Commit 4e79de2a authored by Andy Lutomirski's avatar Andy Lutomirski Committed by Luis Henriques

x86/nmi/64: Reorder nested NMI checks

commit a27507ca 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: default avatarAndy Lutomirski <luto@kernel.org>
Reviewed-by: default avatarSteven 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>
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
[bwh: Backported to 4.0: adjust filename, spacing]
Signed-off-by: default avatarBen Hutchings <ben@decadent.org.uk>
[ luis: backported to 3.16: Used Ben's backport to 4.0 ]
Signed-off-by: default avatarLuis Henriques <luis.henriques@canonical.com>
parent f79aba67
......@@ -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.
*/
......@@ -1612,21 +1629,6 @@ ENTRY(nmi)
CFI_REMEMBER_STATE
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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