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

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On Wed,  1 May 2024 09:29:56 -0700
Stephen Brennan <stephen.s.brennan@xxxxxxxxxx> 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]:
>   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@xxxxxxxxxx>
> ---
> Changes in v3:
>   Don't expose ftrace_is_dead(). Create a "kprobe_ftrace_disabled"
>   variable and check it directly in the kprobe handlers.

Reviewed-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>


-- Steve

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