lkml.org 
[lkml]   [2010]   [Sep]   [27]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [RFC PATCH] x86: Barf when faults happen in NMI
    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.



    \
     
     \ /
      Last update: 2010-09-28 01:55    [W:2.740 / U:1.164 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site