Re: [PATCH 1/1] x86/boot: clear some fields explicitly

[John Hubbard]

On 7/25/19 12:22 AM, Thomas Gleixner wrote:
> On Wed, 24 Jul 2019, hpa@xxxxxxxxx wrote:
>> On July 24, 2019 4:15:28 PM PDT, john.hubbard@xxxxxxxxx wrote:
>>> From: John Hubbard <jhubbard@xxxxxxxxxx>
>>>
>>> Recent gcc compilers (gcc 9.1) generate warnings about an
>>> out of bounds memset, if you trying memset across several fields
>>> of a struct. This generated a couple of warnings on x86_64 builds.
>>>
>>> Because struct boot_params is __packed__, normal variable
>>> variable assignment will work just as well as a memset here.
>>> Change three u32 fields to be cleared to zero that way, and
>>> just memset the _pad4 field.
>>>
>>> This clears up the build warnings for me.
>>>
>>> Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
>>> ---
>>> arch/x86/include/asm/bootparam_utils.h | 11 +++++------
>>> 1 file changed, 5 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/arch/x86/include/asm/bootparam_utils.h
>>> b/arch/x86/include/asm/bootparam_utils.h
>>> index 101eb944f13c..4df87d4a043b 100644
>>> --- a/arch/x86/include/asm/bootparam_utils.h
>>> +++ b/arch/x86/include/asm/bootparam_utils.h
>>> @@ -37,12 +37,11 @@ static void sanitize_boot_params(struct boot_params
>>> *boot_params)
>>> 	if (boot_params->sentinel) {
>>> 		/* fields in boot_params are left uninitialized, clear them */
>>> 		boot_params->acpi_rsdp_addr = 0;
>>> -		memset(&boot_params->ext_ramdisk_image, 0,
>>> -		       (char *)&boot_params->efi_info -
>>> -			(char *)&boot_params->ext_ramdisk_image);
>>> -		memset(&boot_params->kbd_status, 0,
>>> -		       (char *)&boot_params->hdr -
>>> -		       (char *)&boot_params->kbd_status);
>>> +		boot_params->ext_ramdisk_image = 0;
>>> +		boot_params->ext_ramdisk_size = 0;
>>> +		boot_params->ext_cmd_line_ptr = 0;
>>> +
>>> +		memset(&boot_params->_pad4, 0, sizeof(boot_params->_pad4));
>>> 		memset(&boot_params->_pad7[0], 0,
>>> 		       (char *)&boot_params->edd_mbr_sig_buffer[0] -
>>> 			(char *)&boot_params->_pad7[0]);
>>
>> The problem with this is that it will break silently when changes are
>> made to this structure.
> 
> That's not really the worst problem. Changes to that struct which touch any
> of the to be cleared ranges will break anyway if not handled correctly in
> the sanitizer function.
> 
> What's worse is that the patch is broken.  It 'clears' the build warnings,
> but not all the fields which have been cleared before:
> 
> It removes the clearing of the range between kbd_status and hdr without any
> replacement. It neither clears edid_info.


Yes. Somehow I left that chunk out. Not my finest hour. 

> 
> The above approach is doomed and if we have to handle this GCC0.1 madness
> then this needs to be done smarter. Something like the completely untested
> thing below.
> 
> Thanks,
> 
> 	tglx
> 
> 8<--------------
> diff --git a/arch/x86/include/asm/bootparam_utils.h b/arch/x86/include/asm/bootparam_utils.h
> index 101eb944f13c..f5bc4c01b66b 100644
> --- a/arch/x86/include/asm/bootparam_utils.h
> +++ b/arch/x86/include/asm/bootparam_utils.h
> @@ -9,6 +9,18 @@
>   * add completing #includes to make it standalone.
>   */
>  
> +struct boot_params_clear {
> +	unsigned int	offs;
> +	unsigned int	len;
> +};
> +
> +#define BOOT_PARAM_CLEAR(start, end)				\
> +{								\
> +	.offs	= offsetof(struct boot_params, start),		\
> +	.len	= offsetof(struct boot_params, end) -		\
> +		  offsetof(struct boot_params, start),		\
> +}
> +
>  /*
>   * Deal with bootloaders which fail to initialize unknown fields in
>   * boot_params to zero.  The list fields in this list are taken from
> @@ -20,7 +32,19 @@
>   */
>  static void sanitize_boot_params(struct boot_params *boot_params)
>  {
> -	/* 
> +	const struct boot_params_clear toclear[] = {
> +		BOOT_PARAM_CLEAR(acpi_rdsp_addr, _pad3),
> +		BOOT_PARAM_CLEAR(ext_ramdisk_image, efi_info),
> +		BOOT_PARAM_CLEAR(kbd_status, hdr),
> +		BOOT_PARAM_CLEAR(_pad7, edd_mbr_sig_buffer),
> +		BOOT_PARAM_CLEAR(_pad8, eddbuf),
> +		{
> +			.offs	= offsetof(struct boot_params, _pad9),
> +			.len	= sizeof(boot_params->_pad9),
> +		},
> +	}
> +
> +	/*
>  	 * IMPORTANT NOTE TO BOOTLOADER AUTHORS: do not simply clear
>  	 * this field.  The purpose of this field is to guarantee
>  	 * compliance with the x86 boot spec located in
> @@ -36,20 +60,11 @@ static void sanitize_boot_params(struct boot_params *boot_params)
>  	 */
>  	if (boot_params->sentinel) {
>  		/* fields in boot_params are left uninitialized, clear them */
> -		boot_params->acpi_rsdp_addr = 0;
> -		memset(&boot_params->ext_ramdisk_image, 0,
> -		       (char *)&boot_params->efi_info -
> -			(char *)&boot_params->ext_ramdisk_image);
> -		memset(&boot_params->kbd_status, 0,
> -		       (char *)&boot_params->hdr -
> -		       (char *)&boot_params->kbd_status);
> -		memset(&boot_params->_pad7[0], 0,
> -		       (char *)&boot_params->edd_mbr_sig_buffer[0] -
> -			(char *)&boot_params->_pad7[0]);
> -		memset(&boot_params->_pad8[0], 0,
> -		       (char *)&boot_params->eddbuf[0] -
> -			(char *)&boot_params->_pad8[0]);
> -		memset(&boot_params->_pad9[0], 0, sizeof(boot_params->_pad9));
> +		char *p = (char *) boot_params;
> +		int i;
> +
> +		for (i = 0; i < ARRAY_SIZE(toclear); i++)
> +			memset(p + toclear[i].start, 0, toclear[i].len);
>  	}
>  }


Looks nice.


thanks,
-- 
John Hubbard
NVIDIA