Re: [PATCH v2] Add kernel config option for fuzz testing.

From: Jiri Slaby <jslaby@suse.com>
To: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
	Andrew Morton <akpm@linux-foundation.org>
Cc: Matthew Garrett <mjg59@google.com>,
	Andi Kleen <ak@linux.intel.com>,
	"Theodore Y . Ts'o" <tytso@mit.edu>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Petr Mladek <pmladek@suse.com>,
	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	Arnd Bergmann <arnd@arndb.de>,
	Steven Rostedt <rostedt@goodmis.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Dmitry Vyukov <dvyukov@google.com>
Subject: Re: [PATCH v2] Add kernel config option for fuzz testing.
Date: Sat, 7 Mar 2020 17:28:22 +0100	[thread overview]
Message-ID: <6f2e27de-c820-7de3-447d-cd9f7c650add@suse.com> (raw)
In-Reply-To: <20200307135822.3894-1-penguin-kernel@I-love.SAKURA.ne.jp>

On 07. 03. 20, 14:58, Tetsuo Handa wrote:
> While syzkaller is finding many bugs, sometimes syzkaller examines
> stupid operations. Currently we prevent syzkaller from examining
> stupid operations by blacklisting syscall arguments and/or disabling
> whole functionality using existing kernel config options, but it is
> a whack-a-mole approach. We need cooperation from kernel side [1].
> 
> This patch introduces a kernel config option which allows disabling
> only specific operations. This kernel config option should be enabled
> only when building kernels for fuzz testing.
> 
> We discussed possibility of disabling specific operations at run-time
> using some lockdown mechanism [2], but conclusion seems that build-time
> control (i.e. kernel config option) fits better for this purpose.
> Since patches for users of this kernel config option will want a lot of
> explanation [3], this patch provides only kernel config option for them.
> 
> [1] https://github.com/google/syzkaller/issues/1622
> [2] https://lkml.kernel.org/r/CACdnJutc7OQeoor6WLTh8as10da_CN=crs79v3Fp0mJTaO=+yw@mail.gmail.com
> [3] https://lkml.kernel.org/r/20191216163155.GB2258618@kroah.com
> 
> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> Cc: Dmitry Vyukov <dvyukov@google.com>
> ---
>  lib/Kconfig.debug | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> Changes since v1:
>   Drop users of this kernel config option.
>   Update patch description.
> 
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 53e786e0a604..e360090e24c5 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -2208,4 +2208,14 @@ config HYPERV_TESTING
>  
>  endmenu # "Kernel Testing and Coverage"
>  
> +config KERNEL_BUILT_FOR_FUZZ_TESTING
> +       bool "Build kernel for fuzz testing"

If we really want to go this way, I wouldn't limit it for fuzz testing
only. Static analyzers, symbolic executors, formal verifiers, etc. --
all of them should avoid the paths.

So what about KERNEL_BUILT_FOR_ANALYZERS?

> +       default n
> +       help
> +	 Say N unless you are building kernels for fuzz testing.
> +	 Saying Y here disables several things that legitimately cause
> +	 damage under a fuzzer workload (e.g. copying to arbitrary
> +	 user-specified kernel address, changing console loglevel,
> +	 freezing filesystems).
> +
>  endmenu # Kernel hacking
> 

-- 
js
suse labs