Re: [patch 2/2] x86 NMI-safe INT3 and Page Fault

Avi Kivity
2010-07-16T12:30:21Z

On 07/14/2010 06:49 PM, Mathieu Desnoyers wrote:
> Implements an alternative iret with popf and return so trap and exception
> handlers can return to the NMI handler without issuing iret. iret would cause
> NMIs to be reenabled prematurely. x86_32 uses popf and far return. x86_64 has to
> copy the return instruction pointer to the top of the previous stack, issue a
> popf, loads the previous esp and issue a near return (ret).
>
> It allows placing dynamically patched static jumps in asm gotos, which will be
> used for optimized tracepoints, in NMI code since returning from a breakpoint
> would be valid. Accessing vmalloc'd memory, which allows executing module code
> or accessing vmapped or vmalloc'd areas from NMI context, would also be valid.
> This is very useful to tracers like LTTng.
>
> This patch makes all faults, traps and exception safe to be called from NMI
> context*except* single-stepping, which requires iret to restore the TF (trap
> flag) and jump to the return address in a single instruction. Sorry, no kprobes
> support in NMI handlers because of this limitation. This cannot be emulated
> with popf/lret, because lret would be single-stepped. It does not apply to
> "immediate values" because they do not use single-stepping. This code detects if
> the TF flag is set and uses the iret path for single-stepping, even if it
> reactivates NMIs prematurely.
>

You need to save/restore cr2 in addition, otherwise the following hits you

- page fault
- processor writes cr2, enters fault handler
- nmi
- page fault
- cr2 overwritten

I guess you would usually not notice the corruption since you'd just see
a spurious fault on the page the NMI handler touched, but if the first
fault happened in a kvm guest, then we'd corrupt the guest's cr2.

But the whole thing strikes me as overkill. If it's 8k per-cpu, what's
wrong with using a per-cpu pointer to a kmalloc() area?

--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.