[PATCH 2/2] KVM: SEV: Do cache maintenance on the source VM during intra-host migration

[Sean Christopherson <seanjc@google.com>]

Manually perform cache maintenance on the source VM during intra-host
migration to ensure no stale data is left in CPU caches after the VM is
destroyed.  Because the source VM is "converted" to a non-SEV VM, KVM's
memory reclaim flows won't trigger cache maintenance, e.g. when all guest
memory is reclaimed in response to detaching from the mmu_notifier.

Note, relying on the destination VM to do cache maintenance isn't an option
as KVM doesn't require identical guest memory configurations, i.e. the
source VM may have access to memory that the destination VM does not.
Enforcing equivalent memory configurations is infeasible, as it would
require a *deep* comparison of memslots, e.g. to verify that not only are
the memslot identical, but what the memslots point at is also identical.

Fixes: b56639318bb2 ("KVM: SEV: Add support for SEV intra host migration")
Cc: stable@vger.kernel.org
Reported-by: Stefan Teodorescu <fane@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/svm/sev.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index c9242c936a40..71923cb72d1d 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -2048,6 +2048,12 @@ static void sev_migrate_from(struct kvm *dst_kvm, struct kvm *src_kvm)
 	src->pages_locked = 0;
 	src->es_active = false;
 
+	/*
+	 * Do cache maintenance on the source VM as it is no longer an SEV VM,
+	 * i.e. memory reclaim flows won't trigger cache maintenance on the VM.
+	 */
+	sev_writeback_caches(src_kvm);
+
 	list_cut_before(&dst->regions_list, &src->regions_list, &src->regions_list);
 
 	mutex_lock(&sev_mirror_lock);
@@ -2187,6 +2193,10 @@ int sev_vm_move_enc_context_from(struct kvm *kvm, unsigned int source_fd)
 	 * the set of CPUs from the source.  If a CPU was used to run a vCPU in
 	 * the source VM but is never used for the destination VM, then the CPU
 	 * can only have cached memory that was accessible to the source VM.
+	 * Furthermore, KVM *must* perform cache maintenance on the source VM,
+	 * as the source VM may have access to memory that the destination VM
+	 * does not, i.e. KVM could skip flushes if memory is reclaimed from
+	 * the old VM but not the new VM.
 	 */
 	if (!zalloc_cpumask_var(&dst_sev->have_run_cpus, GFP_KERNEL_ACCOUNT)) {
 		ret = -ENOMEM;
-- 
2.55.0.1082.g2b9226bbc0-goog