Re: [PATCH] x86: Remove WARN_ON(in_nmi()) from vmalloc_fault

[Frederic Weisbecker]

On Tue, Oct 15, 2013 at 04:39:06PM -0400, Steven Rostedt wrote:
> Since the NMI iretq nesting has been fixed, there's no reason that
> an NMI handler can not take a page fault for vmalloc'd code. No locks
> are taken in that code path, and the software now handles nested NMIs
> when the fault re-enables NMIs on iretq.
> 
> Not only that, if the vmalloc_fault() WARN_ON_ONCE() is hit, and that
> warn on triggers a vmalloc fault for some reason, then we can go into
> an infinite loop (the WARN_ON_ONCE() does the WARN() before updating
> the variable to make it happen "once").
> 
> Reported-by: "Liu, Chuansheng" <chuansheng.liu@xxxxxxxxx>
> Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>

Thanks! For now we probably indeed want this patch. But I hope it's only
for the short term.

I still think that allowing faults in NMIs is very nasty, as we expect NMIs to never
be disturbed. I'm not even sure if that interacts correctly with the rcu_nmi_enter()
and preempt_count & NMI_MASK things. Not sure how perf is ready for that either (now
hardware events can be interrupted by fault trace events).

So I hope we can think about something else for the long term.

> ---
>  arch/x86/mm/fault.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index 3aaeffc..78926c6 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -268,8 +268,6 @@ static noinline __kprobes int vmalloc_fault(unsigned long address)
>  	if (!(address >= VMALLOC_START && address < VMALLOC_END))
>  		return -1;
>  
> -	WARN_ON_ONCE(in_nmi());
> -
>  	/*
>  	 * Synchronize this task's top level page-table
>  	 * with the 'reference' page table.
> -- 
> 1.8.1.4
> 
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