Re: [PATCH v2 0/3] Randomize free memory

[Dan Williams]

On Thu, Oct 4, 2018 at 9:44 AM Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
>
> Hi Michal,
>
> On Thu, Oct 4, 2018 at 12:53 AM Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> >
> > On Wed 03-10-18 19:15:18, Dan Williams wrote:
> > > Changes since v1:
> > > * Add support for shuffling hot-added memory (Andrew)
> > > * Update cover letter and commit message to clarify the performance impact
> > >   and relevance to future platforms
> >
> > I believe this hasn't addressed my questions in
> > http://lkml.kernel.org/r/20181002143015.GX18290@xxxxxxxxxxxxxxx Namely
> > "
> > It is the more general idea that I am not really sure about. First of
> > all. Does it make _any_ sense to randomize 4MB blocks by default? Why
> > cannot we simply have it disabled?
>
> I'm not aware of any CVE that this would directly preclude, but that
> said the entropy injected at 4MB boundaries raises the bar on heap
> attacks. Environments that want more can adjust that with the boot
> parameter. Given the potential benefits I think it would only make
> sense to default disable it if there was a significant runtime impact,
> from what I have seen there isn't.
>
> > Then and more concerning question is,
> > does it even make sense to have this randomization applied to higher
> > orders than 0? Attacker might fragment the memory and keep recycling the
> > lowest order and get the predictable behavior that we have right now.
>
> Certainly I expect there are attacks that can operate within a 4MB
> window, as I expect there are attacks that could operate within a 4K
> window that would need sub-page randomization to deter. In fact I
> believe that is the motivation for CONFIG_SLAB_FREELIST_RANDOM.
> Combining that with page allocator randomization makes the kernel less
> predictable.
>
> Is that enough justification for this patch on its own? It's
> debatable. Combine that though with the wider availability of
> platforms with memory-side-cache and I think it's a reasonable default
> behavior for the kernel to deploy.

Hi Michal,

Does the above address your concerns? v4.20 is perhaps the last
upstream kernel release in advance of wider hardware availability.