[PATCH v2 09/10] KVM: nVMX: Don't try to load eVMCS12 page when the VM is dying

Don't try to get/load the eVMCS12 page when KVM can't do uaccesses to guest
memory, i.e. when the VM dying, as reading/writing guest memory via the
associated userspace address space is obviously broken if the address space
is inactive and/or has already been torn down.

Hack-a-fix the eVMCS code even though KVM now protects against bad uaccess
reads/writes in the core APIs, as doing so will allow adding WARNs in said
APIs to help detect other buggy code.  Add a TODO to call out that checking
if KVM can do a uaccess for the VM is a hack; nVMX really needs to stop
abusing __nested_vmx_vmexit() when destroying a vCPU.

E.g. without the "fix", adding the sanity checks will trigger:

  ------------[ cut here ]------------
  WARNING: ./include/linux/kvm_host.h:1359 at kvm_read_guest_offset_cached+0x1b0/0x200 [kvm], CPU#14: hyperv_evmcs/27428
  Call Trace:
   <TASK>
   nested_get_evmptr+0x4f/0x80 [kvm_intel]
   nested_vmx_handle_enlightened_vmptrld+0x2a/0x130 [kvm_intel]
   __nested_vmx_vmexit+0xbe/0x8e0 [kvm_intel]
   nested_vmx_free_vcpu+0x36/0x50 [kvm_intel]
   vmx_vcpu_free+0x73/0x120 [kvm_intel]
   kvm_arch_vcpu_destroy+0x81/0x1a0 [kvm]
   kvm_destroy_vcpus+0x9d/0x160 [kvm]
   kvm_arch_destroy_vm+0x25/0x90 [kvm]
   kvm_put_kvm+0x342/0x460 [kvm]
   kvm_vm_stats_release+0x12/0x20 [kvm]
   __fput+0xfe/0x280
   __se_sys_close+0x76/0xe0
   do_syscall_64+0xfe/0x440
   entry_SYSCALL_64_after_hwframe+0x4b/0x53
  RIP: 0033:0x4a3822
   </TASK>
  ---[ end trace 0000000000000000 ]---

Fixes: f5c7e8425f18 ("KVM: nVMX: Always make an attempt to map eVMCS after migration")
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/vmx/nested.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 8e31eba4d9fa..07ec0dd014b5 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c

@@ -5091,8 +5091,13 @@ void __nested_vmx_vmexit(struct kvm_vcpu *vcpu, u32 vm_exit_reason,
 		 * Enlightened VMCS after migration and we still need to
 		 * do that when something is forcing L2->L1 exit prior to
 		 * the first L2 run.
+		 *
+		 * TODO: Drop the explicit check on being able to access guest
+		 *       memory once KVM no longer abuses the nested VM-Exit
+		 *       flow when destroying a vCPU.
 		 */
-		(void)nested_get_evmcs_page(vcpu);
+		if (__kvm_can_do_uaccess(vcpu->kvm))
+			(void)nested_get_evmcs_page(vcpu);
 #endif
 	}
 

-- 
2.56.0.rc1.315.gc6ed9934b7-goog