virtio-net: fix use after unmap/free for sg#1379
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When mergeable buffer is enabled, we try to set the num_buffers after the virtqueue elem has been unmapped. This will lead several issues, E.g a use after free when the descriptor has an address which belongs to the non direct access region. In this case we use bounce buffer that is allocated during address_space_map() and freed during address_space_unmap(). Fixing this by storing the elems temporarily in an array and delay the unmap after we set the the num_buffers. This addresses CVE-2021-3748. Reported-by: Alexander Bulekov <alxndr@bu.edu> Fixes: fbe78f4 ("virtio-net support") Cc: qemu-stable@nongnu.org Signed-off-by: Jason Wang <jasowang@redhat.com>
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Cherry-picked commit bedd7e9 from upstream to address #1378. In general, we're not in the business of backporting upstream qemu code to PANDA, but if this single commit doesn't break anything it's easy enough to merge.