Re: [PATCH v3 2/6] userfaultfd: add /dev/userfaultfd for fine grained access control

[Axel Rasmussen <axelrasmussen@google.com>]

On Mon, Jun 13, 2022 at 4:23 PM Jonathan Corbet <corbet@lwn.net> wrote:
>
> Axel Rasmussen <axelrasmussen@google.com> writes:
>
> > 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.
>
> I take it there's a reason why this can't be done with a security module
> - either a custom module or a policy in one of the existing modules?
> That sort of access control is just what security modules are supposed
> to be for, after all.
>
> Thanks,
>
> jon

Admittedly I haven't tried proposing a patch, but I suspect there
would be pushback against adding an entirely new LSM just for this
case, similarly to the reasons the CAP_USERFAULTFD approach was
rejected.

For existing LSMs, I think SELinux can be used to restrict access to
syscalls. But then again, it's fairly heavy weight / difficult to
configure, and I suspect migrating production servers which don't use
it today would be a nontrivial undertaking. At least to me it seems
unfortunate to say, there isn't an obvious "safe" way to use
userfaultfd, without enabling + configuring selinux. (That assumes by
"safe" we mean, without granting wider-than necessary access to
userfaultfd, or without granting uffd-using processes more permissions
[root or CAP_SYS_PTRACE] to do their job.) I suspect if we do that
then in practice many? most? users will just either run UFFD programs
as root, or toggle the sysctl to allow unprivileged UFFD usage.