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SubjectRe: [PATCH 2/6] KVM: SEV: Explicitly document that there are no TOCTOU races in copy ASID
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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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