linux-kernel - Re: [PATCH 2/4] efi/x86: move efi bgrt init code to early init code

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Message-ID: <20170113022106.GA3266@dhcp-128-65.nay.redhat.com>
Date:   Fri, 13 Jan 2017 10:21:06 +0800
From:   Dave Young <dyoung@...hat.com>
To:     Nicolai Stange <nicstange@...il.com>
Cc:     Matt Fleming <matt@...eblueprint.co.uk>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org,
        x86@...nel.org, Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>, hpa@...or.com,
        Dan Williams <dan.j.williams@...el.com>,
        mika.penttila@...tfour.com, bhsharma@...hat.com
Subject: Re: [PATCH 2/4] efi/x86: move efi bgrt init code to early init code

On 01/13/17 at 12:11am, Nicolai Stange wrote:
> On Fri, Jan 13 2017, Dave Young wrote:
> 
> > On 01/12/17 at 12:54pm, Nicolai Stange wrote:
> >> On Thu, Jan 12 2017, Dave Young wrote:
> >> 
> >> > -void __init efi_bgrt_init(void)
> >> > +void __init efi_bgrt_init(struct acpi_table_header *table)
> >> >  {
> >> > -	acpi_status status;
> >> >  	void *image;
> >> >  	struct bmp_header bmp_header;
> >> >  
> >> >  	if (acpi_disabled)
> >> >  		return;
> >> >  
> >> > -	status = acpi_get_table("BGRT", 0,
> >> > -	                        (struct acpi_table_header **)&bgrt_tab);
> >> > -	if (ACPI_FAILURE(status))
> >> > -		return;
> >> 
> >> 
> >> Not sure, but wouldn't it be safer to reverse the order of this assignment
> >> 
> >> > +	bgrt_tab = *(struct acpi_table_bgrt *)table;
> >
> > Nicolai, sorry, I'm not sure I understand the comment, is it about above line?
> > Could you elaborate a bit?
> >
> >> 
> >> and this length check
> >> 
> >
> > I also do not get this :(
> 
> Ah sorry, my point is this: the length check should perhaps be made
> before doing the assignment to bgrt_tab because otherwise, we might end
> up reading from invalid memory.
> 
> I.e. if (struct acpi_table_bgrt *)table->length < sizeof(bgrt_tab), then
> 
>   bgrt_tab = *(struct acpi_table_bgrt *)table;
> 
> would read past the table's end.
> 
> I'm not sure whether this is a real problem though -- that is, whether
> this read could ever hit some unmapped memory.

Nicolai, thanks for the explanation. It make sense to move it to even later
at the end of the function.

Thanks
Dave