Re: [PATCH v6 2/4] mm/slub: only zero the requested size of buffer for kzalloc

[Feng Tang <feng.tang@intel.com>]

On Tue, Sep 27, 2022 at 04:15:02AM +0800, Kees Cook wrote:
> On Mon, Sep 26, 2022 at 09:11:24PM +0200, Andrey Konovalov wrote:
> > On Tue, Sep 13, 2022 at 8:54 AM Feng Tang <feng.tang@intel.com> wrote:
> > >
> > 
> > Hi Feng,
> > 
> > > kzalloc/kmalloc will round up the request size to a fixed size
> > > (mostly power of 2), so the allocated memory could be more than
> > > requested. Currently kzalloc family APIs will zero all the
> > > allocated memory.
> > >
> > > To detect out-of-bound usage of the extra allocated memory, only
> > > zero the requested part, so that sanity check could be added to
> > > the extra space later.
> > 
> > I still don't like the idea of only zeroing the requested memory and
> > not the whole object. Considering potential info-leak vulnerabilities.
> 
> I really really do not like reducing the zeroing size. We're trying to
> be proactive against _flaws_, which means that when there's a memory
> over-read (or uninitialized use), suddenly the scope of the exposure (or
> control) is wider/looser.
> 
> Imagine the (unfortunately very common) case of use-after-free attacks,
> which leverage type confusion: some object is located in kmalloc-128
> because it's 126 bytes. That slot gets freed and reallocated to, say, a
> 97 byte object going through kzalloc() or zero-on-init. With this patch
> the bytes above the 97 don't get zeroed, and the stale data from the
> prior 126 byte object say there happily to be used again later through
> a dangling pointer, or whatever. Without the proposed patch, the entire
> 128 bytes is wiped, which makes stale data re-use more difficult.

Thanks for the details explaination, which is a valid concern.

And Andrey's suggestion is a good solution: only reduce the zeroing
size for kmalloc-redzone enabled objects, as the extra space will be
redzoned, and no info will be leaked.

Thanks,
Feng