Re: [PATCH v3] kprobe/ftrace: bail out if ftrace was killed

Stephen Brennan
2024-05-15T22:19:47Z

Masami Hiramatsu (Google) <mhiramat@kernel.org> writes:
> On Thu, 2 May 2024 01:35:16 +0800
> Guo Ren <guoren@kernel.org> wrote:
>
>> On Thu, May 2, 2024 at 12:30 AM Stephen Brennan
>> <stephen.s.brennan@oracle.com> wrote:
>> >
>> > If an error happens in ftrace, ftrace_kill() will prevent disarming
>> > kprobes. Eventually, the ftrace_ops associated with the kprobes will be
>> > freed, yet the kprobes will still be active, and when triggered, they
>> > will use the freed memory, likely resulting in a page fault and panic.
>> >
>> > This behavior can be reproduced quite easily, by creating a kprobe and
>> > then triggering a ftrace_kill(). For simplicity, we can simulate an
>> > ftrace error with a kernel module like [1]:
>> >
>> > [1]: https://github.com/brenns10/kernel_stuff/tree/master/ftrace_killer
>> >
>> > sudo perf probe --add commit_creds
>> > sudo perf trace -e probe:commit_creds
>> > # In another terminal
>> > make
>> > sudo insmod ftrace_killer.ko # calls ftrace_kill(), simulating bug
>> > # Back to perf terminal
>> > # ctrl-c
>> > sudo perf probe --del commit_creds
>> >
>> > After a short period, a page fault and panic would occur as the kprobe
>> > continues to execute and uses the freed ftrace_ops. While ftrace_kill()
>> > is supposed to be used only in extreme circumstances, it is invoked in
>> > FTRACE_WARN_ON() and so there are many places where an unexpected bug
>> > could be triggered, yet the system may continue operating, possibly
>> > without the administrator noticing. If ftrace_kill() does not panic the
>> > system, then we should do everything we can to continue operating,
>> > rather than leave a ticking time bomb.
>> >
>> > Signed-off-by: Stephen Brennan <stephen.s.brennan@oracle.com>
>> > ---
>> > Changes in v3:
>> > Don't expose ftrace_is_dead(). Create a "kprobe_ftrace_disabled"
>> > variable and check it directly in the kprobe handlers.
>> > Link to v1/v2 discussion:
>> > https://lore.kernel.org/all/20240426225834.993353-1-stephen.s.brennan@oracle.com/
>> >
>> > arch/csky/kernel/probes/ftrace.c | 3 +++
>> > arch/loongarch/kernel/ftrace_dyn.c | 3 +++
>> > arch/parisc/kernel/ftrace.c | 3 +++
>> > arch/powerpc/kernel/kprobes-ftrace.c | 3 +++
>> > arch/riscv/kernel/probes/ftrace.c | 3 +++
>> > arch/s390/kernel/ftrace.c | 3 +++
>> > arch/x86/kernel/kprobes/ftrace.c | 3 +++
>> > include/linux/kprobes.h | 7 +++++++
>> > kernel/kprobes.c | 6 ++++++
>> > kernel/trace/ftrace.c | 1 +
>> > 10 files changed, 35 insertions(+)
>> >
>> > diff --git a/arch/csky/kernel/probes/ftrace.c b/arch/csky/kernel/probes/ftrace.c
>> > index 834cffcfbce3..7ba4b98076de 100644
>> > --- a/arch/csky/kernel/probes/ftrace.c
>> > +++ b/arch/csky/kernel/probes/ftrace.c
>> > @@ -12,6 +12,9 @@ void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
>> > struct kprobe_ctlblk *kcb;
>> > struct pt_regs *regs;
>> >
>> > + if (unlikely(kprobe_ftrace_disabled))
>> > + return;
>> > +
>> For csky part.
>> Acked-by: Guo Ren <guoren@kernel.org>
>
> Thanks Stephen, Guo and Steve!
>
> Let me pick this to probes/for-next!

Thank you Masami!

I did want to check, is this the correct git tree to be watching?

https://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace.git/log/?h=probes/for-next

( I'm not trying to pressure on timing, as I know the merge window is
hectic. Just making sure I'm watching the correct place! )

Thanks,
Stephen