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

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Message-ID: <20201221171228.GA2672@gate.crashing.org>
Date:   Mon, 21 Dec 2020 11:12:28 -0600
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     David Laight <David.Laight@...lab.com>
Cc:     Christophe Leroy <christophe.leroy@...roup.eu>,
        Xiaoming Ni <nixiaoming@...wei.com>,
        "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 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.

> 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