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@xxxxxxxxxxxxxx. 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.
