Messages in this thread | | | Date | Tue, 16 Nov 2021 12:38:28 +0100 | Subject | Re: [PATCH 2/6] KVM: SEV: Explicitly document that there are no TOCTOU races in copy ASID | From | Paolo Bonzini <> |
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On 11/9/21 22:50, Sean Christopherson wrote: > Deliberately grab the source's SEV info for COPY_ENC_CONTEXT_FROM outside > of kvm->lock and document that doing so is safe due to SEV/SEV-ES info, > e.g. ASID, active, etc... being "write-once" and set atomically with > respect to kvm->lock. > > No functional change intended.
This isn't true anymore with the move-enc-context-from patches, though! I will send a follow-up shortly.
Paolo
> Signed-off-by: Sean Christopherson <seanjc@google.com> > --- > arch/x86/kvm/svm/sev.c | 24 ++++++++++++++++-------- > 1 file changed, 16 insertions(+), 8 deletions(-) > > diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c > index eeec499e4372..6d14e2595c96 100644 > --- a/arch/x86/kvm/svm/sev.c > +++ b/arch/x86/kvm/svm/sev.c > @@ -1737,9 +1737,9 @@ int svm_unregister_enc_region(struct kvm *kvm, > > int svm_vm_copy_asid_from(struct kvm *kvm, unsigned int source_fd) > { > + struct kvm_sev_info *mirror_sev, *source_sev; > struct file *source_kvm_file; > struct kvm *source_kvm; > - struct kvm_sev_info source_sev, *mirror_sev; > int ret; > > source_kvm_file = fget(source_fd); > @@ -1762,9 +1762,6 @@ int svm_vm_copy_asid_from(struct kvm *kvm, unsigned int source_fd) > goto e_source_unlock; > } > > - memcpy(&source_sev, &to_kvm_svm(source_kvm)->sev_info, > - sizeof(source_sev)); > - > /* > * The mirror kvm holds an enc_context_owner ref so its asid can't > * disappear until we're done with it > @@ -1785,14 +1782,25 @@ int svm_vm_copy_asid_from(struct kvm *kvm, unsigned int source_fd) > goto e_mirror_unlock; > } > > + /* > + * Referencing the source's sev_info without holding the source's lock > + * is safe as SEV/SEV-ES activation is a one-way, "atomic" operation. > + * SEV state, e.g. the ASID, is modified under kvm->lock, and cannot be > + * changed after SEV is marked active (here or in normal activation). > + * That same atomicity also prevents TOC-TOU issues with respect to > + * related sanity checks on source_kvm. > + */ > + source_sev = &to_kvm_svm(source_kvm)->sev_info; > + > /* Set enc_context_owner and copy its encryption context over */ > mirror_sev = &to_kvm_svm(kvm)->sev_info; > mirror_sev->enc_context_owner = source_kvm; > + mirror_sev->asid = source_sev->asid; > mirror_sev->active = true; > - mirror_sev->asid = source_sev.asid; > - mirror_sev->fd = source_sev.fd; > - mirror_sev->es_active = source_sev.es_active; > - mirror_sev->handle = source_sev.handle; > + mirror_sev->asid = source_sev->asid; > + mirror_sev->fd = source_sev->fd; > + mirror_sev->es_active = source_sev->es_active; > + mirror_sev->handle = source_sev->handle; > /* > * Do not copy ap_jump_table. Since the mirror does not share the same > * KVM contexts as the original, and they may have different >
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