Re: [PATCH 11/14] MIPS: memblock: Print out kernel virtual mem layout

[Serge Semin]

Hello Matt,

On Wed, Jan 24, 2018 at 09:46:07AM +0000, Matt Redfearn <matt.redfearn@xxxxxxxx> wrote:
> 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@xxxxxxxx> 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@xxxxxxxx> 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@xxxxxxxxx> 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@xxxxxxxxx>
> >>>>>>
> >>>>>>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.
> 

Oh, it isn't the bug then) I'll do as you suggest and replace %pK with
%px closing the code by CONFIG_DEBUG_KERNEL macro.

Regards,
-Sergey

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