[PATCH 1/2] KVM: nVMX: Reset MMU back to the root MMU if loading L2 state fails for !VM-Entry

[Paolo Bonzini <pbonzini@redhat.com>]

From: Amirmohammad Eftekhar <amirmohammadeftekhar@gmail.com>

Reset the vCPU's MMU pointer back to the root MMU if loading L2 state fails
in the "not from VM-Entry" case, i.e. when emulating RSM or handling
KVM_SET_NESTED_STATE.  Failure to unwind the MMU pointer results in KVM
trying to run L1 with a nested MMU, and is especially problematic if the
vCPU manages to free its nested state before the VM crashes.  Constructing
the root for a nested EPT MMU goes through vmcs12, which is dynamically
allocated/freed when nVMX is enabled/disabled.  And so if the nested state
is freed with the MMU still pointing at the nested MMU, KVM will hit a NULL
pointer deref when attempting to get L1=>L2 EPTP.

  BUG: kernel NULL pointer dereference, address: 0000000000000078
  #PF: supervisor read access in kernel mode
  #PF: error_code(0x0000) - not-present page
  PGD 114a2d067 P4D 111743067 PUD 11141f067 PMD 0
  Oops: Oops: 0000 [#1] SMP
  CPU: 20 UID: 1000 PID: 1272 Comm: nested_tdp_faul Not tainted 7.3.0-rc4-8c89f11d1a99b-sink-vm #1914 PREEMPTLAZY
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
  RIP: 0010:nested_ept_get_eptp+0xb/0x10 [kvm_intel]
  Call Trace:
   <TASK>
   kvm_mmu_load+0xcd/0x870 [kvm]
   kvm_arch_vcpu_ioctl_run+0x18f5/0x1f80 [kvm]
   kvm_vcpu_ioctl+0x2c7/0x970 [kvm]
   __x64_sys_ioctl+0x90/0xd0
   do_syscall_64+0x9c/0x4d0
   entry_SYSCALL_64_after_hwframe+0x4b/0x53
  RIP: 0033:0x47491b
   </TASK>
  Modules linked in: kvm_intel kvm irqbypass
  CR2: 0000000000000078
  ---[ end trace 0000000000000000 ]---
  RIP: 0010:nested_ept_get_eptp+0xb/0x10 [kvm_intel]

Cc: David Woodhouse <dwmw2@infradead.org>
Fixes: 72e9cbdb4338 ("KVM: nVMX: fix SMI injection in guest mode")
Fixes: 8fcc4b5923af ("kvm: nVMX: Introduce KVM_CAP_NESTED_STATE")
Cc: stable@vger.kernel.org
Signed-off-by: Amirmohammad Eftekhar <amirmohammadeftekhar@gmail.com>
[sean: write changelog]
Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://patch.msgid.link/20261001214356.3825100-2-seanjc@google.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 arch/x86/kvm/vmx/nested.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index b25216862740..5165275128fc 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -3770,8 +3770,10 @@ enum nvmx_vmentry_status nested_vmx_enter_non_root_mode(struct kvm_vcpu *vcpu,
 vmentry_fail_vmexit:
 	vmx_switch_vmcs(vcpu, &vmx->vmcs01);
 
-	if (!from_vmentry)
+	if (!from_vmentry) {
+		nested_ept_uninit_mmu_context(vcpu);
 		return NVMX_VMENTRY_VMEXIT;
+	}
 
 	nested_put_vmcs12_pages(vcpu);
 
-- 
2.56.0