linux-kernel - Re: [PATCH] x86/stacktrace: Do not access user space memory unnecessarily

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Date:   Sat, 20 Jul 2019 10:44:59 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
cc:     Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Eiichi Tsukata <devel@...ukata.com>, edwintorok@...il.com,
        mingo@...hat.com, bp@...en8.de, hpa@...or.com, x86@...nel.org,
        linux-kernel@...r.kernel.org, Josh Poimboeuf <jpoimboe@...hat.com>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [PATCH] x86/stacktrace: Do not access user space memory
 unnecessarily

On Sat, 20 Jul 2019, Thomas Gleixner wrote:
> On Sat, 20 Jul 2019, Thomas Gleixner wrote:
> > On Fri, 19 Jul 2019, Sean Christopherson wrote:
> > > [    0.680477] Run /sbin/init as init process
> > > [    0.682116] init[1]: segfault at 2926a7ef ip 00007f98a49d9c30 sp 00007fffd83e6af0 error 14 in ld-2.23.so[7f98a49d9000+26000]
> > 
> > That's because the call into the context tracking muck clobbers RDX which
> > contains the CR2 value on pagefault. So the pagefault resolves to crap and
> > kills init.
> > 
> > Brute force fix below. That needs to be conditional on read_cr2 but for now
> > it does the job.
> 
> But it does it just for the context tracking case. TRACE_IRQS_OFF* will do
> the same damage.

Hmm, should not becasue that calls through the thunk which preserves RDX.