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

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