linux-kernel - Re: [PATCH 00/10] Hardening page

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Date:   Wed, 8 Dec 2021 21:05:46 +0000
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, rientjes@...gle.com,
        pjt@...gle.com
Subject: Re: [PATCH 00/10] Hardening page _refcount

On Wed, Dec 08, 2021 at 08:35:34PM +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. There were a number
> of refcount related issues discovered recently [1], [2], [3].
> 
> 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

You're doing a lot more atomic instructions with these patches.  Have you
done any performance measurements with these patches applied and debug
disabled?  I'm really not convinced it's worth closing
one-instruction-wide races of this kind when they are "shouldn't ever
happen" situations.  If the debugging will catch the problem in 99.99%
of cases and miss 0.01% without using atomic instructions, that seems
like a better set of tradeoffs than catching 100% of problems by using
the atomic instructions.