4.1-stable review patch. If anyone has any objections, please let me know.
------------------
From: Andy Lutomirski <l...@kernel.org>
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 <l...@kernel.org>
Reviewed-by: Steven Rostedt <rost...@goodmis.org>
Cc: Borislav Petkov <b...@suse.de>
Cc: Linus Torvalds <torva...@linux-foundation.org>
Cc: Peter Zijlstra <pet...@infradead.org>
Cc: Thomas Gleixner <t...@linutronix.de>
Cc: sta...@vger.kernel.org
Signed-off-by: Ingo Molnar <mi...@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>
---
arch/x86/kernel/entry_64.S | 34 ++++++++++++++++++----------------
1 file changed, 18 insertions(+), 16 deletions(-)
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -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.
*/
--
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