linux-kernel - Re: [PATCH v3 1/9] mm: add overflow and underflow checks for page->

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Date:   Thu, 27 Jan 2022 19:27:23 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Pasha Tatashin <pasha.tatashin@...een.com>,
        Matthew Wilcox <willy@...radead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>,
        linux-m68k@...ts.linux-m68k.org,
        Anshuman Khandual <anshuman.khandual@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        william.kucharski@...cle.com,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        schmitzmic@...il.com, Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <guro@...com>,
        Muchun Song <songmuchun@...edance.com>,
        Wei Xu <weixugc@...gle.com>, Greg Thelen <gthelen@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Paul Turner <pjt@...gle.com>, Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH v3 1/9] mm: add overflow and underflow checks for
 page->_refcount

On 1/26/22 20:22, Pasha Tatashin wrote:
> On Wed, Jan 26, 2022 at 1:59 PM Matthew Wilcox <willy@...radead.org> wrote:
>>
>> On Wed, Jan 26, 2022 at 06:34:21PM +0000, Pasha Tatashin wrote:
>> > The problems with page->_refcount are hard to debug, because usually
>> > when they are detected, the damage has occurred a long time ago. Yet,
>> > the problems with invalid page refcount may be catastrophic and lead to
>> > memory corruptions.
>> >
>> > Reduce the scope of when the _refcount problems manifest themselves by
>> > adding checks for underflows and overflows into functions that modify
>> > _refcount.
>>
>> If you're chasing a bug like this, presumably you turn on page
>> tracepoints.  So could we reduce the cost of this by putting the
>> VM_BUG_ON_PAGE parts into __page_ref_mod() et al?  Yes, we'd need to
>> change the arguments to those functions to pass in old & new, but that
>> should be a cheap change compared to embedding the VM_BUG_ON_PAGE.
> 
> This is not only about chasing a bug. This also about preventing
> memory corruption and information leaking that are caused by ref_count
> bugs from happening.

So you mean it like a security hardening feature, not just debugging? To me
it's dubious to put security hardening under CONFIG_DEBUG_VM. I think it's
just Fedora that uses DEBUG_VM in general production kernels?

> Several months ago a memory corruption bug was discovered by accident:
> an engineer was studying a process core from a production system and
> noticed that some memory does not look like it belongs to the original
> process. We tried to manually reproduce that bug but failed. However,
> later analysis by our team, explained that the problem occured due to
> ref_count bug in Linux, and the bug itself was root caused and fixed
> (mentioned in the cover letter).  This work would have prevented
> similar ref_count bugs from yielding to the memory corruption
> situation.
> 
> Pasha
>