On Thu 04-10-18 09:44:35, Dan Williams 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. I am sorry but this hasn't explained anything (at least to me). I can still see a way to bypass this randomization by fragmenting the memory. With that possibility in place this doesn't really provide the promissed additional security. So either I am missing something or the per-order threshold is simply a wrong interface to a broken security misfeature. > Is that enough justification for this patch on its own? I do not think so from what I have heard so far. > 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. OK, this sounds a bit more interesting. I am going to speculate because memory-side-cache is way too generic of a term for me to imagine anything specific. Many years back while at a university I was playing with page coloring as a method to reach a more stable performance results due to reduced cache conflicts. It was not always a performance gain but it definitely allowed for more stable run-to-run comparable results. I can imagine that a randomization might lead to a similar effect although I am not sure how much and it would be more interesting to hear about that effect. If this is really the case then I would assume on/off knob to control the randomization without something as specific as order. -- Michal Hocko SUSE Labs
