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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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