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

[Ingo Molnar <mingo@elte.hu>]

* 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