[PATCH 2/2] x86/nmi: Fix and optimize the NMI stack check code

[Steven Rostedt <rostedt@goodmis.org>]

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From: "Steven Rostedt (Red Hat)" <rostedt@goodmis.org>

Andy Lutomirski reported an off by one in the NMI stack check
for the nested NMI code, where if the stack pointer was one above
the actual stack (stack start + STACK_SIZE) it would trigger a false
positive. This is not that big of a deal because the stack pointer
should never be that. Even if a stack was using the pages just
above the NMI stack, it would require the stack about to overflow
for this to trigger, which is a much bigger bug than this is fixing.

Also, Linus Torvalds pointed out that doing two compares can be
accomplish with a single compare. That is:

("reg" is top of stack we are comparing "stack" to)

  cmpq reg, stack
  jae label  // note, code had one off "ja" instead of "jae"
  subq size, reg
  cmpq reg, stack
  jb label

Is the same as:

  subq $1, reg
  subq stack, reg
  cmpq size, reg
  jae label

The subq $1 was added into the leaq by doing:

   leaq 5*8+7(%rsp), %rdx

Added more comments as well.

Reported-by: Andy Lutomirski <luto@amacapital.net>
Inspired-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
 arch/x86/entry/entry_64.S | 21 +++++++++++----------
 1 file changed, 11 insertions(+), 10 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 3aad759aace2..1e6ca3740762 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1355,16 +1355,17 @@ ENTRY(nmi)
 	 * if it controls the kernel's RSP.  We set DF before we clear
 	 * "NMI executing".
 	 */
-	lea	6*8(%rsp), %rdx
-	/* Compare the NMI stack (rdx) with the stack we came from (4*8(%rsp)) */
-	cmpq	%rdx, 4*8(%rsp)
-	/* If the stack pointer is above the NMI stack, this is a normal NMI */
-	ja	first_nmi
-
-	subq	$EXCEPTION_STKSZ, %rdx
-	cmpq	%rdx, 4*8(%rsp)
-	/* If it is below the NMI stack, it is a normal NMI */
-	jb	first_nmi
+
+	/* Load address of the top of this stack into rdx */
+	lea 5*8+7(%rsp), %rdx
+	/* Subtract the return stack pointer from it */
+	subq 4*8(%rsp), %rdx
+	/*
+	 * If the result is greater or equal to the stack size,
+	 * then the return stack was not on this stack.
+	 */
+	cmpq $EXCEPTION_STKSZ, %rdx
+	jae first_nmi
 
 	/* Ah, it is within the NMI stack. */
 
-- 
2.10.2