This is a note to let you know that I've just added the patch titled
x86/nmi: Test saved %cs in NMI to determine nested NMI case
to the 3.2-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
x86-nmi-test-saved-cs-in-nmi-to-determine-nested-nmi-case.patch
and it can be found in the queue-3.2 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <[email protected]> know about it.
>From 45d5a1683c04be28abdf5c04c27b1417e0374486 Mon Sep 17 00:00:00 2001
From: Steven Rostedt <[email protected]>
Date: Sun, 19 Feb 2012 16:43:37 -0500
Subject: x86/nmi: Test saved %cs in NMI to determine nested NMI case
From: Steven Rostedt <[email protected]>
commit 45d5a1683c04be28abdf5c04c27b1417e0374486 upstream.
Currently, the NMI handler tests if it is nested by checking the
special variable saved on the stack (set during NMI handling)
and whether the saved stack is the NMI stack as well (to prevent
the race when the variable is set to zero).
But userspace may set their %rsp to any value as long as they do
not derefence it, and it may make it point to the NMI stack,
which will prevent NMIs from triggering while the userspace app
is running. (I tested this, and it is indeed the case)
Add another check to determine nested NMIs by looking at the
saved %cs (code segment register) and making sure that it is the
kernel code segment.
Signed-off-by: Steven Rostedt <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Linus Torvalds <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index 3fe8239..debd851 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -1532,6 +1532,13 @@ ENTRY(nmi)
pushq_cfi %rdx
/*
+ * If %cs was not the kernel segment, then the NMI triggered in user
+ * space, which means it is definitely not nested.
+ */
+ cmp $__KERNEL_CS, 16(%rsp)
+ jne first_nmi
+
+ /*
* Check the special variable on the stack to see if NMIs are
* executing.
*/
Patches currently in stable-queue which might be from [email protected] are
queue-3.2/x86-nmi-test-saved-cs-in-nmi-to-determine-nested-nmi-case.patch
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