Re: [PATCH -tip 0/2] kprobes/x86: Fix bugs for NMI handling

[Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>]

Hi Ingo,

Should I resend this series to your kernel.org address?

Thank you,

(2014/02/20 12:39), Masami Hiramatsu wrote:
> The following series fixes bugs hidden in the ancient code.
> 
> The bugs suddenly appeared when I enabled over 6,000 kprobes
> and ran perf-top with --call-graph. The bugs are hidden in
> the old code and it have woken up by real stress testing.
> 
> Actually, current kprobes doesn't expect an NMI handler
> hits in single-stepping state (including preparation and
> do_debug() handling). Moreover, the NMI handler causing
> a page fault by trying to access user pages, is out of
> imagination! :) But perf does it.
> 
> Thus the previous code optimistically check the current
> running kprobe state, and if it is in the singlestep state,
> it changes the IP address to probed address and return,
> because it expects the page fault happened on the single
> stepped code.
> However, in fact, the perf's NMI can interrupt the
> do_debug or somewhere around that and it may cause a
> page fault. In this case, putting the IP address to
> probed address is simply wrong. It causes unexpected
> kernel crash.
> To handle this correctly, this patch fixes it to ensure
> the page-fault address is actually same to the single-
> stepping address, and only if so, set the IP address
> to the probed address.
> 
> I also found another small mistake which gives up the
> recovery from reentered kprobes in single-stepping state,
> but it also assumes that there is no NMI handler interrupts
> in that state. It should gives up only when the nested
> reentering happens.
> 
> Thanks to Ingo and Frank for encouraging me to start
> stress testing with massive multiple kprobes. :)
> 
> Thank you,
> 
> ---
> 
> Masami Hiramatsu (2):
>       [BUGFIX]kprobes/x86: Fix page-fault handling logic
>       kprobes/x86: Allow to handle reentered kprobe on singlestepping
> 
> 
>  arch/x86/kernel/kprobes/core.c |   16 ++++++----------
>  1 file changed, 6 insertions(+), 10 deletions(-)
> 
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-- 
Masami HIRAMATSU
IT Management Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@hitachi.com