Re: [PATCH] efifb: ignore frame buffer with physical address 0

[Jan Beulich <jbeulich@xxxxxxxx>]

On 18.11.2022 13:39, Roger Pau Monne wrote:
> On one of my boxes when the HDMI cable is not plugged in the
> FrameBufferBase of the EFI_GRAPHICS_OUTPUT_PROTOCOL_MODE structure is
> set to 0 by the firmware (while some of the other fields looking
> plausible).
> 
> Such (bogus address) ends up mapped in vesa_init(), and since it
> overlaps with a RAM region the whole system goes down pretty badly,
> see:
> 
> (XEN) vesafb: framebuffer at 0x0000000000000000, mapped to 
> 0xffff82c000201000, using 35209k, total 35209k
> (XEN) vesafb: mode is 0x37557x32, linelength=960, font 8x16

Interesting mode - should we check for non-zero values there as well,
perhaps?

> (XEN) vesafb: Truecolor: size=8:8:8:8, shift=24:0:8:16
> (XEN) (XEN) (XEN) (XEN) (XEN) (XEN) (XEN) (XEN) �ERROR: Class:0; Subclass:0; 
> Operation: 0
> ERROR: No ConOut
> ERROR: No ConIn
> 
> Do like Linux and prevent using the EFI Frame Buffer if the base
> address is 0.  This is inline with the logic in Linuxes
> fb_base_is_valid() function at drivers/video/fbdev/efifb.c v6.0.9.
> 
> See also Linux commit 133bb070e94ab41d750c6f2160c8843e46f11b78 for
> further reference.
> 
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> ---
> Other options would be doing the check in vesa_init(), but that would
> also then apply to other framebuffers and won't be strictly limited to
> the EFI fb.

Well, zero is wrong uniformly, so it wouldn't seem unreasonable to
put the check there. But I'm happy to keep it in EFI code for now.

> We could also check in vesa_init() whether the framebuffer overlaps
> with any RAM region, but I think that should be in addition to the
> change done here.

Indeed.

> --- a/xen/arch/x86/efi/efi-boot.h
> +++ b/xen/arch/x86/efi/efi-boot.h
> @@ -552,7 +552,7 @@ static void __init 
> efi_arch_video_init(EFI_GRAPHICS_OUTPUT_PROTOCOL *gop,
>          bpp  = 0;
>          break;
>      }
> -    if ( bpp > 0 )
> +    if ( bpp > 0 && gop->Mode->FrameBufferBase )
>      {
>          vga_console_info.video_type = XEN_VGATYPE_EFI_LFB;
>          vga_console_info.u.vesa_lfb.gbl_caps = 2; /* possibly non-VGA */

A few lines up from here, just out of patch context, there is a
PrintErr() which imo is bogus/misleading when also encountering a
zero fb base. I'd like to suggest that you put the new check early
in the function (perhaps extended by a zero check of other
applicable fields, as per above), returning right away alongside
another new PrintErr().

Jan