On Jun 14, 2022, at 5:55 PM, Axel Rasmussen <axelrasmussen@xxxxxxxxxx> wrote: > ⚠ External Email > > On Mon, Jun 13, 2022 at 5:10 PM Nadav Amit <namit@xxxxxxxxxx> wrote: >> On Jun 13, 2022, at 3:38 PM, Axel Rasmussen <axelrasmussen@xxxxxxxxxx> wrote: >> >>> 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. >> >> If this direction is pursued, I think that it would be better to set it as >> /proc/[pid]/userfaultfd, which would allow remote monitors (processes) to >> hook into userfaultfd of remote processes. I have a patch for that which >> extends userfaultfd syscall, but /proc/[pid]/userfaultfd may be cleaner. > > Hmm, one thing I'm unsure about - > > If a process is able to control another process' memory like this, > then this seems like exactly what CAP_SYS_PTRACE is intended to deal > with, right? So I'm not sure this case is directly related to the one > I'm trying to address. > > This also seems distinct to me versus the existing way you'd do this, > which is open a userfaultfd and register a shared memory region, and > then fork(). Now you can control your child's memory with userfaultfd. > But, attaching to some other, previously-unrelated process with > /proc/[pid]/userfaultfd seems like a clear case for CAP_SYS_PTRACE. I agree about CAP_SYS_PTRACE. I just know that if the /dev approach is taken, there would be even more pushback for userfaultfd2. Whatever.
