This is a note to let you know that I've just added the patch titled
KVM: nVMX: Fix races when sending nested PI while dest enters/leaves L2
to the 4.9-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git%3Ba=su...
The filename of the patch is: kvm-nvmx-fix-races-when-sending-nested-pi-while-dest-enters-leaves-l2.patch and it can be found in the queue-4.9 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree, please let stable@vger.kernel.org know about it.
From 6b6977117f50d60455ace86b2d256f6fb4f3de05 Mon Sep 17 00:00:00 2001
From: Liran Alon liran.alon@oracle.com Date: Thu, 9 Nov 2017 20:27:20 +0200 Subject: KVM: nVMX: Fix races when sending nested PI while dest enters/leaves L2 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit
From: Liran Alon liran.alon@oracle.com
commit 6b6977117f50d60455ace86b2d256f6fb4f3de05 upstream.
Consider the following scenario: 1. CPU A calls vmx_deliver_nested_posted_interrupt() to send an IPI to CPU B via virtual posted-interrupt mechanism. 2. CPU B is currently executing L2 guest. 3. vmx_deliver_nested_posted_interrupt() calls kvm_vcpu_trigger_posted_interrupt() which will note that vcpu->mode == IN_GUEST_MODE. 4. Assume that before CPU A sends the physical POSTED_INTR_NESTED_VECTOR IPI, CPU B exits from L2 to L0 during event-delivery (valid IDT-vectoring-info). 5. CPU A now sends the physical IPI. The IPI is received in host and it's handler (smp_kvm_posted_intr_nested_ipi()) does nothing. 6. Assume that before CPU A sets pi_pending=true and KVM_REQ_EVENT, CPU B continues to run in L0 and reach vcpu_enter_guest(). As KVM_REQ_EVENT is not set yet, vcpu_enter_guest() will continue and resume L2 guest. 7. At this point, CPU A sets pi_pending=true and KVM_REQ_EVENT but it's too late! CPU B already entered L2 and KVM_REQ_EVENT will only be consumed at next L2 entry!
Another scenario to consider: 1. CPU A calls vmx_deliver_nested_posted_interrupt() to send an IPI to CPU B via virtual posted-interrupt mechanism. 2. Assume that before CPU A calls kvm_vcpu_trigger_posted_interrupt(), CPU B is at L0 and is about to resume into L2. Further assume that it is in vcpu_enter_guest() after check for KVM_REQ_EVENT. 3. At this point, CPU A calls kvm_vcpu_trigger_posted_interrupt() which will note that vcpu->mode != IN_GUEST_MODE. Therefore, do nothing and return false. Then, will set pi_pending=true and KVM_REQ_EVENT. 4. Now CPU B continue and resumes into L2 guest without processing the posted-interrupt until next L2 entry!
To fix both issues, we just need to change vmx_deliver_nested_posted_interrupt() to set pi_pending=true and KVM_REQ_EVENT before calling kvm_vcpu_trigger_posted_interrupt().
It will fix the first scenario by chaging step (6) to note that KVM_REQ_EVENT and pi_pending=true and therefore process nested posted-interrupt.
It will fix the second scenario by two possible ways: 1. If kvm_vcpu_trigger_posted_interrupt() is called while CPU B has changed vcpu->mode to IN_GUEST_MODE, physical IPI will be sent and will be received when CPU resumes into L2. 2. If kvm_vcpu_trigger_posted_interrupt() is called while CPU B hasn't yet changed vcpu->mode to IN_GUEST_MODE, then after CPU B will change vcpu->mode it will call kvm_request_pending() which will return true and therefore force another round of vcpu_enter_guest() which will note that KVM_REQ_EVENT and pi_pending=true and therefore process nested posted-interrupt.
Fixes: 705699a13994 ("KVM: nVMX: Enable nested posted interrupt processing") Signed-off-by: Liran Alon liran.alon@oracle.com Reviewed-by: Nikita Leshenko nikita.leshchenko@oracle.com Reviewed-by: Krish Sadhukhan krish.sadhukhan@oracle.com [Add kvm_vcpu_kick to also handle the case where L1 doesn't intercept L2 HLT and L2 executes HLT instruction. - Paolo] Signed-off-by: Paolo Bonzini pbonzini@redhat.com Signed-off-by: Radim Krčmář rkrcmar@redhat.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- arch/x86/kvm/vmx.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-)
--- a/arch/x86/kvm/vmx.c +++ b/arch/x86/kvm/vmx.c @@ -4967,14 +4967,15 @@ static int vmx_deliver_nested_posted_int
if (is_guest_mode(vcpu) && vector == vmx->nested.posted_intr_nv) { - /* the PIR and ON have been set by L1. */ - kvm_vcpu_trigger_posted_interrupt(vcpu); /* * If a posted intr is not recognized by hardware, * we will accomplish it in the next vmentry. */ vmx->nested.pi_pending = true; kvm_make_request(KVM_REQ_EVENT, vcpu); + /* the PIR and ON have been set by L1. */ + if (!kvm_vcpu_trigger_posted_interrupt(vcpu)) + kvm_vcpu_kick(vcpu); return 0; } return -1;
Patches currently in stable-queue which might be from liran.alon@oracle.com are
queue-4.9/kvm-nvmx-fix-races-when-sending-nested-pi-while-dest-enters-leaves-l2.patch