linux-kernel - Re: NMI vs #PF clash

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Date:	Wed, 23 May 2012 04:56:51 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Brian Gerst <brgerst@...il.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Avi Kivity <avi@...hat.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Paul Turner <pjt@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: NMI vs #PF clash

On Tue, 2012-05-22 at 21:26 -0400, Brian Gerst wrote:
> 
> You could save cr2 in a callee-saved register (like r12) instead of
> putting it on the stack.
> 
Much simpler!

-- Steve

diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index f41b3b1..59bae47 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -1725,10 +1725,30 @@ end_repeat_nmi:
 	 */
 	call save_paranoid
 	DEFAULT_FRAME 0
+
+	/*
+	 * Save off the CR2 register. If we take a page fault in the NMI then
+	 * it could corrupt the CR2 value. If the NMI preempts a page fault
+	 * handler before it was able to read the CR2 register, and then the
+	 * NMI itself takes a page fault, the page fault that was preempted
+	 * will read the information from the NMI page fault and not the
+	 * origin fault. Save it off and restore it if it changes.
+	 * Use the r12 callee-saved register.
+	 */
+	movq %cr2, %r12
+
 	/* paranoidentry do_nmi, 0; without TRACE_IRQS_OFF */
 	movq %rsp,%rdi
 	movq $-1,%rsi
 	call do_nmi
+
+	/* Did the NMI take a page fault? Restore cr2 if it did */
+	movq %cr2, %rcx
+	cmpq %rcx, %r12
+	je 1f
+	movq %r12, %cr2
+1:
+	
 	testl %ebx,%ebx				/* swapgs needed? */
 	jnz nmi_restore
 nmi_swapgs:


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