Re: [PATCH 3/3] drm: allow render capable master with DRM_AUTH ioctls

[Daniel Vetter <daniel@ffwll.ch>]

On Thu, Dec 20, 2018 at 03:16:15PM +0000, Emil Velikov wrote:
> On Wed, 19 Dec 2018 at 20:34, Daniel Vetter <daniel@ffwll.ch> wrote:
> >
> > On Wed, Dec 19, 2018 at 07:22:47PM +0000, Emil Velikov wrote:
> > > From: Emil Velikov <emil.velikov@collabora.com>
> > >
> > > There are cases (in mesa and applications) where one would open the
> > > primary node without properly authenticating the client.
> > >
> > > Sometimes we don't check if the authentication succeeds, but there's
> > > also cases we simply forget to do it. Mesa has been fixed recently
> > > although, there's the question of older drivers or other apps that
> > > exbibit this behaviour.
> >
> > Would be good to have links to mesa where these bugs are fixed (or
> > wherever those bugs where).
> >
> The error checking for drmGetMagic() was missing :-\
> https://gitlab.freedesktop.org/mesa/mesa/blob/2bc1f5c2e70fe3b4d41f060af9859bc2a94c5b62/src/egl/drivers/dri2/platform_wayland.c#L1136
> 
> > >
> > > To workaround this, some users resort to running their apps under sudo.
> > > Which admittedly isn't always a good idea.
> > >
> One sudo example:
> https://lists.freedesktop.org/archives/libva/2016-July/004185.html
> 
> Note: libva itself doesn't authenticate the DRM client and the
> vaGetDisplayDRM documentation doesn't mention if the app should
> either.
> Fairly neither one does as it - as in the example above.
> 
> Typical output when auth handling is missing somewhere in the stack:
> https://gitlab.freedesktop.org/mesa/kmscube/issues/1

Yup, the above is exactly what I was looking for. I think pasting the
entire thing into the commit message is all we need.
> 
> > > Since any DRIVER_RENDER driver has sufficient isolation between clients,
> > > we can use that, for unauthenticated [primary node] ioctls that require
> > > DRM_AUTH. But only if the respective ioctl is tagged as DRM_RENDER_ALLOW.
> > >
> > > As an added bonus this allows us to use vgem in userspace with zero
> > > change to some (but not all) existing programs.
> >
> > How/what/where?
> >
> Not quite sure what you mean here. Are you interested in what
> userspace will work unmodified with vgem, or something else?
> 
> Userspace (such as some IGT vgem tests) that uses dumb buffers,
> fences, etc yet lacks drm authentication will work.
> Admittedly IGT is the first client (or ran as root), so it's
> implicitly authenticated - others may not.

igt isn't really a use-case, there's explicit checks that nothing else is
running and that you're root. The tests don't work otherwise. Some might,
but you can't make that assumption at all.

> Admittedly, I did not search the webs for explicit vgem examples,
> although an educated assumption is that people will take the IGT code
> as norm.

I think clearer to remove this "added bonus" from the commit message.

> > > Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
> > > ---
> > >  drivers/gpu/drm/drm_ioctl.c | 8 ++++++--
> > >  1 file changed, 6 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c
> > > index 2221c8857fb0..4c775b775395 100644
> > > --- a/drivers/gpu/drm/drm_ioctl.c
> > > +++ b/drivers/gpu/drm/drm_ioctl.c
> > > @@ -521,13 +521,17 @@ int drm_version(struct drm_device *dev, void *data,
> > >   */
> > >  int drm_ioctl_permit(u32 flags, struct drm_file *file_priv)
> > >  {
> > > +     const struct drm_device *dev = file_priv->minor->dev;
> > > +
> > >       /* ROOT_ONLY is only for CAP_SYS_ADMIN */
> > >       if (unlikely((flags & DRM_ROOT_ONLY) && !capable(CAP_SYS_ADMIN)))
> > >               return -EACCES;
> > >
> > > -     /* AUTH is only for authenticated or render client */
> > > +     /* AUTH is only for authenticated/render capable master or render client */
> > >       if (unlikely((flags & DRM_AUTH) && !drm_is_render_client(file_priv) &&
> > > -                  !file_priv->authenticated))
> > > +                  !file_priv->authenticated &&
> > > +                  !(drm_core_check_feature(dev, DRIVER_RENDER) &&
> > > +                    (flags & DRM_RENDER_ALLOW))))
> >
> > Gets a bit unreadable but looks correct.
> >
> Agreed. If you prefer I could add an helper, say:
> 
> +static inline bool
> +drm_is_render_driver_and_ioctl(...)
> +{
> +  return drm_core_check_feature(dev, DRIVER_RENDER) && (flags &
> DRM_RENDER_ALLOW));
> +}
> 
> [snip]
> 
> -     /* AUTH is only for authenticated or render client */
> +     /* AUTH is only for authenticated/render capable master or
> render client */
>       if (unlikely((flags & DRM_AUTH) && !drm_is_render_client(file_priv) &&
> -                  !file_priv->authenticated))
> +                  !file_priv->authenticated &&
> !drm_is_render_driver_and_ioctl(...)))

Not sure that's really better, was thinking more of splitting this up into
a set of separate if checks, e.g.

if ((flags & DRM_AUTH) && drm_is_primary_client(file_priv)) {
	if (unlikely(!file_priv->authenticated))
		return -EACCESS;
	if (unlikely(!drm_render_driver_and_ioctl(...))
		return -EACCESS;
}

> > With the commit message improved (since this is new uapi, so needs those
> > pesky userspace links):
> >
> > Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> >
> Smashing thank you. Please let me know if the above information and
> links are sufficient.

Ack.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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