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Date:   Sat, 20 Jul 2019 01:01:08 +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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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>
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On Sat, 20 Jul 2019, Thomas Gleixner wrote:

> On Fri, 19 Jul 2019, Sean Christopherson wrote:
> > On Tue, Jul 02, 2019 at 01:39:05PM -0400, Steven Rostedt wrote:
> > 
> > I'm hitting a similar panic that bisects to commit
> > 
> >   a0d14b8909de ("x86/mm, tracing: Fix CR2 corruption")
> > 
> > except I'm experiencing death immediately after starting init.
> > 
> > Through sheer dumb luck, I tracked (pun intended) this down to forcing
> > context tracking:
> > 
> >   CONFIG_CONTEXT_TRACKING=y
> >   CONFIG_CONTEXT_TRACKING_FORCE=y
> >   CONFIG_VIRT_CPU_ACCOUNTING_GEN=y
> > 
> > I haven't attempted to debug further and I'll be offline for most of the
> > next few days.  Hopefully this is enough to root cause the badness.
> > 
> > [    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.

Fix is not pretty, but ...

Thanks,

	tglx

8<-----------
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -876,6 +876,7 @@ apicinterrupt IRQ_WORK_VECTOR			irq_work
 
 	.if \read_cr2
 	GET_CR2_INTO(%rdx);			/* can clobber %rax */
+	pushq	%rdx
 	.endif
 
 	.if \shift_ist != -1
@@ -885,12 +886,20 @@ apicinterrupt IRQ_WORK_VECTOR			irq_work
 	.endif
 
 	.if \paranoid == 0
+	.if \read_cr2
+	testb	$3, CS + 8(%rsp)
+	.else
 	testb	$3, CS(%rsp)
+	.endif
 	jz	.Lfrom_kernel_no_context_tracking_\@
 	CALL_enter_from_user_mode
 .Lfrom_kernel_no_context_tracking_\@:
 	.endif
 
+	.if \read_cr2
+	popq	%rdx
+	.endif
+
 	movq	%rsp, %rdi			/* pt_regs pointer */
 
 	.if \has_error_code