Messages in this thread | | | Date | Tue, 28 Sep 2010 01:52:08 +0200 | From | Frederic Weisbecker <> | Subject | Re: [RFC PATCH] x86: Barf when faults happen in NMI |
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On Mon, Sep 27, 2010 at 05:14:01PM -0400, Mathieu Desnoyers wrote: > * Frederic Weisbecker (fweisbec@gmail.com) wrote: > > In x86, faults exit by executing the iret instruction, which then > > reenables NMIs if we faulted in NMI context. Then if a fault > > happens in NMI, another NMI can nest after the fault exits. > > > > But we don't yet support nested NMIs because we have only one NMI > > stack. To prevent that, trigger a bug when a fault happens in NMI > > context. > > > > Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com> > > Cc: Ingo Molnar <mingo@elte.hu> > > Cc: Thomas Gleixner <tglx@linutronix.de> > > Cc: H. Peter Anvin <hpa@zytor.com> > > Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> > > Cc: Peter Zijlstra <a.p.zijlstra@chello.nl> > > --- > > > > I first thought about putting it in the vmalloc fault path only. > > But then I saw more occasions for the kernel to fault (kmemcheck > > or so), and so I thought it should be better put in the all in one > > path. But I suspect you won't like that conditional in the big > > x86 fault path. > > > > > > arch/x86/mm/fault.c | 2 ++ > > 1 files changed, 2 insertions(+), 0 deletions(-) > > > > diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c > > index 4c4508e..80c997e 100644 > > --- a/arch/x86/mm/fault.c > > +++ b/arch/x86/mm/fault.c > > @@ -955,6 +955,8 @@ do_page_fault(struct pt_regs *regs, unsigned long error_code) > > int write; > > int fault; > > > > + BUG_ON(in_nmi()); > > Alternative idea: we could put the test at the beginning of the NMI handler, so > if a NMI handler nests over a processor already "in_nmi", then we bug. I agree > that this will trigger less easily than bugging in the fault handler (because we > need to hit the actual nmi-coming-in-because-iret-reenabled-them-too-early > scenario, but it's far less intrusive. > > Thoughts ?
In fact we have that already in nmi_enter(). Now as you said that alone is probably too light to find the reason of a nested NMI or to prevent it.
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