For all intents and purposes, revert commit b4b65b5642d6 ("KVM: x86: cleanup freeing of nested state") and go back to manually tearing down nested state when freeing a vCPU. When a vCPU is destroyed while L2 is active, KVM synthesizes a nested VM-Exit, which flushes the cached shadow VMCS12 back to guest memory: vmx_vcpu_free() |-> nested_vmx_free_vcpu() |-> vmx_leave_nested() |-> nested_vmx_vmexit(vcpu, -1, 0, 0) |-> nested_flush_cached_shadow_vmcs12() |-> kvm_write_guest_cached() |-> __copy_to_user(ghc->hva, ...) Accessing user memory via a memslot during VM destruction is broken, as there are no guarantees that current->mm == kvm->mm when the VM is dying, because the last reference to the VM can be put from a different process than the original creating processes. And even if the original process does put the final reference, during process exit, do_exit() calls exit_mm() before closing file descriptors, so vCPU destruction runs with current->mm == NULL on a borrowed lazy TLB active_mm. If the borrowed address space has a writable mapping at the to-be-written userspace address, KVM will corrupt an unrelated task's memory since uaccess APIs, including __copy_to_user(), don't sanity check current->mm (and *can't* sanity perform KVM's current->mm == kvm->mm check since that is firmly a KVM-only concept). Furthermore, the shadow VMCS flaw is only the tip of the iceberg, as the MSR load/store lists mean that emulating a nested VM-Exit can read or write an arbitrary set of MSRs, e.g. if MSR_KVM_SYSTEM_TIME_NEW is in L1's MSR load list: kvm_arch_vcpu_destroy() kvmclock_reset() - takes pv_time off kvm->gpc_list vmx_vcpu_free() nested_vmx_free_vcpu() vmx_leave_nested() nested_vmx_vmexit() __nested_vmx_vmexit() load_vmcs12_host_state() nested_vmx_load_msr() - writes the MSRs that L1 listed kvm_cpu_has_injectable_intr() - reads Xen's vcpu_info kvm_xen_destroy_vcpu() - deactivates the vcpu_info cache processing the MSR write puts the "pv_time" cache back on kvm->gpc_list, and the next list_del() writes into the freed vCPU. And as effectively pointed out by syzbot[*], attempting to fix each case individually is a losing game of whack-a-mole. Deliberately leave "nested_run_pending" as-is, because even though doing so creates potentitally inconsistent state, KVM should *never* consume said state while freeing a vCPU. I.e. if leaving nested_run_pending set trips a sanity check or bug, then that just means there's more fixing to be done. Open code only the "free vCPU" path, i.e. keep emulating a nested VM-Exit when forcing a vCPU out of L2 via vmx_leave_nested(), as there are a number of updates buried in the nested VM-Exit flow that are needed if the vCPU is going to run in L1, e.g. clearing pending exceptions and MTF traps, setting the TSC offset+scale information back to L1, and so on and so forth. Note, this means svm_leave_nested() is all but guaranteed to be buggy, but that's basically an open secret at this point and at least any bugs "only" break L1, i.e. don't corrupt host state as is the case for abusing nested VM-Exit when freeing a vCPU. Link: https://lore.kernel.org/all/6ac0145f.34119e79.2f92f3.0003.GAE@google.com [*] Fixes: b4b65b5642d6 ("KVM: x86: cleanup freeing of nested state") Cc: stable@vger.kernel.org Co-developed-by: Chris Mason Signed-off-by: Chris Mason Signed-off-by: Sean Christopherson --- arch/x86/kvm/vmx/nested.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-) diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c index 9b0bfa2f854c..49c4529901c5 100644 --- a/arch/x86/kvm/vmx/nested.c +++ b/arch/x86/kvm/vmx/nested.c @@ -385,8 +385,15 @@ static void free_nested(struct kvm_vcpu *vcpu) */ void nested_vmx_free_vcpu(struct kvm_vcpu *vcpu) { + struct vcpu_vmx *vmx = to_vmx(vcpu); + vcpu_load(vcpu); - vmx_leave_nested(vcpu); + if (is_guest_mode(vcpu)) { + hrtimer_cancel(&vmx->nested.preemption_timer); + leave_guest_mode(vcpu); + vmx_switch_vmcs(vcpu, &vmx->vmcs01); + } + free_nested(vcpu); vcpu_put(vcpu); } -- 2.56.0.385.gac671c819a-goog