: { 4 41 57 62 77} (detected by 15, t=60004 jiffies, g=899, c=898, q=15073)
 Call Trace:
   flush_tlb_mm_range+0x68/0x140
   tlb_flush_mmu.part.75+0x37/0xe0
   tlb_finish_mmu+0x55/0x60
   zap_page_range+0x142/0x190
   SyS_madvise+0x3cd/0x9c0
   system_call_fastpath+0x1c/0x21

swait_active() sustains to be true before finish_swait() is called in 
kvm_vcpu_block(), voluntarily preempted vCPUs are taken into account 
by kvm_vcpu_on_spin() loop greatly increases the probability condition 
kvm_arch_vcpu_runnable(vcpu) is checked and can be true, when APICv 
is enabled the yield-candidate vCPU's VMCS RVI field leaks(by 
vmx_sync_pir_to_irr()) into spinning-on-a-taken-lock vCPU's current 
VMCS.

This patch fixes it by reverting the kvm_arch_vcpu_runnable() condition 
in kvm_vcpu_on_spin() loop.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Fixes: 98f4a1467 (KVM: add kvm_arch_vcpu_runnable() test to kvm_vcpu_on_spin() loop)
Cc: stable@vger.kernel.org
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
---
 virt/kvm/kvm_main.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index ed061d8..12f2c91 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2506,7 +2506,7 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me, bool yield_to_kernel_mode)
 				continue;
 			if (vcpu == me)
 				continue;
-			if (swait_active(&vcpu->wq) && !kvm_arch_vcpu_runnable(vcpu))
+			if (swait_active(&vcpu->wq))
 				continue;
 			if (READ_ONCE(vcpu->preempted) && yield_to_kernel_mode &&
 				!kvm_arch_vcpu_in_kernel(vcpu))
-- 
2.7.4