[PATCH 3.16.y-ckt 054/118] x86/nmi/64: Reorder nested NMI checks

[Luis Henriques]

3.16.7-ckt16 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Andy Lutomirski <luto@xxxxxxxxxx>

commit a27507ca2d796cfa8d907de31ad730359c8a6d06 upstream.

Check the repeat_nmi .. end_repeat_nmi special case first.  The
next patch will rework the RSP check and, as a side effect, the
RSP check will no longer detect repeat_nmi .. end_repeat_nmi, so
we'll need this ordering of the checks.

Note: this is more subtle than it appears.  The check for
repeat_nmi .. end_repeat_nmi jumps straight out of the NMI code
instead of adjusting the "iret" frame to force a repeat.  This
is necessary, because the code between repeat_nmi and
end_repeat_nmi sets "NMI executing" and then writes to the
"iret" frame itself.  If a nested NMI comes in and modifies the
"iret" frame while repeat_nmi is also modifying it, we'll end up
with garbage.  The old code got this right, as does the new
code, but the new code is a bit more explicit.

If we were to move the check right after the "NMI executing"
check, then we'd get it wrong and have random crashes.

( Because the "NMI executing" check would jump to the code that would
  modify the "iret" frame without checking if the interrupted NMI was
  currently modifying it. )

Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
Reviewed-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
[bwh: Backported to 4.0: adjust filename, spacing]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
[ luis: backported to 3.16: Used Ben's backport to 4.0 ]
Signed-off-by: Luis Henriques <luis.henriques@xxxxxxxxxxxxx>
---
 arch/x86/kernel/entry_64.S | 34 ++++++++++++++++++----------------
 1 file changed, 18 insertions(+), 16 deletions(-)

diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index 0fdb80df2cac..8a6866b2f2a6 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -1586,7 +1586,24 @@ ENTRY(nmi)
 	/*
 	 * Determine whether we're a nested NMI.
 	 *
-	 * First check "NMI executing".  If it's set, then we're nested.
+	 * If we interrupted kernel code between repeat_nmi and
+	 * end_repeat_nmi, then we are a nested NMI.  We must not
+	 * modify the "iret" frame because it's being written by
+	 * the outer NMI.  That's okay: the outer NMI handler is
+	 * about to about to call do_nmi anyway, so we can just
+	 * resume the outer NMI.
+	 */
+
+	movq	$repeat_nmi, %rdx
+	cmpq	8(%rsp), %rdx
+	ja	1f
+	movq	$end_repeat_nmi, %rdx
+	cmpq	8(%rsp), %rdx
+	ja	nested_nmi_out
+1:
+
+	/*
+	 * Now check "NMI executing".  If it's set, then we're nested.
 	 * This will not detect if we interrupted an outer NMI just
 	 * before IRET.
 	 */
@@ -1613,21 +1630,6 @@ ENTRY(nmi)
 
 nested_nmi:
 	/*
-	 * If we interrupted an NMI that is between repeat_nmi and
-	 * end_repeat_nmi, then we must not modify the "iret" frame
-	 * because it's being written by the outer NMI.  That's okay:
-	 * the outer NMI handler is about to call do_nmi anyway,
-	 * so we can just resume the outer NMI.
-	 */
-	movq $repeat_nmi, %rdx
-	cmpq 8(%rsp), %rdx
-	ja 1f
-	movq $end_repeat_nmi, %rdx
-	cmpq 8(%rsp), %rdx
-	ja nested_nmi_out
-
-1:
-	/*
 	 * Modify the "iret" frame to point to repeat_nmi, forcing another
 	 * iteration of NMI handling.
 	 */
--
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