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@xxxxxxxxxx>
---
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
