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 :-)


Xiaoming Ni

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