Re: [PATCH] mm/slub: Add slub_debug option to panic on memory corruption

From: Kees Cook <keescook@chromium.org>
To: Vlastimil Babka <vbabka@suse.cz>
Cc: Georgi Djakov <georgi.djakov@linaro.org>,
	linux-mm@kvack.org, akpm@linux-foundation.org, cl@linux.com,
	penberg@kernel.org, rientjes@google.com, iamjoonsoo.kim@lge.com,
	corbet@lwn.net, linux-doc@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm/slub: Add slub_debug option to panic on memory corruption
Date: Wed, 17 Mar 2021 22:48:58 -0700	[thread overview]
Message-ID: <202103172244.D5ADB06A96@keescook> (raw)
In-Reply-To: <8fd43de6-71e4-cfe7-8208-32753cf1c363@suse.cz>

On Tue, Mar 09, 2021 at 07:18:32PM +0100, Vlastimil Babka wrote:
> On 3/9/21 7:14 PM, Georgi Djakov wrote:
> > Hi Vlastimil,
> > 
> > Thanks for the comment!
> > 
> > On 3/9/21 17:09, Vlastimil Babka wrote:
> >> On 3/9/21 2:47 PM, Georgi Djakov wrote:
> >>> Being able to stop the system immediately when a memory corruption
> >>> is detected is crucial to finding the source of it. This is very
> >>> useful when the memory can be inspected with kdump or other tools.
> >>
> >> Is this in some testing scenarios where you would also use e.g. panic_on_warn?
> >> We could hook to that. If not, we could introduce a new
> >> panic_on_memory_corruption that would apply also for debug_pagealloc and whatnot?
> > 
> > I would prefer that we not tie it with panic_on_warn - there might be lots of
> > new code in multiple subsystems, so hitting some WARNing while testing is not
> > something unexpected.
> > 
> > Introducing an additional panic_on_memory_corruption would work, but i noticed
> > that we already have slub_debug and thought to re-use that. But indeed, аdding
> > an option to panic in for example bad_page() sounds also useful, if that's what
> > you suggest.
> 
> Yes, that would be another example.
> Also CCing Kees for input, as besides the "kdump ASAP for debugging" case, I can
> imagine security hardening folks could be interested in the "somebody might have
> just failed to pwn the kernel, better panic than let them continue" angle. But
> I'm naive wrt security, so it might be a stupid idea :)

I've really wanted such things, but Linus has been pretty adamant about
not wanting to provide new "panic" paths (or even BUG usage[1]). It
seems that panic_on_warn remains the way to get this behavior,
with the understanding that WARN should only be produced on
expected-to-be-impossible situations[1].

Hitting a WARN while testing should result in either finding and fixing
a real bug, or removing the WARN in favor of pr_warn(). :)

-Kees

[1] https://www.kernel.org/doc/html/latest/process/deprecated.html#bug-and-bug-on

-- 
Kees Cook