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

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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)


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.

Thanks,
Matt


Regards,
-Sergey


Thanks,
Matt


--
Florian


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