hw/uefi: clear uefi-vars buffer in uefi_vars_write callback

hw/uefi: clear uefi-vars buffer in uefi_vars_write callback

When the guest writes to register UEFI_VARS_REG_BUFFER_SIZE, the .write
callback `uefi_vars_write` is invoked. The function allocates a
heap buffer without zeroing the memory, leaving the buffer filled with
residual data from prior allocations. When the guest later reads from
register UEFI_VARS_REG_PIO_BUFFER_TRANSFER, the .read callback
`uefi_vars_read` returns leftover metadata or other sensitive process
memory from the previously allocated buffer, leading to an information
disclosure vulnerability.

Fixes: CVE-2025-8860
Fixes: 90ca4e03c27d ("hw/uefi: add var-service-core.c")
Reported-by: ZDI <zdi-disclosures@trendmicro.com>
Suggested-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Mauro Matteo Cascella <mcascell@redhat.com>
Message-ID: <20250811101128.17661-1-mcascell@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>