linux-kernel - Re: [PATCH v3 0/6] implement KASLR for powerpc/fsl

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Date:   Sun, 01 Mar 2020 21:24:00 -0600
From:   Scott Wood <oss@...error.net>
To:     Jason Yan <yanaijie@...wei.com>, Daniel Axtens <dja@...ens.net>,
        mpe@...erman.id.au, linuxppc-dev@...ts.ozlabs.org,
        diana.craciun@....com, christophe.leroy@....fr,
        benh@...nel.crashing.org, paulus@...ba.org, npiggin@...il.com,
        keescook@...omium.org, kernel-hardening@...ts.openwall.com,
        me@...in.cc
Cc:     linux-kernel@...r.kernel.org, zhaohongjiang@...wei.com
Subject: Re: [PATCH v3 0/6] implement KASLR for powerpc/fsl_booke/64

On Mon, 2020-03-02 at 10:17 +0800, Jason Yan wrote:
> 
> 在 2020/3/1 6:54, Scott Wood 写道:
> > On Sat, 2020-02-29 at 15:27 +0800, Jason Yan wrote:
> > > 
> > > 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.

I'm more concerned about the stack address for wading through a raw stack dump
(to find function call arguments, etc).  The return address does help confirm
that I'm on the right stack frame though, and also makes looking up a line
number slightly easier than having to look up a symbol address and then add
the offset (at least for non-module addresses).

As a random aside, the mismatch between Linux printing a hex offset and GDB
using decimal in disassembly is annoying...

-Scott