On Mon, Jun 13, 2022 at 3:29 PM Peter Xu <peterx@xxxxxxxxxx> wrote: > > On Mon, Jun 13, 2022 at 02:55:40PM -0700, Andrew Morton wrote: > > On Wed, 1 Jun 2022 14:09:47 -0700 Axel Rasmussen <axelrasmussen@xxxxxxxxxx> wrote: > > > > > To achieve this, add a /dev/userfaultfd misc device. This device > > > provides an alternative to the userfaultfd(2) syscall for the creation > > > of new userfaultfds. The idea is, any userfaultfds created this way will > > > be able to handle kernel faults, without the caller having any special > > > capabilities. Access to this mechanism is instead restricted using e.g. > > > standard filesystem permissions. > > > > The use of a /dev node isn't pretty. Why can't this be done by > > tweaking sys_userfaultfd() or by adding a sys_userfaultfd2()? I think for any approach involving syscalls, we need to be able to control access to who can call a syscall. Maybe there's another way I'm not aware of, but I think today the only mechanism to do this is capabilities. I proposed adding a CAP_USERFAULTFD for this purpose, but that approach was rejected [1]. So, I'm not sure of another way besides using a device node. One thing that could potentially make this cleaner is, as one LWN commenter pointed out, we could have open() on /dev/userfaultfd just return a new userfaultfd directly, instead of this multi-step process of open /dev/userfaultfd, NEW ioctl, then you get a userfaultfd. When I wrote this originally it wasn't clear to me how to get that to happen - open() doesn't directly return the result of our custom open function pointer, as far as I can tell - but it could be investigated. [1]: https://lore.kernel.org/lkml/686276b9-4530-2045-6bd8-170e5943abe4@xxxxxxxxxxxxxxxx/T/ > > > > Peter, will you be completing review of this patchset? > > Sorry to not have reviewed it proactively.. > > I think it's because I never had a good picture/understanding of what > should be the best security model for uffd, meanwhile I am (it seems) just > seeing more and more ways to "provide a safer uffd" by different people > using different ways.. and I never had time (and probably capability too..) > to figure out the correct approach if not to accept all options provided. Agreed, what we have right now is a bit of a mess of different approaches. I think the reason for this is, there is no "perfect" way to control access to features like this, so what we now have is several different approaches with different tradeoffs. >From my perspective, the existing controls were simpler to implement, but are not ideal because they require us to grant access to UFFD *plus more stuff too*. The approach I've proposed is the most granular, so it doesn't require adding any extra permissions. But, I agree the interface is sort of overcomplicated. :/ But, from my perspective, security in shared Cloud computing environments where UFFD is used for live migration is critical, so I prefer this tradeoff - I'll put up with a slightly messier interface, if the gain is a very minimal set of privileges. > > I think I'll just assume the whole thing is acked already from you > generally, then I'll read at least the implementation before the end of > tomorrow. > > Thanks, > > -- > Peter Xu >
