KVM: SEV: Do cache maintenance on the source VM *before* clearing SEV state

Explicitly flush caches after intra-host migration before clearing "SEV
active" on the source VM, as doing cache maintenance afterwards creates a
tiny window where memory reclaim could return memory to the host without
performing a cache flush, e.g. as pointed out by Sashiko:

  CPU1 in sev_migrate_from():
    src->active = false;

  CPU2 running concurrent unmap:
    Since active is false, the automatic cache flush in
    sev_guest_memory_reclaimed is skipped.
    The host frees and reallocates the page.

  CPU1 in sev_migrate_from():
    sev_writeback_caches(src_kvm);
    Executes a hardware cache flush (wbnoinvd), which writes the guest's old
    dirty ciphertext over the new page owner's data.

Fixes: 93de2a6a4b91 ("KVM: SEV: Do cache maintenance on the source VM during intra-host migration")
Cc: stable@vger.kernel.org
Reported-by: Sashiko Bot <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/all/20260923165304.1662E1F000FF@smtp.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20260928154644.2559454-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 7 insertions, 6 deletions

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index d3a2e6a51efc..0c1ebb16cec6 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -2045,6 +2045,13 @@ static void sev_migrate_from(struct kvm *dst_kvm, struct kvm *src_kvm)
 	struct kvm_sev_info *mirror;
 	unsigned long i;
 
+	/*
+	 * Do cache maintenance on the source VM *before* clearing "SEV active",
+	 * as memory reclaim flows won't trigger cache maintenance on the VM
+	 * once it's no longer an SEV VM.
+	 */
+	sev_writeback_caches(src_kvm);
+
 	dst->active = true;
 	dst->asid = src->asid;
 	dst->handle = src->handle;
@@ -2058,12 +2065,6 @@ 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);