Patch "x86/nmi/64: Reorder nested NMI checks" has been added to the 4.1-stable tree

This is a note to let you know that I've just added the patch titled

x86/nmi/64: Reorder nested NMI checks

to the 4.1-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:

     x86-nmi-64-reorder-nested-nmi-checks.patch

and it can be found in the queue-4.1 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Andy Lutomirski <[email protected]>
Date: Wed, 15 Jul 2015 10:29:37 -0700
Subject: x86/nmi/64: Reorder nested NMI checks

From: Andy Lutomirski <[email protected]>

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. )

---
 arch/x86/kernel/entry_64.S |   34 ++++++++++++++++++----------------
 1 file changed, 18 insertions(+), 16 deletions(-)

@@ -1535,7 +1535,24 @@ 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.
 	 */

@@ -1562,21 +1579,6 @@ 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.
 	 */

Patches currently in stable-queue which might be from [email protected] are

queue-4.1/x86-nmi-enable-nested-do_nmi-handling-for-64-bit-kernels.patch queue-4.1/x86-nmi-64-switch-stacks-on-userspace-nmi-entry.patch queue-4.1/x86-nmi-64-remove-asm-code-that-saves-cr2.patch queue-4.1/x86-nmi-64-use-df-to-avoid-userspace-rsp-confusing-nested-nmi-detection.patch queue-4.1/x86-asm-entry-64-remove-pointless-jump-to-irq_return.patch queue-4.1/x86-nmi-64-reorder-nested-nmi-checks.patch queue-4.1/x86-nmi-64-improve-nested-nmi-comments.patch