[PATCH 0/2] KVM: SEV: Fix intra-host migration cache bugs

[PATCH 0/2] KVM: SEV: Fix intra-host migration cache bugs

[Sean Christopherson]

Fix a memory leak and a cache maintenance issue related to doing intra-host
migration on an SEV guest.  TL;DR: the source VM becomes a non-SEV guest after
intra-host migration, and so KVM fails to do the right thing because its cache
maintenance stuff keys off sev_guest().

Sean Christopherson (2):
  KVM: SEV: Free have_run_cpus during VM destruction even if VM is no
    longer SEV
  KVM: SEV: Do cache maintenance on the source VM during intra-host
    migration

 arch/x86/kvm/svm/sev.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)


base-commit: 30b5175943e709911702d8a9364145e911f57e3f
-- 
2.55.0.1082.g2b9226bbc0-goog

[PATCH 1/2] KVM: SEV: Free have_run_cpus during VM destruction even if VM is no longer SEV

[Sean Christopherson]

Unconditionally free SEV's "have run CPUs" cpumask in the VM destroy path,
i.e. even for what appear to be non-SEV VMs, as an SEV VM becomes a non-SEV
VM if its state is intra-host migrated.  Alternatively, the mask could be
freed in sev_migrate_from() when "converting" the source VM, but that gets
annoying because ideally KVM would nullify the mask to guard against UAF,
and nullifying the mask would need be conditioned on CPUMASK_OFFSTACK=y.

Freeing the mask during sev_migrate_from() is also not robust against other
KVM bugs, though that's kind of a moot point since any such bugs would show
up even if sev->active is never set.  I.e. KVM must get that side of things
correct.  But, that's not a great reason to add more code just to make
things marginally less robust.

Fixes: 6f38f8c57464 ("KVM: SVM: Flush cache only on CPUs running SEV guest")
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 | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 63e8cfa9bf55..c9242c936a40 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -2981,13 +2981,17 @@ void sev_vm_destroy(struct kvm *kvm)
 	struct list_head *head = &sev->regions_list;
 	struct list_head *pos, *q;
 
+	/*
+	 * Free the mask even if the VM is not *currently* an SEV VM, as it may
+	 * have been an SEV VM prior to intra-host migration.
+	 */
+	free_cpumask_var(sev->have_run_cpus);
+
 	if (!sev_guest(kvm))
 		return;
 
 	WARN_ON(!list_empty(&sev->mirror_vms));
 
-	free_cpumask_var(sev->have_run_cpus);
-
 	/*
 	 * If this is a mirror VM, remove it from the owner's list of a mirrors
 	 * and skip ASID cleanup (the ASID is tied to the lifetime of the owner).
-- 
2.55.0.1082.g2b9226bbc0-goog

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

[Sean Christopherson]

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