KVM: SEV: Flush cache on non-coherent systems before RECEIVE_UPDATE_DATA

Flush the destination page before invoking RECEIVE_UPDATE_DATA, as the
PSP encrypts the data with the guest's key when writing to guest memory.
If the target memory was not previously encrypted, the cache may contain
dirty, unecrypted data that will persist on non-coherent systems.

Fixes: 15fb7de1 ("KVM: SVM: Add KVM_SEV_RECEIVE_UPDATE_DATA command")
Cc: stable@vger.kernel.org
Cc: Peter Gonda <pgonda@google.com>
Cc: Marc Orr <marcorr@google.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Signed-off-by: Masahiro Kozuka <masa.koz@kozuka.jp>
[sean: converted bug report to changelog]
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20210914210951.2994260-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/svm/sev.c

@@ -1484,6 +1484,13 @@ static int sev_receive_update_data(struct kvm *kvm, struct kvm_sev_cmd *argp)
 		goto e_free_trans;
 	}
 
+	/*
+	 * Flush (on non-coherent CPUs) before RECEIVE_UPDATE_DATA, the PSP
+	 * encrypts the written data with the guest's key, and the cache may
+	 * contain dirty, unencrypted data.
+	 */
+	sev_clflush_pages(guest_page, n);
+
 	/* The RECEIVE_UPDATE_DATA command requires C-bit to be always set. */
 	data.guest_address = (page_to_pfn(guest_page[0]) << PAGE_SHIFT) + offset;
 	data.guest_address |= sev_me_mask;