From: Matt Redfearn <matt.redfearn@mips.com>
To: Serge Semin <fancer.lancer@gmail.com>
Cc: Florian Fainelli <f.fainelli@gmail.com>, <ralf@linux-mips.org>,
<miodrag.dinic@mips.com>, <jhogan@kernel.org>,
<goran.ferenc@mips.com>, <david.daney@cavium.com>,
<paul.gortmaker@windriver.com>, <paul.burton@mips.com>,
<alex.belits@cavium.com>, <Steven.Hill@cavium.com>,
<alexander.sverdlin@nokia.com>, <kumba@gentoo.org>,
<marcin.nowakowski@mips.com>, <James.hogan@mips.com>,
<Peter.Wotton@mips.com>, <Sergey.Semin@t-platforms.ru>,
<linux-mips@linux-mips.org>, <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 11/14] MIPS: memblock: Print out kernel virtual mem layout
Date: Wed, 24 Jan 2018 09:46:07 +0000 [thread overview]
Message-ID: <e751ccda-ab57-dfe8-0a16-25bb5368337c@mips.com> (raw)
In-Reply-To: <20180123191051.GA28147@mobilestation>
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
next prev parent reply other threads:[~2018-01-24 9:48 UTC|newest]
Thread overview: 73+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-01-17 22:22 [PATCH 00/14] MIPS: memblock: Switch arch code to NO_BOOTMEM Serge Semin
2018-01-17 22:22 ` [PATCH 01/14] MIPS: memblock: Add RESERVED_NOMAP memory flag Serge Semin
2018-01-17 22:23 ` [PATCH 02/14] MIPS: memblock: Surely map BSS kernel memory section Serge Semin
2018-01-22 16:35 ` Matt Redfearn
2018-01-22 21:47 ` Serge Semin
2018-01-23 11:03 ` Matt Redfearn
2018-01-23 19:27 ` Serge Semin
2018-01-24 9:49 ` Matt Redfearn
2018-01-24 10:03 ` Serge Semin
2018-01-17 22:23 ` [PATCH 03/14] MIPS: memblock: Reserve initrd memory in memblock Serge Semin
2018-01-17 22:23 ` [PATCH 04/14] MIPS: memblock: Discard bootmem initialization Serge Semin
2018-01-17 22:23 ` [PATCH 05/14] MIPS: memblock: Add reserved memory regions to memblock Serge Semin
2018-01-17 22:23 ` [PATCH 06/14] MIPS: memblock: Reserve kdump/crash regions in memblock Serge Semin
2018-01-17 22:23 ` [PATCH 07/14] MIPS: memblock: Mark present sparsemem sections Serge Semin
2018-01-24 6:13 ` Marcin Nowakowski
2018-01-24 7:27 ` Serge Semin
2018-01-17 22:23 ` [PATCH 08/14] MIPS: memblock: Simplify DMA contiguous reservation Serge Semin
2018-01-17 22:23 ` [PATCH 09/14] MIPS: memblock: Allow memblock regions resize Serge Semin
2018-01-17 22:23 ` [PATCH 10/14] MIPS: memblock: Perform early low memory test Serge Semin
2018-01-17 22:23 ` [PATCH 11/14] MIPS: memblock: Print out kernel virtual mem layout Serge Semin
2018-01-18 20:03 ` Florian Fainelli
2018-01-18 20:18 ` Serge Semin
2018-01-19 7:59 ` Matt Redfearn
2018-01-19 14:27 ` Serge Semin
2018-01-23 15:35 ` Matt Redfearn
2018-01-23 19:10 ` Serge Semin
2018-01-24 9:46 ` Matt Redfearn [this message]
2018-01-24 10:08 ` Serge Semin
2018-01-17 22:23 ` [PATCH 12/14] MIPS: memblock: Discard bootmem from Loongson3 code Serge Semin
2018-01-23 22:28 ` Jiaxun Yang
2018-01-23 19:36 ` Serge Semin
2018-01-17 22:23 ` [PATCH 13/14] MIPS: memblock: Discard bootmem from SGI IP27 code Serge Semin
2018-01-17 22:23 ` [PATCH 14/14] MIPS: memblock: Deactivate bootmem allocator Serge Semin
2018-01-23 23:59 ` James Hogan
2018-01-24 8:28 ` Serge Semin
2018-01-22 16:36 ` [PATCH 00/14] MIPS: memblock: Switch arch code to NO_BOOTMEM Matt Redfearn
2018-01-22 21:33 ` Serge Semin
2018-01-23 11:29 ` Mathieu Malaterre
2018-01-23 14:01 ` Matt Redfearn
2018-01-25 17:58 ` Alexander Sverdlin
2018-01-25 20:17 ` Serge Semin
2018-01-31 0:21 ` Serge Semin
2018-02-02 3:54 ` [PATCH v2 00/15] " Serge Semin
2018-02-02 3:54 ` [PATCH v2 01/15] MIPS: memblock: Add RESERVED_NOMAP memory flag Serge Semin
2018-02-13 11:21 ` Matt Redfearn
2018-02-02 3:54 ` [PATCH v2 02/15] MIPS: memblock: Surely map BSS kernel memory section Serge Semin
2018-02-13 11:22 ` Matt Redfearn
2018-02-02 3:54 ` [PATCH v2 03/15] MIPS: memblock: Reserve initrd memory in memblock Serge Semin
2018-02-13 11:22 ` Matt Redfearn
2018-02-02 3:54 ` [PATCH v2 04/15] MIPS: memblock: Discard bootmem initialization Serge Semin
2018-02-13 11:28 ` Matt Redfearn
2018-02-02 3:54 ` [PATCH v2 05/15] MIPS: KASLR: Drop relocatable fixup from reservation_init Serge Semin
2018-02-13 11:30 ` Matt Redfearn
2018-02-02 3:54 ` [PATCH v2 06/15] MIPS: memblock: Add reserved memory regions to memblock Serge Semin
2018-02-13 13:44 ` Matt Redfearn
2018-02-02 3:54 ` [PATCH v2 07/15] MIPS: memblock: Reserve kdump/crash regions in memblock Serge Semin
2018-02-13 13:45 ` Matt Redfearn
2018-02-02 3:54 ` [PATCH v2 08/15] MIPS: memblock: Mark present sparsemem sections Serge Semin
2018-02-13 13:50 ` Matt Redfearn
2018-02-02 3:54 ` [PATCH v2 09/15] MIPS: memblock: Simplify DMA contiguous reservation Serge Semin
2018-02-13 13:51 ` Matt Redfearn
2018-02-02 3:54 ` [PATCH v2 10/15] MIPS: memblock: Allow memblock regions resize Serge Semin
2018-02-13 13:55 ` Matt Redfearn
2018-02-02 3:54 ` [PATCH v2 11/15] MIPS: memblock: Perform early low memory test Serge Semin
2018-02-13 14:01 ` Matt Redfearn
2018-02-02 3:54 ` [PATCH v2 12/15] MIPS: memblock: Print out kernel virtual mem layout Serge Semin
2018-02-13 14:05 ` Matt Redfearn
2018-02-02 3:54 ` [PATCH v2 13/15] MIPS: memblock: Discard bootmem from Loongson3 code Serge Semin
2018-02-13 14:09 ` Matt Redfearn
2018-02-02 3:54 ` [PATCH v2 14/15] MIPS: memblock: Discard bootmem from SGI IP27 code Serge Semin
2018-02-13 14:17 ` Matt Redfearn
2018-02-02 3:54 ` [PATCH v2 15/15] MIPS: memblock: Deactivate bootmem allocator Serge Semin
2018-02-13 14:18 ` Matt Redfearn
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