linux-kernel - Re: [RFC 0/1] BPF tracing for arm64 using fprobe

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Date:   Mon, 21 Nov 2022 16:29:54 +0100
From:   KP Singh <kpsingh@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Chris Mason <clm@...a.com>, Mark Rutland <mark.rutland@....com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Florent Revest <revest@...omium.org>,
        bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Brendan Jackman <jackmanb@...gle.com>, markowsky@...gle.com,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Xu Kuohai <xukuohai@...wei.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Christoph Hellwig <hch@...radead.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC 0/1] BPF tracing for arm64 using fprobe

On Mon, Nov 21, 2022 at 4:15 PM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Mon, 21 Nov 2022 14:47:10 +0100
> KP Singh <kpsingh@...nel.org> wrote:
>
> > This annotation already exists, i.e. ALLOW_ERROR_INJECTION
> >
> > Users, with CONFIG_FUNCTION_ERROR_INJECTION, can already modify return
> > values of kernel functions using kprobes and the failure injection
> > framework [1] for functions annotated with ALLOW_ERROR_INJECTION.
> >
> > BPF just provides another way to do the same thing with "modify
> > return" programs and this also respects the error injection list [2]
> > and users can *only* attach these programs to the functions annotated
> > with ALLOW_ERROR_INJECTION.
>
> WAIT!
>
> Looking at the Kconfigs, I see
>
> CONFIG_FUNCTION_ERROR_INJECTION is set when
> CONFIG_HAVE_FUNCTION_ERROR_INJECTION is set, and when CONFIG_KPROBES is set.
>
> And ALLOW_ERROR_INJECTION() is set when CONFIG_FUNCTION_ERROR_INJECTION is.
>
> There's no way to turn it off on x86 except by disabling kprobes!
>
> WTF!
>
> I don't want a kernel that can add error injection just because kprobes is
> enabled. There's two kinds of kprobes. One that is for visibility only (for
> tracing) and one that can be used for functional changes. I want the
> visibility without the ability to change the kernel. The visibility portion
> is very useful for security, where as the modifying one can be used to
> circumvent security.

I am not sure how they can circumvent security since this needs root /
root equivalent permissions. Fault injection is actually a very useful
debugging tool.

>
> As kprobes are set in most production environments, so is error injection.
> Do we really want error injection enabled on production environments?
> I don't.
>
> I think we need this patch ASAP!
>
> -- Steve
>
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index c3c0b077ade3..9ee72d8860c3 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -1874,8 +1874,14 @@ config NETDEV_NOTIFIER_ERROR_INJECT
>           If unsure, say N.
>
>  config FUNCTION_ERROR_INJECTION
> -       def_bool y
> +       bool "Fault-injections of functions"
>         depends on HAVE_FUNCTION_ERROR_INJECTION && KPROBES
> +       help
> +         Add fault injections into various functions that are annotated with
> +         ALLOW_ERROR_INJECTION() in the kernel. BPF may also modify the return
> +         value of theses functions. This is useful to test error paths of code.
> +
> +         If unsure, say N
>
>  config FAULT_INJECTION
>         bool "Fault-injection framework"