Re: [PATCH v5 2/2] Add user-mode only option to unprivileged_userfaultfd sysctl knob

[Lokesh Gidra]

On Fri, Oct 23, 2020 at 7:48 PM Andrea Arcangeli <aarcange@xxxxxxxxxx> wrote:
>
> Hello everyone,
>
> On Sat, Oct 10, 2020 at 11:24:56PM -0700, Lokesh Gidra wrote:
> > With this change, when the knob is set to 0, it allows unprivileged
> > users to call userfaultfd, like when it is set to 1, but with the
> > restriction that page faults from only user-mode can be handled.
> > In this mode, an unprivileged user (without SYS_CAP_PTRACE capability)
> > must pass UFFD_USER_MODE_ONLY to userfaultd or the API will fail with
> > EPERM.
> >
> > This enables administrators to reduce the likelihood that
> > an attacker with access to userfaultfd can delay faulting kernel
> > code to widen timing windows for other exploits.
> >
> > The default value of this knob is changed to 0. This is required for
> > correct functioning of pipe mutex. However, this will fail postcopy
> > live migration, which will be unnoticeable to the VM guests. To avoid
> > this, set 'vm.userfault = 1' in /sys/sysctl.conf. For more details,
> > refer to Andrea's reply [1].
> >
> > [1] https://lore.kernel.org/lkml/20200904033438.GI9411@xxxxxxxxxx/
> >
> > Signed-off-by: Lokesh Gidra <lokeshgidra@xxxxxxxxxx>
>
> Nobody commented so it seems everyone is on board with this change to
> synchronize the kernel default with the post-boot Android default.
>
> The email in the link above was pretty long, so the below would be a
> summary that could be added to the commit header:
>
> ==
>
> The main reason this change is desirable as in the short term is that
> the Android userland will behave as with the sysctl set to zero. So
> without this commit, any Linux binary using userfaultfd to manage its
> memory would behave differently if run within the Android userland.
>
> ==

Sure. I'll add it in the next revision.
>
> Reviewed-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
>
Thanks so much for the review. I hope it's ok to add your
'reviewed-by' in the next revision?
>
> BTW, this is still a minor nitpick, but a printk_once of the 1/2 could
> be added before the return -EPERM too, that's actually what I meant
> when I suggested to add a printk_once :), however the printk_once you
> added can turn out to be useful too for devs converting code to use
> bounce buffers, so it's fine too, just it could go under DEBUG_VM and
> to be ratelimited (similarly to the "FAULT_FLAG_ALLOW_RETRY missing
> %x\n" printk).

I'll move the printk_once from 1/2 to this patch, as you suggested.
>
> Thanks,
> Andrea
>