Re: [PATCH] x86_64/efi: enforce 32 bit address for command line buffer

[Matt Fleming <matt@codeblueprint.co.uk>]

On Tue, 14 Apr, at 05:45:52PM, Roy Franz wrote:
> The boot_params structure has a 32 bit field for storing the address of
> the kernel command line.  When the EFI stub allocates memory for the command
> line, it allocates at as low and address as possible, but does not ensure
> that the address of memory allocated is below 4G.
> This patch enforces this limit, and the stub now returns an error if the
> command line buffer is allocated at too high of an address.
> For 32 bit systems, the EFI mandated 1-1 memory mapping ensures
> that all memory is 32 bit addressable, so we don't have a problem.
> Also, mixed-mode booting on EFI platforms does not use the stub
> code, so we don't need to handle the case of booting a 32 bit
> kernel on a 64 bit EFI platform.
> 
> Signed-off-by: Roy Franz <roy.franz@linaro.org>
> ---
>  arch/x86/boot/compressed/eboot.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/arch/x86/boot/compressed/eboot.c b/arch/x86/boot/compressed/eboot.c
> index ef17683..82dbe27 100644
> --- a/arch/x86/boot/compressed/eboot.c
> +++ b/arch/x86/boot/compressed/eboot.c
> @@ -1108,6 +1108,19 @@ struct boot_params *make_boot_params(struct efi_config *c)
>  	cmdline_ptr = efi_convert_cmdline(sys_table, image, &options_size);
>  	if (!cmdline_ptr)
>  		goto fail;
> +
> +#ifdef CONFIG_X86_64
> +	/*
> +	 * hdr->cmd_line_ptr is a 32 bit field, so on 64 bit systems we need
> +	 * to ensure that the allocated buffer for the commandline is 32 bit
> +	 * addressable.
> +	  */
> +	if ((u64)(cmdline_ptr) + options_size > (u64)U32_MAX) {
> +		efi_printk(sys_table, "Failed to alloc lowmem for command line\n");
> +		efi_free(sys_table, options_size, (unsigned long)cmdline_ptr);
> +		goto fail;
> +	}
> +#endif /* CONFIG_X86_64 */
>  	hdr->cmd_line_ptr = (unsigned long)cmdline_ptr;
>  
>  	hdr->ramdisk_image = 0;

Good catch. But actually, we have boot_params->ext_cmd_line_ptr for
exactly this problem.

So yes, that's a valid bug, but I don't think this is how we should fix
it.

-- 
Matt Fleming, Intel Open Source Technology Center