On Wed, Oct 30, 2019 at 1:40 AM Mike Rapoport <rppt@xxxxxxxxxx> wrote: > > On Tue, Oct 29, 2019 at 10:00:55AM -0700, Andy Lutomirski wrote: > > On Tue, Oct 29, 2019 at 2:33 AM Mike Rapoport <rppt@xxxxxxxxxx> wrote: > > > > > > On Mon, Oct 28, 2019 at 02:44:23PM -0600, Andy Lutomirski wrote: > > > > > > > > > On Oct 27, 2019, at 4:17 AM, Mike Rapoport <rppt@xxxxxxxxxx> wrote: > > > > > > > > > > From: Mike Rapoport <rppt@xxxxxxxxxxxxx> > > > > > > > > > > Hi, > > > > > > > > > > The patch below aims to allow applications to create mappins that have > > > > > pages visible only to the owning process. Such mappings could be used to > > > > > store secrets so that these secrets are not visible neither to other > > > > > processes nor to the kernel. > > > > > > > > > > I've only tested the basic functionality, the changes should be verified > > > > > against THP/migration/compaction. Yet, I'd appreciate early feedback. > > > > > > > > I’ve contemplated the concept a fair amount, and I think you should > > > > consider a change to the API. In particular, rather than having it be a > > > > MAP_ flag, make it a chardev. You can, at least at first, allow only > > > > MAP_SHARED, and admins can decide who gets to use it. It might also play > > > > better with the VM overall, and you won’t need a VM_ flag for it — you > > > > can just wire up .fault to do the right thing. > > > > > > I think mmap()/mprotect()/madvise() are the natural APIs for such > > > interface. > > > > Then you have a whole bunch of questions to answer. For example: > > > > What happens if you mprotect() or similar when the mapping is already > > in use in a way that's incompatible with MAP_EXCLUSIVE? > > Then we refuse to mprotect()? Like in any other case when vm_flags are not > compatible with required madvise()/mprotect() operation. > I'm not talking about flags. I'm talking about the case where one thread (or RDMA or whatever) has get_user_pages()'d a mapping and another thread mprotect()s it MAP_EXCLUSIVE. > > Is it actually reasonable to malloc() some memory and then make it exclusive? > > > > Are you permitted to map a file MAP_EXCLUSIVE? What does it mean? > > I'd limit MAP_EXCLUSIVE only to anonymous memory. > > > What does MAP_PRIVATE | MAP_EXCLUSIVE do? > > My preference is to have only mmap() and then the semantics is more clear: > > MAP_PRIVATE | MAP_EXCLUSIVE creates a pre-populated region, marks it locked > and drops the pages in this region from the direct map. > The pages are returned back on munmap(). > Then there is no way to change an existing area to be exclusive or vice > versa. And what happens if you fork()? Limiting it to MAP_SHARED | MAP_EXCLUSIVE would about this particular nasty question. > > > How does one pass exclusive memory via SCM_RIGHTS? (If it's a > > memfd-like or chardev interface, it's trivial. mmap(), not so much.) > > Why passing such memory via SCM_RIGHTS would be useful? Suppose I want to put a secret into exclusive memory and then send that secret to some other process. The obvious approach would be to SCM_RIGHTS an fd over, but you can't do that with MAP_EXCLUSIVE as you've defined it. In general, there are lots of use cases for memfd and other fd-backed memory. > > > And finally, there's my personal giant pet peeve: a major use of this > > will be for virtualization. I suspect that a lot of people would like > > the majority of KVM guest memory to be unmapped from the host > > pagetables. But people might also like for guest memory to be > > unmapped in *QEMU's* pagetables, and mmap() is a basically worthless > > interface for this. Getting fd-backed memory into a guest will take > > some possibly major work in the kernel, but getting vma-backed memory > > into a guest without mapping it in the host user address space seems > > much, much worse. > > Well, in my view, the MAP_EXCLUSIVE is intended to keep small secrets > rather than use it for the entire guest memory. I even considered adding a > limit for the mapping size, but then I decided that since RLIMIT_MEMLOCK is > anyway enforced there is no need for a new one. > > I agree that getting fd-backed memory into a guest would be less pain that > VMA, but KVM can already use memory outside the control of the kernel via > /dev/map [1]. That series doesn't address the problem I'm talking about at all. I'm saying that there is a legitimate use case where QEMU should *not* have a mapping of the memory. So QEMU would create some exclusive memory using /dev/exclusive_memory and would tell KVM to map it into the guest without mapping it into QEMU's address space at all. (In fact, the way that SEV currently works is *functionally* like this, except that there's a bogus incoherent mapping in the QEMU process that is a giant can of worms. IMO a major benefit of a chardev approach is that you don't need a new VM_ flag and you don't need to worry about wiring it up everywhere in the core mm code.
