Re: [PATCH v2 1/1] userfaultfd/sysctl: add vm.unprivileged_userfaultfd

[Peter Xu <peterx@redhat.com>]

On Tue, Mar 19, 2019 at 06:28:23PM +0000, Dr. David Alan Gilbert wrote:
> * Andrew Morton (akpm@linux-foundation.org) wrote:
> > On Tue, 19 Mar 2019 11:07:22 +0800 Peter Xu <peterx@redhat.com> wrote:
> > 
> > > Add a global sysctl knob "vm.unprivileged_userfaultfd" to control
> > > whether userfaultfd is allowed by unprivileged users.  When this is
> > > set to zero, only privileged users (root user, or users with the
> > > CAP_SYS_PTRACE capability) will be able to use the userfaultfd
> > > syscalls.
> > 
> > Please send along a full description of why you believe Linux needs
> > this feature, for me to add to the changelog.  What is the benefit to
> > our users?  How will it be used?
> > 
> > etcetera.  As it was presented I'm seeing no justification for adding
> > the patch!
> 
> How about:
> 
> ---
> Userfaultfd can be misued to make it easier to exploit existing use-after-free
> (and similar) bugs that might otherwise only make a short window
> or race condition available.  By using userfaultfd to stall a kernel
> thread, a malicious program can keep some state, that it wrote, stable
> for an extended period, which it can then access using an existing
> exploit.   While it doesn't cause the exploit itself, and while it's not
> the only thing that can stall a kernel thread when accessing a memory location,
> it's one of the few that never needs priviledge.
> 
> Add a flag, allowing userfaultfd to be restricted, so that in general 
> it won't be useable by arbitrary user programs, but in environments that
> require userfaultfd it can be turned back on.

Thanks for the quick write up, Dave!  I definitely should have some
justification in the cover letter and carry it until the last version.
Sorry to be unclear at the first glance.

-- 
Peter Xu