Re: [PATCH] KVM: SVM: Fix SNP AP destroy race with VMRUN

[Sean Christopherson]

On Mon, Mar 17, 2025, Tom Lendacky wrote:
> On 3/17/25 12:20, Tom Lendacky wrote:
> > An AP destroy request for a target vCPU is typically followed by an
> > RMPADJUST to remove the VMSA attribute from the page currently being
> > used as the VMSA for the target vCPU. This can result in a vCPU that
> > is about to VMRUN to exit with #VMEXIT_INVALID.
> > 
> > This usually does not happen as APs are typically sitting in HLT when
> > being destroyed and therefore the vCPU thread is not running at the time.
> > However, if HLT is allowed inside the VM, then the vCPU could be about to
> > VMRUN when the VMSA attribute is removed from the VMSA page, resulting in
> > a #VMEXIT_INVALID when the vCPU actually issues the VMRUN and causing the
> > guest to crash. An RMPADJUST against an in-use (already running) VMSA
> > results in a #NPF for the vCPU issuing the RMPADJUST, so the VMSA
> > attribute cannot be changed until the VMRUN for target vCPU exits. The
> > Qemu command line option '-overcommit cpu-pm=on' is an example of allowing
> > HLT inside the guest.
> > 
> > Use kvm_test_request() to ensure that the target vCPU sees the AP destroy
> > request before returning to the initiating vCPU.
> > 
> > Fixes: e366f92ea99e ("KVM: SEV: Support SEV-SNP AP Creation NAE event")
> > Signed-off-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
> 
> Sean,
> 
> If you're ok with this approach for the fix, this patch may need to be
> adjusted given your series around AP creation fixes, unless you want to
> put this as an early patch in your series. Let me know what you'd like
> to do.

This is unsafe as it requires userspace to do KVM_RUN _and_ for the vCPU to get
far enough along to consume the request.

Very off-the-cuff, but I assume KVM_REQ_UPDATE_PROTECTED_GUEST_STATE just needs
to be annotated with KVM_REQUEST_WAIT.

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 04e6c5604bc3..67abfe97c600 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -124,7 +124,8 @@
        KVM_ARCH_REQ_FLAGS(31, KVM_REQUEST_WAIT | KVM_REQUEST_NO_WAKEUP)
 #define KVM_REQ_HV_TLB_FLUSH \
        KVM_ARCH_REQ_FLAGS(32, KVM_REQUEST_WAIT | KVM_REQUEST_NO_WAKEUP)
-#define KVM_REQ_UPDATE_PROTECTED_GUEST_STATE   KVM_ARCH_REQ(34)
+#define KVM_REQ_UPDATE_PROTECTED_GUEST_STATE \
+       KVM_ARCH_REQ_FLAGS(34, KVM_REQUEST_WAIT)
 
 #define CR0_RESERVED_BITS                                               \
        (~(unsigned long)(X86_CR0_PE | X86_CR0_MP | X86_CR0_EM | X86_CR0_TS \