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

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Date:   Tue, 22 Dec 2020 21:45:03 +0800
From:   Xiaoming Ni <nixiaoming@...wei.com>
To:     Segher Boessenkool <segher@...nel.crashing.org>,
        David Laight <David.Laight@...lab.com>
CC:     Christophe Leroy <christophe.leroy@...roup.eu>,
        "ravi.bangoria@...ux.ibm.com" <ravi.bangoria@...ux.ibm.com>,
        "mikey@...ling.org" <mikey@...ling.org>,
        "yanaijie@...wei.com" <yanaijie@...wei.com>,
        "haren@...ux.ibm.com" <haren@...ux.ibm.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "npiggin@...il.com" <npiggin@...il.com>,
        "wangle6@...wei.com" <wangle6@...wei.com>,
        "paulus@...ba.org" <paulus@...ba.org>,
        "aneesh.kumar@...ux.ibm.com" <aneesh.kumar@...ux.ibm.com>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH] powerpc:Don't print raw EIP/LR hex values in dump_stack()
 and show_regs()

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