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

[Frederic Weisbecker <fweisbec@gmail.com>]

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