Commit 5932ca41 authored by Paolo Bonzini's avatar Paolo Bonzini
Browse files

KVM: x86: disallow pre-fault for SNP VMs before initialization 



KVM_PRE_FAULT_MEMORY for an SNP guest can race with
sev_gmem_post_populate() in bad ways. The following sequence for
instance can potentially trigger an RMP fault:

  thread A, sev_gmem_post_populate: called
  thread B, sev_gmem_prepare: places below 'pfn' in a private state in RMP
  thread A, sev_gmem_post_populate: *vaddr = kmap_local_pfn(pfn + i);
  thread A, sev_gmem_post_populate: copy_from_user(vaddr, src + i * PAGE_SIZE, PAGE_SIZE);
  RMP #PF

Fix this by only allowing KVM_PRE_FAULT_MEMORY to run after a guest's
initial private memory contents have been finalized via
KVM_SEV_SNP_LAUNCH_FINISH.

Beyond fixing this issue, it just sort of makes sense to enforce this,
since the KVM_PRE_FAULT_MEMORY documentation states:

  "KVM maps memory as if the vCPU generated a stage-2 read page fault"

which sort of implies we should be acting on the same guest state that a
vCPU would see post-launch after the initial guest memory is all set up.

Co-developed-by: default avatarMichael Roth <michael.roth@amd.com>
Signed-off-by: default avatarMichael Roth <michael.roth@amd.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent c2adcf05

Documentation/virt/kvm/api.rst

+6 −0
Original line number Diff line number Diff line
@@ -6402,6 +6402,12 @@ for the current vCPU state. KVM maps memory as if the vCPU generated a 
stage-2 read page fault, e.g. faults in memory as needed, but doesn't break

CoW.  However, KVM does not mark any newly created stage-2 PTE as Accessed.



In the case of confidential VM types where there is an initial set up of

private guest memory before the guest is 'finalized'/measured, this ioctl

should only be issued after completing all the necessary setup to put the

guest into a 'finalized' state so that the above semantics can be reliably

ensured.



In some cases, multiple vCPUs might share the page tables.  In this

case, the ioctl can be called in parallel.

arch/x86/include/asm/kvm_host.h

+1 −0
Original line number Diff line number Diff line
@@ -1305,6 +1305,7 @@ struct kvm_arch { 
	u8 vm_type;

	bool has_private_mem;

	bool has_protected_state;

	bool pre_fault_allowed;

	struct hlist_head mmu_page_hash[KVM_NUM_MMU_PAGES];

	struct list_head active_mmu_pages;

	struct list_head zapped_obsolete_pages;

arch/x86/kvm/mmu/mmu.c

+3 −0
Original line number Diff line number Diff line
@@ -4743,6 +4743,9 @@ long kvm_arch_vcpu_pre_fault_memory(struct kvm_vcpu *vcpu, 
	u64 end;

	int r;



	if (!vcpu->kvm->arch.pre_fault_allowed)

		return -EOPNOTSUPP;



	/*

	 * reload is efficient when called repeatedly, so we can do it on

	 * every iteration.

arch/x86/kvm/svm/sev.c

+8 −0
Original line number Diff line number Diff line
@@ -2549,6 +2549,14 @@ static int snp_launch_finish(struct kvm *kvm, struct kvm_sev_cmd *argp) 
	data->gctx_paddr = __psp_pa(sev->snp_context);

	ret = sev_issue_cmd(kvm, SEV_CMD_SNP_LAUNCH_FINISH, data, &argp->error);



	/*

	 * Now that there will be no more SNP_LAUNCH_UPDATE ioctls, private pages

	 * can be given to the guest simply by marking the RMP entry as private.

	 * This can happen on first access and also with KVM_PRE_FAULT_MEMORY.

	 */

	if (!ret)

		kvm->arch.pre_fault_allowed = true;



	kfree(id_auth);



e_free_id_block:

arch/x86/kvm/svm/svm.c

+1 −0
Original line number Diff line number Diff line
@@ -4949,6 +4949,7 @@ static int svm_vm_init(struct kvm *kvm) 
		to_kvm_sev_info(kvm)->need_init = true;



		kvm->arch.has_private_mem = (type == KVM_X86_SNP_VM);

		kvm->arch.pre_fault_allowed = !kvm->arch.has_private_mem;

	}



	if (!pause_filter_count || !pause_filter_thresh)