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);
...
651 out:
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:

651 out:
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

virtio-iommu.c.patch
Description: Text Data