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From:   Pasha Tatashin <pasha.tatashin@soleen.com>
Date:   Tue, 26 Oct 2021 14:30:25 -0400
Message-ID: <CA+CK2bBbGEJa4s7HtgQkG_tZmN66UaVcDRovgLz4q+a0VGSqZg@mail.gmail.com>
Subject: Re: [RFC 0/8] Hardening page _refcount
To:     Matthew Wilcox <willy@infradead.org>
Cc:     LKML <linux-kernel@vger.kernel.org>, linux-mm <linux-mm@kvack.org>,
        linux-m68k@lists.linux-m68k.org,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        william.kucharski@oracle.com,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        schmitzmic@gmail.com, Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@redhat.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Roman Gushchin <guro@fb.com>, songmuchun@bytedance.com,
        weixugc@google.com, Greg Thelen <gthelen@google.com>
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On Tue, Oct 26, 2021 at 2:24 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Tue, Oct 26, 2021 at 05:38:14PM +0000, Pasha Tatashin wrote:
> > It is hard to root cause _refcount problems, because they usually
> > manifest after the damage has occurred.  Yet, they can lead to
> > catastrophic failures such memory corruptions.
> >
> > Improve debugability by adding more checks that ensure that
> > page->_refcount never turns negative (i.e. double free does not
> > happen, or free after freeze etc).
> >
> > - Check for overflow and underflow right from the functions that
> >   modify _refcount
> > - Remove set_page_count(), so we do not unconditionally overwrite
> >   _refcount with an unrestrained value
> > - Trace return values in all functions that modify _refcount
>
> I think this is overkill.  Won't we get exactly the same protection
> by simply testing that page->_refcount == 0 in set_page_count()?
> Anything which triggers that BUG_ON would already be buggy because
> it can race with speculative gets.

We can't because set_page_count(v) is used for
1. changing _refcount form a current value to unconstrained v
2.  initialize _refcount from undefined state to v.

In this work we forbid the first case, and reduce the second case to
initialize only to 1.

Pasha