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

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Date: Mon, 6 May 2024 14:46:57 +0000
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Stephen Brennan <stephen.s.brennan@...cle.com>, Steven Rostedt
	<rostedt@...dmis.org>, Masami Hiramatsu <mhiramat@...nel.org>, Mark Rutland
	<mark.rutland@....com>
CC: Guo Ren <guoren@...nel.org>, Huacai Chen <chenhuacai@...nel.org>, WANG
 Xuerui <kernel@...0n.name>, "James E.J. Bottomley"
	<James.Bottomley@...senPartnership.com>, Helge Deller <deller@....de>,
	Michael Ellerman <mpe@...erman.id.au>, Nicholas Piggin <npiggin@...il.com>,
	Aneesh Kumar K.V <aneesh.kumar@...nel.org>, "Naveen N. Rao"
	<naveen.n.rao@...ux.ibm.com>, Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>, Heiko
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Subject: Re: [PATCH v3] kprobe/ftrace: bail out if ftrace was killed



Le 01/05/2024 à 18:29, Stephen Brennan a écrit :
> 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@...cle.com>
> ---
> Changes in v3:
>    Don't expose ftrace_is_dead(). Create a "kprobe_ftrace_disabled"
>    variable and check it directly in the kprobe handlers.

Isn't it safer to provide a fonction rather than a direct access to a 
variable ?

By the way, wouldn't it be more performant to use a static branch (jump 
label) ?

Christophe