linux-kernel - Re: [PATCH 16/16] pstore/platform: Disable automatic updates by default

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Message-ID: <CAGXu5jK9xL57zaqYfFDiBY7t+8MCSsQSvv8E4J7u-Rhtb-xKag@mail.gmail.com>
Date:	Tue, 22 May 2012 08:42:06 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Anton Vorontsov <anton.vorontsov@...aro.org>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Colin Cross <ccross@...roid.com>,
	Tony Luck <tony.luck@...el.com>, Arnd Bergmann <arnd@...db.de>,
	John Stultz <john.stultz@...aro.org>,
	Shuah Khan <shuahkhan@...il.com>, arve@...roid.com,
	Rebecca Schultz Zavin <rebecca@...roid.com>,
	Jesper Juhl <jj@...osbits.net>,
	Randy Dunlap <rdunlap@...otime.net>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Thomas Meyer <thomas@...3r.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Marco Stornelli <marco.stornelli@...il.com>,
	WANG Cong <xiyou.wangcong@...il.com>,
	linux-kernel@...r.kernel.org, devel@...verdev.osuosl.org,
	linaro-kernel@...ts.linaro.org, patches@...aro.org,
	kernel-team@...roid.com
Subject: Re: [PATCH 16/16] pstore/platform: Disable automatic updates by default

On Tue, May 22, 2012 at 7:17 AM, Anton Vorontsov
<anton.vorontsov@...aro.org> wrote:
> Having automatic updates seems pointless for production system, and
> even dangerous and thus counter-productive:
>
> 1. If we can mount pstore, or read files, we can as well read
>   /proc/kmsg. So, there's little point in duplicating the
>   functionality and present the same information but via another
>   userland ABI;
>
> 2. Expecting the kernel to behave sanely after oops/panic is naive.
>   It might work, but you'd rather not try it. Screwed up kernel
>   can do rather bad things, like recursive faults[1]; and pstore
>   rather provoking bad things to happen. It uses:
>
>   1. Timers (assumes sane interrupts state);
>   2. Workqueues and mutexes (assumes scheduler in a sane state);
>   3. kzalloc (a working slab allocator);
>
>   That's too much for a dead kernel, so the debugging facility
>   itself might just make debugging harder, which is not what
>   we want.
>
> Maybe for non-oops message types it would make sense to re-enable
> automatic updates, but so far I don't see any use case for this.
> Even for tracing, it has its own run-time/normal ABI, so we're
> only interested in pstore upon next boot, to retrieve what has
> gone wrong with HW or SW.
>
> So, let's disable the updates by default.

I'll let Tony ack this, but I'm fine with it -- making this
configurable is sufficient for my needs. :)

> diff --git a/fs/pstore/platform.c b/fs/pstore/platform.c
> index 4f49bb4..1dbf49d 100644
> --- a/fs/pstore/platform.c
> +++ b/fs/pstore/platform.c
> @@ -41,10 +41,11 @@
>  * whether the system is actually still running well enough
>  * to let someone see the entry
>  */
> -static int pstore_update_ms = 60000;
> +static int pstore_update_ms = -1;
>  module_param_named(update_ms, pstore_update_ms, int, 0600);
>  MODULE_PARM_DESC(update_ms, "milliseconds before pstore updates its content "
> -                "(default is 60000; -1 means runtime updates are disabled)");
> +                "(default is -1, which means runtime updates are disabled; "
> +                "enabling this option is not safe)");

Perhaps "enabling this option may lead to further corruption on
Oopses" or something more specific?

-Kees

-- 
Kees Cook
Chrome OS Security
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