Re: [PATCH v3 0/6] implement KASLR for powerpc/fsl_booke/64

[Jason Yan]

å 2020/3/1 6:54, Scott Wood åé:
> On Sat, 2020-02-29 at 15:27 +0800, Jason Yan wrote:
> > å 2020/2/29 12:28, Scott Wood åé:
> > > On Fri, 2020-02-28 at 14:47 +0800, Jason Yan wrote:
> > > > å 2020/2/28 13:53, Scott Wood åé:
> > > > > I don't see any debug setting for %pK (or %p) to always print the
> > > > > actual
> > > > > address (closest is kptr_restrict=1 but that only works in certain
> > > > > contexts)... from looking at the code it seems it hashes even if kaslr
> > > > > is
> > > > > entirely disabled?  Or am I missing something?
> > > >
> > > > Yes, %pK (or %p) always hashes whether kaslr is disabled or not. So if
> > > > we want the real value of the address, we cannot use it. But if you only
> > > > want to distinguish if two pointers are the same, it's ok.
> > >
> > > Am I the only one that finds this a bit crazy?  If you want to lock a
> > > system
> > > down then fine, but why wage war on debugging even when there's no
> > > randomization going on?  Comparing two pointers for equality is not always
> > > adequate.
> >
> > AFAIK, %p hashing is only exist because of many legacy address printings
> > and force who really want the raw values to switch to %px or even %lx.
> > It's not the opposite of debugging. Raw address printing is not
> > forbidden, only people need to estimate the risk of adrdress leaks.
>
> Yes, but I don't see any format specifier to switch to that will hash in a
> randomized production environment, but not in a debug or other non-randomized
> environment which seems like the ideal default for most debug output.

Sorry I have no idea why there is no format specifier considered for 
switching of randomized or non-randomized environment. May they think 
that raw address should not leak in non-randomized environment too. May 
be Kees or Tobin can answer this question.

Kees? Tobin?

> > Turnning to %p may not be a good idea in this situation. So
> > for the REG logs printed when dumping stack, we can disable it when
> > KASLR is open. For the REG logs in other places like show_regs(), only
> > privileged can trigger it, and they are not combind with a symbol, so
> > I think it's ok to keep them.
> >
> > diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
> > index fad50db9dcf2..659c51f0739a 100644
> > --- a/arch/powerpc/kernel/process.c
> > +++ b/arch/powerpc/kernel/process.c
> > @@ -2068,7 +2068,10 @@ void show_stack(struct task_struct *tsk, unsigned
> > long *stack)
> >                   newsp = stack[0];
> >                   ip = stack[STACK_FRAME_LR_SAVE];
> >                   if (!firstframe || ip != lr) {
> > -                       printk("["REG"] ["REG"] %pS", sp, ip, (void *)ip);
> > +                       if (IS_ENABLED(CONFIG_RANDOMIZE_BASE))
> > +                               printk("%pS", (void *)ip);
> > +                       else
> > +                               printk("["REG"] ["REG"] %pS", sp, ip,
> > (void *)ip);
>
> This doesn't deal with "nokaslr" on the kernel command line.  It also doesn't
> seem like something that every callsite should have to opencode, versus having
> an appropriate format specifier behaves as I described above (and I still
> don't see why that format specifier should not be "%p").

Actually I still do not understand why we should print the raw value 
here. When KALLSYMS is enabled we have symbol name  and  offset like 
put_cred_rcu+0x108/0x110, and when KALLSYMS is disabled we have the raw 
address.

> -Scott
>
>
>
> .