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SubjectRe: [PATCH][GIT PULL][v3.3] x86: Test saved %rip in NMI to determine nested NMI

* Steven Rostedt <rostedt@goodmis.org> wrote:

> Ingo,
>
> I found that it is possible for userspace to prevent an NMI
> from triggering while it is running by setting its stack
> pointer to that of the NMI stack. This tricks the NMI nested
> algorithm in thinking that the NMI is nested. The easy
> solution to this is to test the %rip to make sure that the NMI
> happened in kernel mode before testing for nesting.

Ouch...

> I've tested a program that exhibits the missing NMIs and this
> patch corrects that behavior.

Does it need a -stable tag?

> Please pull the latest tip/perf/urgent tree, which can be
> found at:
>
> git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace.git
> tip/perf/urgent
>
> Head SHA1: b80ddc7b1636474297815d47fbfed7552f9b8f2c
>
>
> Steven Rostedt (1):
> x86: Test saved %rip in NMI to determine nested NMI
>
> ----
> arch/x86/kernel/entry_64.S | 8 ++++++++
> 1 files changed, 8 insertions(+), 0 deletions(-)
> ---------------------------
> commit b80ddc7b1636474297815d47fbfed7552f9b8f2c
> Author: Steven Rostedt <srostedt@redhat.com>
> Date: Sat Feb 18 20:26:52 2012 -0500
>
> x86: Test saved %rip in NMI to determine nested NMI
>
> Currently, the NMI handler tests if it is nested by checking the
> special variable saved no 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 the 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
> %rip and making sure that it is a kernel pointer (negative).
>
> Cc: H. Peter Anvin <hpa@zytor.com>
> Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
>
> diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
> index 3fe8239..7c35a7a 100644
> --- a/arch/x86/kernel/entry_64.S
> +++ b/arch/x86/kernel/entry_64.S
> @@ -1532,6 +1532,14 @@ ENTRY(nmi)
> pushq_cfi %rdx
>
> /*
> + * If the RIP is not negative then we are in userspace where this is not
> + * a nested NMI.
> + */
> + movq 8(%rsp), %rdx
> + testq %rdx, %rdx
> + jns first_nmi

Does this do the right thing for the vDSO as well? It is in
negative addresses:

ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0 [vsyscall]
Thanks,

Ingo


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