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SubjectRe: [tip:x86/urgent] x86/nmi: Test saved %cs in NMI to determine nested NMI case
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On Mon, 2012-02-20 at 03:47 -0800, tip-bot for Steven Rostedt wrote:
> Commit-ID: 45d5a1683c04be28abdf5c04c27b1417e0374486
> Gitweb: http://git.kernel.org/tip/45d5a1683c04be28abdf5c04c27b1417e0374486
> Author: Steven Rostedt <rostedt@goodmis.org>
> AuthorDate: Sun, 19 Feb 2012 16:43:37 -0500
> Committer: Ingo Molnar <mingo@elte.hu>
> CommitDate: Mon, 20 Feb 2012 09:09:57 +0100
>
> x86/nmi: Test saved %cs in NMI to determine nested NMI case
>
> 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 <rostedt@goodmis.org>
> Cc: H. Peter Anvin <hpa@zytor.com>
> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: <stable@kernel.org>

Just so that we do not confuse Greg, the bug was introduced in the 3.3
merge window. It does not exist in 3.2 or earlier.

-- Steve



> Link: http://lkml.kernel.org/r/1329687817.1561.27.camel@acer.local.home
> Signed-off-by: Ingo Molnar <mingo@elte.hu>
> ---
> arch/x86/kernel/entry_64.S | 7 +++++++
> 1 files changed, 7 insertions(+), 0 deletions(-)
>
> 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.
> */




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