linux-kernel - Re: [PATCH] x86, boot: access cmdline when loaded high

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Date:	Tue, 11 Jun 2013 13:54:04 -0700
From:	Yinghai Lu <yinghai@...nel.org>
To:	Kees Cook <keescook@...omium.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Gokul Caushik <caushik1@...il.com>,
	Josh Triplett <josh@...htriplett.org>
Subject: Re: [PATCH] x86, boot: access cmdline when loaded high

On Tue, Jun 11, 2013 at 12:26 PM, Kees Cook <keescook@...omium.org> wrote:
> On Tue, Jun 11, 2013 at 12:17 PM, Yinghai Lu <yinghai@...nel.org> wrote:
>> On Tue, Jun 11, 2013 at 11:41 AM, Kees Cook <keescook@...omium.org> wrote:
>>> The __cmdline_find_option routine requires that the kernel cmdline be
>>> located under 0x10000. When running the compressed boot code, if the
>>> cmdline is loaded above this range, it will be ignored. This breaks
>>> recognition of things like "earlyprintk=...". Since the compressed boot
>>> code is already tricking the cmdline routines about locations, take it all
>>> the way and access the cmdline via offset instead of via actual location.
>>
>> Hi,
>>
>> What is point of the patch?
>> It looks like it try to fix one bug, but it doesn't,
>> as there is no bug out there.
>>
>> Before the patch, command line that is loaded high
>> can be processed correctly in arch/x86/boot/compressed/cmdline.c.
>
> It can't. Here is the code from arch/x86/boot/cmdline.c:

Are you sure?

I tested that with kexec loaded second kernel high.

How do you test that?

>
> int __cmdline_find_option(unsigned long cmdline_ptr, const char
> *option, char *buffer, int bufsize)
> ...
>         if (!cmdline_ptr)
>                 return -1;      /* No command line */
>
>         cptr = cmdline_ptr & 0xf;
>         set_fs(cmdline_ptr >> 4);
>
>         while (cptr < 0x10000 && (c = rdfs8(cptr++))) {
>
>
> This means that if get_cmd_line_ptr() returns an address >= 0x10000,
> __cmdline_find_option will simply ignore the cmdline.

No, you read the code wrong. you miss the

cptr = cmdline_ptr & 0xf;

Yinghai
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