Re: [PATCH] powerpc:Don't print raw EIP/LR hex values in dump_stack() and show_regs()

[Christophe Leroy]

Le 22/12/2020 à 18:29, Segher Boessenkool a écrit :
> On Tue, Dec 22, 2020 at 09:45:03PM +0800, Xiaoming Ni wrote:
> > On 2020/12/22 1:12, Segher Boessenkool wrote:
> > > On Mon, Dec 21, 2020 at 04:42:23PM +0000, David Laight wrote:
> > > > From: Segher Boessenkool
> > > > > Sent: 21 December 2020 16:32
> > > > >
> > > > > On Mon, Dec 21, 2020 at 04:17:21PM +0100, Christophe Leroy wrote:
> > > > > > Le 21/12/2020 à 04:27, Xiaoming Ni a écrit :
> > > > > > > Since the commit 2b0e86cc5de6 ("powerpc/fsl_booke/32: implement KASLR
> > > > > > > infrastructure"), the powerpc system is ready to support KASLR.
> > > > > > > To reduces the risk of invalidating address randomization, don't print
> > > > > > > the
> > > > > > > EIP/LR hex values in dump_stack() and show_regs().
> > > > >
> > > > > > I think your change is not enough to hide EIP address, see below a dump
> > > > > > with you patch, you get "Faulting instruction address: 0xc03a0c14"
> > > > >
> > > > > As far as I can see the patch does nothing to the GPR printout.  Often
> > > > > GPRs contain code addresses.  As one example, the LR is moved via a GPR
> > > > > (often GPR0, but not always) for storing on the stack.
> > > > >
> > > > > So this needs more work.
> > > >
> > > > If the dump_stack() is from an oops you need the real EIP value
> > > > on order to stand any chance of making headway.
> > >
> > > Or at least the function name + offset, yes.
> > When the system is healthy, only symbols and offsets are printed,
> > Output address and symbol + offset when the system is dying
> > Does this meet both debugging and security requirements?
>
> If you have the vmlinux, sym+off is enough to find what instruction
> caused the crash.
>
> It does of course not give all the information you get in a crash dump
> with all the registers, so it does hinder debugging a bit.  This is a
> tradeoff.
>
> Most debugging will need xmon or similar (or printf-style debugging)
> anyway; and otoh the register dump will render KASLR largely
> ineffective.
>
> > For example:
> >
> > +static void __show_regs_ip_lr(const char *flag, unsigned long addr)
> > +{
> > + if (system_going_down()) { /* panic oops reboot */
> > +         pr_cont("%s["REG"] %pS", flag, addr, (void *)addr);
> > + } else {
> > +         pr_cont("%s%pS", flag, (void *)addr);
> > + }
> > +}
>
> *If* you are certain the system goes down immediately, and you are also
> certain this information will not help defeat ASLR after a reboot, you
> could just print whatever, sure.
>
> Otherwise, you only want to show some very few registers.  Or, make sure
> no attackers can ever see these dumps (which is hard, many systems trust
> all (local) users with it!)  Which means we first will need some very
> different patches, before any of this can be much useful :-(

So IIUC, on one side we enlarge the dumping of registers with commits like 
https://github.com/linuxppc/linux/commit/bf13718bc57ada25016d9fe80323238d0b94506e#diff-8b965e0e62fc1b6ad5e51bf0a539941e929754cdb716041b06b4f4a5f73590f9, 
and on the other side we want to narrow it and hide registers ? I'm lost.

Christophe