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: Michael Roth <michael.roth@xxxxxxx> Signed-off-by: Michael Roth <michael.roth@xxxxxxx> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx> --- Documentation/virt/kvm/api.rst | 6 ++++++ arch/x86/include/asm/kvm_host.h | 1 + arch/x86/kvm/mmu/mmu.c | 3 +++ arch/x86/kvm/svm/sev.c | 8 ++++++++ arch/x86/kvm/svm/svm.c | 1 + arch/x86/kvm/x86.c | 3 +++ 6 files changed, 22 insertions(+) diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst index ec1cd8aa1d56..7b512286f8d2 100644 --- a/Documentation/virt/kvm/api.rst +++ b/Documentation/virt/kvm/api.rst @@ -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. diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h index 950a03e0181e..94e7b5a4fafe 100644 --- a/arch/x86/include/asm/kvm_host.h +++ b/arch/x86/include/asm/kvm_host.h @@ -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; diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index 901be9e420a4..26ef5b6ac3c1 100644 --- a/arch/x86/kvm/mmu/mmu.c +++ b/arch/x86/kvm/mmu/mmu.c @@ -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. diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c index a16c873b3232..6589091e8ce0 100644 --- a/arch/x86/kvm/svm/sev.c +++ b/arch/x86/kvm/svm/sev.c @@ -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: diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c index c115d26844f7..d6f252555ab3 100644 --- a/arch/x86/kvm/svm/svm.c +++ b/arch/x86/kvm/svm/svm.c @@ -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) diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index af6c8cf6a37a..52778689cef4 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -12646,6 +12646,9 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type) kvm->arch.vm_type = type; kvm->arch.has_private_mem = (type == KVM_X86_SW_PROTECTED_VM); + /* Decided by the vendor code for other VM types. */ + kvm->arch.pre_fault_allowed = + type == KVM_X86_DEFAULT_VM || type == KVM_X86_SW_PROTECTED_VM; ret = kvm_page_track_init(kvm); if (ret) -- 2.43.0