As one of the QEMU developers put it,
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However this is hardly security critical as ati-vga is
experimental and not fully implemented yet so anyone
using it will likely get other problems (such as drivers
not loading) before a guest could exploit this.
I think QEMU only considers bugs in parts that are used
for virtualisation via KVM as security problems so maybe
this does not even need a CVE and could be normally
reported/discussed on the mailing list.
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See https://lists.gnu.org/archive/html/qemu-devel/2020-08/msg05528.html
/mjt