On Fri, May 19, 2023 at 10:32:13AM -0700, Axel Rasmussen wrote: > On Fri, May 19, 2023 at 9:20 AM Peter Xu <peterx@xxxxxxxxxx> wrote: > > > > Hi, Jiaqi, > > > > On Fri, May 19, 2023 at 08:04:09AM -0700, Jiaqi Yan wrote: > > > I don't think CAP_ADMIN is something we can work around: a VMM must be > > > a good citizen to avoid introducing any vulnerability to the host or > > > guest. > > > > > > On the other hand, "Userfaults allow the implementation of on-demand > > > paging from userland and more generally they allow userland to take > > > control of various memory page faults, something otherwise only the > > > kernel code could do." [3]. I am not familiar with the UFFD internals, > > > but our use case seems to match what UFFD wants to provide: without > > > affecting the whole world, give a specific userspace (without > > > CAP_ADMIN) the ability to handle page faults (indirectly emulate a > > > HWPOISON page (in my mind I treat it as SetHWPOISON(page) + > > > TestHWPOISON(page) operation in kernel's PF code)). So is it fair to > > > say what Axel provided here is "provide !ADMIN somehow"? > > > > > > [3]https://docs.kernel.org/admin-guide/mm/userfaultfd.html > > > > Userfault keywords on "user", IMHO. We don't strictly need userfault to > > resolve anything regarding CAP_ADMIN problems. MADV_DONTNEED also dosn't > > need CAP_ADMIN, same to any new madvise() if we want to make it useful for > > injecting poisoned ptes with !ADMIN and limit it within current->mm. > > > > But I think you're right that userfaultfd always tried to avoid having > > ADMIN and keep everything within its own scope of permissions. > > > > So again, totally no objection on make it uffd specific for now if you guys > > are all happy with it, but just to be clear that it's (to me) mostly for > > avoiding another WAKE, and afaics that's not really for solving the ADMIN > > issue here. > > How about this plan: > > Since the concrete use case we have (postcopy live migration) is > UFFD-specific, let's leave it as a UFFDIO_* operation for now. > > If in the future we come up with a non-UFFD use case, we can add a new > MADV_* which does this operation at that point. From my perspective > they could even share most of the same implementation code. > > I don't think it's a big problem keeping the UFFDIO_* version too at > that point, because it still provides some (perhaps small) value: > > - Combines the operation + waking into one syscall > - It allows us to support additional UFFD flags which modify / extend > the operation in UFFD-specific ways, if we want to add those in the > future > > Seem reasonable? Ok here. > > If so, I'll send a v2 with documentation updates. I've reviewed v1 in this case, please have a look, thanks. -- Peter Xu
