Commit a27507ca authored Jul 15, 2015 by Andy Lutomirski Committed by Ingo Molnar Jul 17, 2015

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>

parent 0b22930e

arch/x86/entry/entry_64.S

+18 −16

Original line number	Diff line number	Diff line
		@@ -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.
		*/
		@@ -1386,21 +1403,6 @@ ENTRY(nmi)
		/* Ah, it is within the NMI stack, treat it as nested */

		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.