Hello everyone, On Fri, May 08, 2020 at 12:54:03PM -0400, Michael S. Tsirkin wrote: > On Fri, May 08, 2020 at 12:52:34PM -0400, Michael S. Tsirkin wrote: > > On Wed, Apr 22, 2020 at 05:26:32PM -0700, Daniel Colascione wrote: > > > This sysctl can be set to either zero or one. When zero (the default) > > > the system lets all users call userfaultfd with or without > > > UFFD_USER_MODE_ONLY, modulo other access controls. When > > > unprivileged_userfaultfd_user_mode_only is set to one, users without > > > CAP_SYS_PTRACE must pass UFFD_USER_MODE_ONLY to userfaultfd or the API > > > will fail with EPERM. This facility allows administrators to reduce > > > the likelihood that an attacker with access to userfaultfd can delay > > > faulting kernel code to widen timing windows for other exploits. > > > > > > Signed-off-by: Daniel Colascione <dancol@xxxxxxxxxx> > > > > The approach taken looks like a hard-coded security policy. > > For example, it won't be possible to set the sysctl knob > > in question on any sytem running kvm. So this is > > no good for any general purpose system. > > > > What's wrong with using a security policy for this instead? > > In fact I see the original thread already mentions selinux, > so it's just a question of making this controllable by > selinux. I agree it'd be preferable if it was not hardcoded, but then this patchset is also much simpler than the previous controlling it through selinux.. I was thinking, an alternative policy that could control it without hard-coding it, is a seccomp-bpf filter, then you can drop 2/2 as well, not just 1/6-4/6. If you keep only 1/2, can't seccomp-bpf enforce userfaultfd to be always called with flags==0x1 without requiring extra modifications in the kernel? Can't you get the feature party with the CAP_SYS_PTRACE capability too, if you don't wrap those tasks with the ptrace capability under that seccomp filter? As far as I can tell, it's unprecedented to create a flag for a syscall API, with the only purpose of implementing a seccomp-bpf filter verifying such flag is set, but then if you want to control it with LSM it's even more complex than doing it with seccomp-bpf, and it requires more kernel code too. We could always add 2/2 later, such possibility won't disappear, in fact we could also add 1/6-4/6 later too if that is not enough. If we could begin by merging only 1/2 from this new series and be done with the kernel changes, because we offload the rest of the work to the kernel eBPF JIT, I think it'd be ideal. Thanks, Andrea
