Re: [PATCH] slub: Relocate freelist pointer to middle of object

[Kees Cook]

On Wed, Mar 11, 2020 at 02:48:05PM +0000, David Laight wrote:
> From: Christopher Lameter
> > Sent: 08 March 2020 19:21
> 
> > 
> > On Thu, 5 Mar 2020, Kees Cook wrote:
> > 
> > > Instead of having the freelist pointer at the very beginning of an
> > > allocation (offset 0) or at the very end of an allocation (effectively
> > > offset -sizeof(void *) from the next allocation), move it away from
> > > the edges of the allocation and into the middle. This provides some
> > > protection against small-sized neighboring overflows (or underflows),
> > > for which the freelist pointer is commonly the target. (Large or well
> > > controlled overwrites are much more likely to attack live object contents,
> > > instead of attempting freelist corruption.)
> > 
> > Sounds good. You could even randomize the position to avoid attacks on via
> > the freelist pointer.
> 
> Random overwrites could be detected (fairly cheaply) by putting two
> copies of the pointer into the same cacheline in the buffer.
> Or better make the second one 'pointer xor constant'.

My sense is that this starts to stray closer to "too much overhead" vs
the mitigation benefit against known heap metadata attacks. I'm open to
seeing patches, of course, though! :)

-- 
Kees Cook