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? If so, I'll send a v2 with documentation updates. > > Thanks, > > -- > Peter Xu >
