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

[Kees Cook]

On Tue, Jun 11, 2013 at 1:54 PM, Yinghai Lu <yinghai@xxxxxxxxxx> wrote:
> On Tue, Jun 11, 2013 at 12:26 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>> On Tue, Jun 11, 2013 at 12:17 PM, Yinghai Lu <yinghai@xxxxxxxxxx> wrote:
>>> On Tue, Jun 11, 2013 at 11:41 AM, Kees Cook <keescook@xxxxxxxxxxxx> 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;

Ah-ha! I see what I missed. I didn't notice 16a4baa6 which did this:
-       if (!cmdline_ptr || cmdline_ptr >= 0x100000)
-               return -1;      /* No command line, or inaccessible */
+       if (!cmdline_ptr)
+               return -1;      /* No command line */

That fixes the problem I was seeing. :)

Thanks!

-Kees

--
Kees Cook
Chrome OS Security
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