From: Stephen Brennan <stephen.s.brennan@oracle.com>
To: Christophe Leroy <christophe.leroy@csgroup.eu>,
Steven Rostedt <rostedt@goodmis.org>,
Masami Hiramatsu <mhiramat@kernel.org>,
Mark Rutland <mark.rutland@arm.com>
Cc: Guo Ren <guoren@kernel.org>, Huacai Chen <chenhuacai@kernel.org>,
WANG Xuerui <kernel@xen0n.name>,
"James E.J. Bottomley" <James.Bottomley@HansenPartnership.com>,
Helge Deller <deller@gmx.de>,
Michael Ellerman <mpe@ellerman.id.au>,
Nicholas Piggin <npiggin@gmail.com>,
"Aneesh Kumar K.V" <aneesh.kumar@kernel.org>,
"Naveen N. Rao" <naveen.n.rao@linux.ibm.com>,
Paul Walmsley <paul.walmsley@sifive.com>,
Palmer Dabbelt <palmer@dabbelt.com>,
Albert Ou <aou@eecs.berkeley.edu>,
Heiko Carstens <hca@linux.ibm.com>,
Vasily Gorbik <gor@linux.ibm.com>,
Alexander Gordeev <agordeev@linux.ibm.com>,
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Subject: Re: [PATCH v3] kprobe/ftrace: bail out if ftrace was killed
Date: Tue, 07 May 2024 11:36:22 -0700 [thread overview]
Message-ID: <87ttj9h3kp.fsf@oracle.com> (raw)
In-Reply-To: <fd8283ac-232e-49e8-a5be-60e54d87b9eb@csgroup.eu>
Christophe Leroy <christophe.leroy@csgroup.eu> writes:
> 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@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.
>
> Isn't it safer to provide a fonction rather than a direct access to a
> variable ?
Is the concern that other code could modify this variable? If so, then I
suppose the function call is safer. But the variable is not exported and
I think built-in code can be trusted not to muck with it. Maybe I'm
missing your point about safety though?
> By the way, wouldn't it be more performant to use a static branch (jump
> label) ?
I agree with Steven's concern that text modification would unfortunately
not be a good way to handle an error in text modification. Especially, I
believe there could be deadlock risks, as static key enablement requires
taking the text_mutex and the jump_label_mutex. I'd be concerned that
the text_mutex could already be held in some situations where
ftrace_kill() is called. But I'm not certain about that.
Thanks for taking a look!
Stephen
prev parent reply other threads:[~2024-05-07 18:37 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-05-01 16:29 [PATCH v3] kprobe/ftrace: bail out if ftrace was killed Stephen Brennan
2024-05-01 17:24 ` Steven Rostedt
2024-05-01 17:35 ` Guo Ren
2024-05-02 2:03 ` Masami Hiramatsu
2024-05-15 22:18 ` Stephen Brennan
2024-05-16 0:23 ` Masami Hiramatsu
2024-05-06 14:46 ` Christophe Leroy
2024-05-06 22:50 ` Steven Rostedt
2024-05-07 18:36 ` Stephen Brennan [this message]
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