x86/nmi/64: Reorder nested NMI checks

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@kernel.org>
Reviewed-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Borislav Petkov <bp@suse.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 18 insertions, 16 deletions

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index f54d63a60a3b..5c4ab384b84f 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1361,7 +1361,24 @@ 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.
          */
@@ -1387,21 +1404,6 @@ 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.
          */