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

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Message-ID: <ea740a0a-6225-3d6c-095d-1c456e497e3a@suse.cz>
Date:   Thu, 18 Mar 2021 13:56:05 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Kees Cook <keescook@...omium.org>
Cc:     Georgi Djakov <georgi.djakov@...aro.org>, linux-mm@...ck.org,
        akpm@...ux-foundation.org, cl@...ux.com, penberg@...nel.org,
        rientjes@...gle.com, iamjoonsoo.kim@....com, corbet@....net,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/slub: Add slub_debug option to panic on memory
 corruption

On 3/18/21 6:48 AM, Kees Cook wrote:
> 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(). :)

I was going to suggest adding a panic_on_taint parameter... but turns out it was
already added last year! And various memory corruption detections already use
TAINT_BAD_PAGE, including SLUB.
If anything's missing an add_taint() it can be added, and with the parameter you
should get what you want.

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