RE: [PATCH 1/2] x86/kexec: Use up-to-dated screen_info copy to fill boot params

[Dexuan Cui]

> From: Kairui Song <kasong@xxxxxxxxxx>
> Sent: Wednesday, October 14, 2020 2:24 AM
> To: linux-kernel@xxxxxxxxxxxxxxx
> 
> kexec_file_load now just reuse the old boot_params.screen_info.
> But if drivers have change the hardware state, boot_param.screen_info
> could contain invalid info.
> 
> For example, the video type might be no longer VGA, or frame buffer
> address changed. If kexec kernel keep using the old screen_info,
> kexec'ed kernel may attempt to write to an invalid framebuffer
> memory region.
> 
> There are two screen_info globally available, boot_params.screen_info
> and screen_info. Later one is a copy, and could be updated by drivers.
> 
> So let kexec_file_load use the updated copy.
> 
> Signed-off-by: Kairui Song <kasong@xxxxxxxxxx>
> ---
>  arch/x86/kernel/kexec-bzimage64.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/kexec-bzimage64.c
> b/arch/x86/kernel/kexec-bzimage64.c
> index 57c2ecf43134..ce831f9448e7 100644
> --- a/arch/x86/kernel/kexec-bzimage64.c
> +++ b/arch/x86/kernel/kexec-bzimage64.c
> @@ -200,8 +200,7 @@ setup_boot_parameters(struct kimage *image, struct
> boot_params *params,
>  	params->hdr.hardware_subarch = boot_params.hdr.hardware_subarch;
> 
>  	/* Copying screen_info will do? */
> -	memcpy(&params->screen_info, &boot_params.screen_info,
> -				sizeof(struct screen_info));
> +	memcpy(&params->screen_info, &screen_info, sizeof(struct screen_info));
> 
>  	/* Fill in memsize later */
>  	params->screen_info.ext_mem_k = 0;
> --

Hi Kairui,
According to "man kexec", kdump/kexec can use 2 different syscalls to set up the
kdump kernel:

-s (--kexec-file-syscall)
       Specify that the new KEXEC_FILE_LOAD syscall should be used exclusively.

-c (--kexec-syscall)
       Specify that the old KEXEC_LOAD syscall should be used exclusively (the default).

It looks I can only reproduce the call-trace 
(https://bugzilla.redhat.com/show_bug.cgi?id=1867887#c5) with KEXEC_FILE_LOAD:
I did kdump tests in Ubuntu 20.04 VM and by default the VM used the KEXEC_LOAD
syscall and I couldn't reproduce the call-trace; after I added the "-s" parameter to use
the KEXEC_FILE_LOAD syscall, I could reproduce the call-trace and I can confirm your
patch can eliminate the call-trace because the "efifb" driver doesn't even load with 
your patch.

Your patch is only for the KEXEC_FILE_LOAD syscall, and I'm sure it's not used in the
code path of the KEXEC_LOAD syscall.

So, in the case of the KEXEC_LOAD syscall, do you know how the *kexec* kernel's
boot_params.screen_info.lfb_base is intialized? I haven't figured it out yet.

Thanks,
-- Dexuan