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Date:   Sat, 20 Jul 2019 10:44:59 +0200 (CEST)
From:   Thomas Gleixner <tglx@linutronix.de>
To:     Sean Christopherson <sean.j.christopherson@intel.com>
cc:     Steven Rostedt <rostedt@goodmis.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Eiichi Tsukata <devel@etsukata.com>, edwintorok@gmail.com,
        mingo@redhat.com, bp@alien8.de, hpa@zytor.com, x86@kernel.org,
        linux-kernel@vger.kernel.org, Josh Poimboeuf <jpoimboe@redhat.com>,
        Joel Fernandes <joel@joelfernandes.org>
Subject: Re: [PATCH] x86/stacktrace: Do not access user space memory
 unnecessarily
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Message-ID: <alpine.DEB.2.21.1907201044150.1782@nanos.tec.linutronix.de>
References: <20190702053151.26922-1-devel@etsukata.com> <20190702072821.GX3419@hirez.programming.kicks-ass.net> <alpine.DEB.2.21.1907021400350.1802@nanos.tec.linutronix.de> <20190702113355.5be9ebfe@gandalf.local.home> <20190702133905.1482b87e@gandalf.local.home>
 <20190719202836.GB13680@linux.intel.com> <alpine.DEB.2.21.1907200018050.1782@nanos.tec.linutronix.de> <alpine.DEB.2.21.1907200039110.1782@nanos.tec.linutronix.de>
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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.