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