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SubjectRe: [PATCH 11/14] MIPS: memblock: Print out kernel virtual mem layout
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Hi Serge,

On 23/01/18 19:10, Serge Semin wrote:
> Hello Matt,
>
> On Tue, Jan 23, 2018 at 03:35:14PM +0000, Matt Redfearn <matt.redfearn@mips.com> wrote:
>> Hi Serge,
>>
>> On 19/01/18 14:27, Serge Semin wrote:
>>> On Fri, Jan 19, 2018 at 07:59:43AM +0000, Matt Redfearn <matt.redfearn@mips.com> wrote:
>>>
>>> Hello Matt,
>>>
>>>> Hi Serge,
>>>>
>>>>
>>>>
>>>> On 18/01/18 20:18, Serge Semin wrote:
>>>>> On Thu, Jan 18, 2018 at 12:03:03PM -0800, Florian Fainelli <f.fainelli@gmail.com> wrote:
>>>>>> On 01/17/2018 02:23 PM, Serge Semin wrote:
>>>>>>> It is useful to have the kernel virtual memory layout printed
>>>>>>> at boot time so to have the full information about the booted
>>>>>>> kernel. In some cases it might be unsafe to have virtual
>>>>>>> addresses freely visible in logs, so the %pK format is used if
>>>>>>> one want to hide them.
>>>>>>>
>>>>>>> Signed-off-by: Serge Semin <fancer.lancer@gmail.com>
>>>>>>
>>>>>> I personally like having that information because that helps debug and
>>>>>> have a quick reference, but there appears to be a trend to remove this
>>>>>> in the name of security:
>>>>>>
>>>>>> https://patchwork.kernel.org/patch/10124007/
>>>>>>
>>>>>> maybe hide this behind a configuration option?
>>>>>
>>>>> Yeah, arm code was the place I picked the function up.) But in my case
>>>>> I've used %pK so the pointers would disappear from logging when
>>>>> kptr_restrict sysctl is 1 or 2.
>>>>> I agree, that we might need to make the printouts optional. If there is
>>>>> any kernel config, which for instance increases the kernel security we
>>>>> could also use it or anything else to discard the printouts at compile
>>>>> time.
>>>>
>>>>
>>>> Certainly, when KASLR is active it would be preferable to hide this
>>>> information, so you could use CONFIG_RELOCATABLE. The existing KASLR stuff
>>>> additionally hides this kind of information behind CONFIG_DEBUG_KERNEL, so
>>>> that only people actively debugging the kernel see it:
>>>>
>>>> http://elixir.free-electrons.com/linux/v4.15-rc8/source/arch/mips/kernel/setup.c#L604
>>>
>>> Ok. I'll hide the printouts behind both of that config macros in the next patchset
>>> version.
>>
>>
>> Another thing to note - since ad67b74d2469d ("printk: hash addresses printed
>> with %p") %pK at this time in the boot process is useless since the RNG is
>> not sufficiently initialised and all prints end up being "(ptrval)". Hence
>> after v4.15-rc2 we end up with output like:
>>
>> [ 0.000000] Kernel virtual memory layout:
>> [ 0.000000] lowmem : 0x(ptrval) - 0x(ptrval) ( 256 MB)
>> [ 0.000000] .text : 0x(ptrval) - 0x(ptrval) (7374 kB)
>> [ 0.000000] .data : 0x(ptrval) - 0x(ptrval) (1901 kB)
>> [ 0.000000] .init : 0x(ptrval) - 0x(ptrval) (1600 kB)
>> [ 0.000000] .bss : 0x(ptrval) - 0x(ptrval) ( 415 kB)
>> [ 0.000000] vmalloc : 0x(ptrval) - 0x(ptrval) (1023 MB)
>> [ 0.000000] fixmap : 0x(ptrval) - 0x(ptrval) ( 68 kB)
>>
>
> It must be some bug in the algo. What point in the %pK then? According to
> the documentation the only way to see the pointers is when (kptr_restrict == 0).
> But if it is we don't get into the restricted_pointer() method at all:
> http://elixir.free-electrons.com/linux/v4.15-rc9/source/lib/vsprintf.c#L1934
> In this case the vsprintf() executes the method ptr_to_id(), which of course
> default to _not_ leak addresses, and hash it before printing.
>
> Really %pK isn't supposed to be dependent from RNG at all since kptr_restrict
> doesn't do any value randomization.


That was true until v4.15-rc2. The behavior of %pK was changed without
that being reflected in the documentation. A patch
(https://patchwork.kernel.org/patch/10124413/) is in progress to update
this.

>
>>
>> The %px format specifier was added for cases such as this, where we really
>> want to print the unmodified address. And as long as this function is
>> suitably guarded to only do this when KASLR is deactivated /
>> CONFIG_DEBUG_KERNEL is activated, etc, then we are not unwittingly leaking
>> information - we are deliberately making it available.
>>
>
> If %pK would work as it's stated by the kernel documentation:
> https://www.kernel.org/doc/Documentation/printk-formats.txt
> then the only change I'd suggest to have here is to close the kernel memory
> layout printout method by the CONFIG_DEBUG_KERNEL ifdef-macro. The kptr_restrict
> should default to 1/2 if the KASLR is activated:
> https://lwn.net/Articles/444556/

Yeah, again, the documentation is no longer correct, and %pK will always
be hashed, and before the RNG is initialized it does not even hash it,
just returning "(ptrval)". So I'd recommend guarding with
CONFIG_DEBUG_KERNEL and switching the format specifier to %px.

Thanks,
Matt

>
> Regards,
> -Sergey
>
>> Thanks,
>> Matt
>>
>>>
>>> Regards,
>>> -Sergey
>>>
>>>>
>>>> Thanks,
>>>> Matt
>>>>
>>>>>
>>>>>> --
>>>>>> Florian

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