Re: [PATCH] hw/nvram/fw_cfg: fix FWCfgDataGeneratorClass::get_data() consumption

[Daniel P . Berrangé]

On Wed, Sep 16, 2020 at 05:15:10PM +0200, Laszlo Ersek wrote:
> The documentation on g_byte_array_free()
> <https://developer.gnome.org/glib/stable/glib-Byte-Arrays.html#g-byte-array-free>
> says:
> 
> > Returns
> >
> > the element data if free_segment is FALSE, otherwise NULL. The element
> > data should be freed using g_free().
> 
> Because we currently call g_byte_array_free() with free_segment=TRUE, we
> end up passing data=NULL to fw_cfg_add_file().
> 
> On the plus side, fw_cfg_data_read() and fw_cfg_dma_transfer() both deal
> with NULL data gracefully: QEMU does not crash when the guest reads such
> an item, the guest just gets a properly sized, but zero-filled blob.
> 
> However, the bug breaks UEFI HTTPS boot, as the IANA_TLS_CIPHER array,
> generated otherwise correctly by the "tls-cipher-suites" object, is in
> effect replaced with a zero blob.
> 
> Fix the issue by passing free_segment=FALSE to g_byte_array_free():
> 
> - the caller (fw_cfg_add_from_generator()) temporarily assumes ownership
>   of the generated byte array,
> 
> - then ownership of the byte array is transfered to fw_cfg, as
>   fw_cfg_add_file() links (not copies) "data" into fw_cfg.
> 
> Cc: "Daniel P. Berrangé" <berrange@redhat.com>
> Cc: "Philippe Mathieu-Daudé" <philmd@redhat.com>
> Cc: Gerd Hoffmann <kraxel@redhat.com>
> Fixes: 3203148917d035b09f71986ac2eaa19a352d6d9d
> Signed-off-by: Laszlo Ersek <lersek@redhat.com>
> ---
>  hw/nvram/fw_cfg.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>


Regards,
Daniel
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