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SubjectRe: [PATCH v2 1/1] userfaultfd/sysctl: add vm.unprivileged_userfaultfd
On Tue, Mar 19, 2019 at 06:28:23PM +0000, Dr. David Alan Gilbert wrote:
> * Andrew Morton ( wrote:
> > On Tue, 19 Mar 2019 11:07:22 +0800 Peter Xu <> 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

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