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

[Xiaoming Ni]

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?
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);
+ }
+}
+
 static void __show_regs(struct pt_regs *regs)
 {
        int i, trap;

-   printk("NIP:  "REG" LR: "REG" CTR: "REG"\n",
-          regs->nip, regs->link, regs->ctr);
+ __show_regs_ip_lr("NIP: ", regs->nip);
+ __show_regs_ip_lr(" LR: ", regs->link);
+ pr_cont(" CTR: "REG"\n", regs->ctr);
        printk("REGS: %px TRAP: %04lx   %s  (%s)\n",
               regs, regs->trap, print_tainted(), init_utsname()->release);
        printk("MSR:  "REG" ", regs->msr);




> > Otherwise you might just as well just print 'borked - tough luck'.
>
> Yes.  ASLR is a house of cards.  But that isn't constructive wrt this
> patch :-)
>
>
> Segher
> .

Thanks
Xiaoming Ni