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SubjectRe: [RFC PATCH] x86: Barf when faults happen in NMI
On Tue, Sep 28, 2010 at 12:36:18PM +0200, Ingo Molnar 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());
> > +
>
> Hm, this is a fastpath, so it would be nice to put this into the vmalloc
> and kmemcheck paths (even though it's less clean that way).
>
> Also, a WARN_ON_ONCE() would probably help more in getting bug reports
> out of such systems, than a BUG_ON() crashed kernel.
>
> Thanks,
>
> Ingo


Ok, will resend then.

Thanks.



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