linux-kernel - Re: [PATCH] slab: implement kmalloc guard

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Date:	Mon, 8 Sep 2014 10:46:03 -0400 (EDT)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	Christoph Lameter <cl@...ux.com>
cc:	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	"Alasdair G. Kergon" <agk@...hat.com>,
	Mike Snitzer <msnitzer@...hat.com>,
	Milan Broz <gmazyland@...il.com>, kkolasa@...soft.pl,
	dm-devel@...hat.com
Subject: Re: [PATCH] slab: implement kmalloc guard



On Mon, 8 Sep 2014, Christoph Lameter wrote:

> On Fri, 5 Sep 2014, Mikulas Patocka wrote:
> 
> > This patch adds a new option DEBUG_KMALLOC that makes it possible to
> > detect writes beyond the end of space allocated with kmalloc. Normally,
> > the kmalloc function rounds the size to the next power of two (there is
> > exception to this rule - sizes 96 and 192). Slab debugging detects only
> > writes beyond the end of the slab object, it is unable to detect writes
> > beyond the end of kmallocated size that fit into the slab object.
> >
> > The motivation for this patch was this: There was 6 year old bug in
> > dm-crypt (d49ec52ff6ddcda178fc2476a109cf1bd1fa19ed) where the driver would
> > write beyond the end of kmallocated space, but the bug went undetected
> > because the write would fit into the power-of-two-sized slab object. Only
> > recent changes to dm-crypt made the bug show up. There is similar problem
> > in the nx-crypto driver in the function nx_crypto_ctx_init - again,
> > because kmalloc rounds the size up to the next power of two, this bug went
> > undetected.
> 
> The problem with the kmalloc array for debugging is inded that it is
> only for powers of two for efficiency purposes. In the debug
> situation we may not have the need for that efficiency. Maybe simply
> creating kmalloc arrays for each size will do the trick?

I don't know what you mean. If someone allocates 10000 objects with sizes 
from 1 to 10000, you can't have 10000 slab caches - you can't have a slab 
cache for each used size. Also - you can't create a slab cache in 
interrupt context.

> > This patch works for slab, slub and slob subsystems. The end of slab block
> > can be found out with ksize (this patch renames it to __ksize). At the end
> > of the block, we put a structure kmalloc_guard. This structure contains a
> > magic number and a real size of the block - the exact size given to
> 
> We already have a redzone structure to check for writes over the end of
> the object. Lets use that.

So, change all three slab subsystems to use that.

Mikulas
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