Fix a potential Use-after-free in virtio_iommu_handle_command() (v6.2.0).

[wliang]

Hi all,

I find a potential Use-after-free in QEMU 6.2.0, which is in 
virtio_iommu_handle_command() (./hw/virtio/virtio-iommu.c).



Specifically, in the loop body, the variable 'buf' allocated at line 639 can be 
freed by g_free() at line 659. However, if the execution path enters the loop 
body again and the if branch takes true at line 616, the control will directly 
jump to 'out' at line 651. At this time, 'buf' is a freed pointer, which is not 
assigned with an allocated memory but used at line 653. As a result, a UAF bug 
is triggered.





599    for (;;) {
...
615        sz = iov_to_buf(iov, iov_cnt, 0, &head, sizeof(head));
616        if (unlikely(sz != sizeof(head))) {
617            tail.status = VIRTIO_IOMMU_S_DEVERR;
618            goto out;
619        }
...
639            buf = g_malloc0(output_size);
...
651out:
652        sz = iov_from_buf(elem->in_sg, elem->in_num, 0,
653                          buf ? buf : &tail, output_size);
...
659        g_free(buf);
660    }






We can fix it by set ‘buf‘ to NULL after freeing it:



651out:
652        sz = iov_from_buf(elem->in_sg, elem->in_num, 0,
653                          buf ? buf : &tail, output_size);
...
659        g_free(buf);
+++buf = NULL;
660    }


I'm looking forward to your confirmation.


Best,
Wentao

--- ./hw/virtio/virtio-iommu.c	2022-02-23 15:06:32.040727196 +0800
+++ ./hw/virtio/virtio-iommu-PATCH.c	2022-02-23 21:12:24.605032121 +0800
@@ -657,6 +657,7 @@
         virtio_notify(vdev, vq);
         g_free(elem);
         g_free(buf);
+        buf = NULL;
     }
 }