Re: [RFC PATCH] x86: Barf when faults happen in NMI

[Mathieu Desnoyers]

* Frederic Weisbecker (fweisbec@xxxxxxxxx) 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@xxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: H. Peter Anvin <hpa@xxxxxxxxx>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> ---
> 
> 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 ?

Mathieu

> +
>  	tsk = current;
>  	mm = tsk->mm;
>  
> -- 
> 1.6.2.3
> 

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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