Re: [PATCH v2] KVM: nVMX: do not use dangling shadow VMCS after guest reset

[Liran Alon]

> On 20 Jul 2019, at 0:39, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
> 
> If a KVM guest is reset while running a nested guest, free_nested will
> disable the shadow VMCS execution control in the vmcs01.  However,
> on the next KVM_RUN vmx_vcpu_run would nevertheless try to sync
> the VMCS12 to the shadow VMCS which has since been freed.
> 
> This causes a vmptrld of a NULL pointer on my machime, but Jan reports
> the host to hang altogether.  Let's see how much this trivial patch fixes.
> 
> Reported-by: Jan Kiszka <jan.kiszka@xxxxxxxxxxx>
> Cc: Liran Alon <liran.alon@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>

1) Are we sure we prefer WARN_ON() instead of WARN_ON_ONCE()?
2) Should we also check for WARN_ON(!vmcs12)? As free_nested() also kfree(vmx->nested.cached_vmcs12).
In fact, because free_nested() donât put NULL in cached_vmcs12 after kfree() it, I wonder if we shouldnât create a separate patch that does:
(a) Modify free_nested() to put NULL in cached_vmcs12 after kfree().
(b) Put BUG_ON(!cached_vmcs12) in get_vmcs12() before returning value.

-Liran

> ---
> arch/x86/kvm/vmx/nested.c | 8 +++++++-
> 1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index 4f23e34f628b..0f1378789bd0 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -194,6 +194,7 @@ static void vmx_disable_shadow_vmcs(struct vcpu_vmx *vmx)
> {
> 	secondary_exec_controls_clearbit(vmx, SECONDARY_EXEC_SHADOW_VMCS);
> 	vmcs_write64(VMCS_LINK_POINTER, -1ull);
> +	vmx->nested.need_vmcs12_to_shadow_sync = false;
> }
> 
> static inline void nested_release_evmcs(struct kvm_vcpu *vcpu)
> @@ -1341,6 +1342,9 @@ static void copy_shadow_to_vmcs12(struct vcpu_vmx *vmx)
> 	unsigned long val;
> 	int i;
> 
> +	if (WARN_ON(!shadow_vmcs))
> +		return;
> +
> 	preempt_disable();
> 
> 	vmcs_load(shadow_vmcs);
> @@ -1373,6 +1377,9 @@ static void copy_vmcs12_to_shadow(struct vcpu_vmx *vmx)
> 	unsigned long val;
> 	int i, q;
> 
> +	if (WARN_ON(!shadow_vmcs))
> +		return;
> +
> 	vmcs_load(shadow_vmcs);
> 
> 	for (q = 0; q < ARRAY_SIZE(fields); q++) {
> @@ -4436,7 +4443,6 @@ static inline void nested_release_vmcs12(struct kvm_vcpu *vcpu)
> 		/* copy to memory all shadowed fields in case
> 		   they were modified */
> 		copy_shadow_to_vmcs12(vmx);
> -		vmx->nested.need_vmcs12_to_shadow_sync = false;
> 		vmx_disable_shadow_vmcs(vmx);
> 	}
> 	vmx->nested.posted_intr_nv = -1;
> -- 
> 1.8.3.1
>