Messages in this thread | | | Subject | Re: [PATCH 11/14] MIPS: memblock: Print out kernel virtual mem layout | From | Matt Redfearn <> | Date | Wed, 24 Jan 2018 09:46:07 +0000 |
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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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