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Message-ID: <4c0e7fec63dbc7b91fa6c24692c73c256c131f51.camel@buserror.net>
Date: Fri, 28 Feb 2020 22:28:39 -0600
From: Scott Wood <oss@...error.net>
To: Jason Yan <yanaijie@...wei.com>, Daniel Axtens <dja@...ens.net>,
mpe@...erman.id.au, linuxppc-dev@...ts.ozlabs.org, diana.craciun@....com,
christophe.leroy@....fr, benh@...nel.crashing.org, paulus@...ba.org,
npiggin@...il.com, keescook@...omium.org,
kernel-hardening@...ts.openwall.com
Cc: linux-kernel@...r.kernel.org, zhaohongjiang@...wei.com
Subject: Re: [PATCH v3 0/6] implement KASLR for powerpc/fsl_booke/64
On Fri, 2020-02-28 at 14:47 +0800, Jason Yan wrote:
>
> 在 2020/2/28 13:53, Scott Wood 写道:
> > On Wed, 2020-02-26 at 16:18 +0800, Jason Yan wrote:
> > > Hi Daniel,
> > >
> > > 在 2020/2/26 15:16, Daniel Axtens 写道:
> > > > Maybe replacing the REG format string in KASLR mode would be
> > > > sufficient?
> > > >
> > >
> > > Most archs have removed the address printing when dumping stack. Do we
> > > really have to print this?
> > >
> > > If we have to do this, maybe we can use "%pK" so that they will be
> > > hidden from unprivileged users.
> >
> > I've found the addresses to be useful, especially if I had a way to dump
> > the
> > stack data itself. Wouldn't the register dump also be likely to give away
> > the
> > addresses?
>
> If we have to print the address, then kptr_restrict and dmesg_restrict
> must be set properly so that unprivileged users cannot see them.
And how does that work with crash dumps that could be from any context?
dmesg_restrict is irrelevant as it just controls who can see the dmesg, not
what goes into it. kptr_restrict=1 will only get the value if you're not in
any sort of IRQ, *and* if the crashing context happened to have CAP_SYSLOG.
No other value of kptr_restrict will ever get you the raw value.
> >
> > I don't see any debug setting for %pK (or %p) to always print the actual
> > address (closest is kptr_restrict=1 but that only works in certain
> > contexts)... from looking at the code it seems it hashes even if kaslr is
> > entirely disabled? Or am I missing something?
> >
>
> Yes, %pK (or %p) always hashes whether kaslr is disabled or not. So if
> we want the real value of the address, we cannot use it. But if you only
> want to distinguish if two pointers are the same, it's ok.
Am I the only one that finds this a bit crazy? If you want to lock a system
down then fine, but why wage war on debugging even when there's no
randomization going on? Comparing two pointers for equality is not always
adequate.
-Scott
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