linux-kernel - Re: [RFC 0/8] Hardening page

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Date:   Tue, 26 Oct 2021 19:23:39 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Pasha Tatashin <pasha.tatashin@...een.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-m68k@...ts.linux-m68k.org, anshuman.khandual@....com,
        akpm@...ux-foundation.org, william.kucharski@...cle.com,
        mike.kravetz@...cle.com, vbabka@...e.cz, geert@...ux-m68k.org,
        schmitzmic@...il.com, rostedt@...dmis.org, mingo@...hat.com,
        hannes@...xchg.org, guro@...com, songmuchun@...edance.com,
        weixugc@...gle.com, gthelen@...gle.com
Subject: Re: [RFC 0/8] Hardening page _refcount

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.