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

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Date:   Thu, 27 Jan 2022 14:38:52 -0500
From:   Pasha Tatashin <pasha.tatashin@...een.com>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Matthew Wilcox <willy@...radead.org>,
        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

> > 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?

In our (Google) internal kernel, I added another macro:
PAGE_REF_BUG(cond, page) to replace VM_BUG_ON_PAGE() in page_ref.h.
The new macro keeps the asserts always enabled.  I was thinking of
adding something like this to the upstream kernel as well, however, I
am worried about performance implications of having extra conditions
in these routines, so I think we would need yet another config which
decouples DEBUG_VM and some security crucial VM asserts. However, to
reduce controversial discussions, I decided not to do this as part of
this series, and perhaps do it as a follow-up work.

Pasha