linux-kernel - Re: [PATCH] mm: Add additional consistency check

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Date:   Tue, 11 Apr 2017 20:55:56 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Christoph Lameter <cl@...ux.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: Add additional consistency check

On Tue 11-04-17 13:44:02, Cristopher Lameter wrote:
> On Tue, 11 Apr 2017, Michal Hocko wrote:
> 
> > > So we are already handling that condition. Why change things? Add a BUG_ON
> > > if you want to make SLAB consistent.
> >
> > I hate to repeat myself but let me do it for the last time in this
> > thread. BUG_ON for something that is recoverable is completely
> > inappropriate. And I consider kfree with a bogus pointer something that
> > we can easily recover from. There are other cases where the internal
> > state of the allocator is compromised to the point where continuing is
> > not possible and BUGing there is acceptable but kfree(garbage) is not
> > that case.
> 
> kfree(garbage) by the core kernel has so far been taken as a sign of
> severe memory corruption and the kernels have been oopsing when this
> occurred. This has been that way for a decade or so.

which doesn't make it a valid decision. We just overuse BUG*

> kfree() is used by
> the allocators and various other core kernel components. If the metadata
> of the core kernel is compromised then it is safest to stop right there.
> 
> If you want to change things then someone has to do some work. What you
> are saying is not the way things are implemented. Sorry.

I didn't say anything like that. Hence the proposed patch which still
needs some more thinking and evaluation.

-- 
Michal Hocko
SUSE Labs