Re: [PATCH v2 2/2] drm: allow render capable master with DRM_AUTH ioctls

[Emil Velikov <emil.l.velikov@gmail.com>]

On 2019/01/14, Daniel Vetter wrote:
> On Mon, Jan 14, 2019 at 08:54:08AM +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.
> > 
> > The former was a case for Mesa where it did not not check the return
> > value of drmGetMagic() [1]. That was fixed recently although, there's
> > the question of older drivers or other apps that exbibit this behaviour.
> > 
> > While omitting the call results in issues as seen in [2] and [3].
> > 
> > In the libva case, libva itself doesn't authenticate the DRM client and
> > the vaGetDisplayDRM documentation doesn't mention if the app should
> > either.
> > 
> > As of today, the official vainfo utility doesn't authenticate.
> > 
> > To workaround issues like these, some users resort to running their apps
> > under sudo. Which admittedly isn't always a good idea.
> > 
> > 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.
> > 
> > v2:
> > - Rework/simplify if check (Daniel V)
> > - Add examples to commit messages, elaborate. (Daniel V)
> > 
> > [1] https://gitlab.freedesktop.org/mesa/mesa/blob/2bc1f5c2e70fe3b4d41f060af9859bc2a94c5b62/src/egl/drivers/dri2/platform_wayland.c#L1136
> > [2] https://lists.freedesktop.org/archives/libva/2016-July/004185.html
> > [3] https://gitlab.freedesktop.org/mesa/kmscube/issues/1
> > Testcase: igt/core_unauth_vs_render
> > Cc: intel-gfx@lists.freedesktop.org
> > Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
> > ---
> > Daniel, the if conditionals did not work exactly as you put them.
> > This is the closest thing that I can think of.
> > ---
> >  drivers/gpu/drm/drm_ioctl.c | 20 ++++++++++++++++----
> >  1 file changed, 16 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c
> > index 94bd872d56c4..08a0b4cc3a76 100644
> > --- a/drivers/gpu/drm/drm_ioctl.c
> > +++ b/drivers/gpu/drm/drm_ioctl.c
> > @@ -507,6 +507,13 @@ int drm_version(struct drm_device *dev, void *data,
> >  	return err;
> >  }
> >  
> > +static inline bool
> > +drm_render_driver_and_ioctl(const struct drm_device *dev, u32 flags)
> > +{
> > +	return drm_core_check_feature(dev, DRIVER_RENDER) &&
> > +		(flags & DRM_RENDER_ALLOW);
> > +}
> > +
> >  /**
> >   * drm_ioctl_permit - Check ioctl permissions against caller
> >   *
> > @@ -521,14 +528,19 @@ 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 */
> > -	if (unlikely((flags & DRM_AUTH) && !drm_is_render_client(file_priv) &&
> > -		     !file_priv->authenticated))
> > -		return -EACCES;
> > +	/* AUTH is only for master ... */
> > +	if ((flags & DRM_AUTH) && drm_is_primary_client(file_priv)) {
> > +		/* authenticated ones, or render capable on DRM_RENDER_ALLOW. */
> > +		if (unlikely(!file_priv->authenticated) &&
> > +		    unlikely(!drm_render_driver_and_ioctl(dev, flags)))
> 
> The double-unlikely looks a bit strange, I'd move it out so there's only
> one. But this is correct too (because unlikely() && unlikely == unlikely(
> && )). Either way:
> 
> Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> 
Ack, thanks. To stay consistent with the surrounding code I'll use:

	if (unlikely((flags & DRM_AUTH) && drm_is_primary_client(file_priv))) {
		/* authenticated ones, or render capable on DRM_RENDER_ALLOW. */
		if (!file_priv->authenticated &&
		    !drm_render_driver_and_ioctl(dev, flags))



Btw, if you can skim through the IGT test that would be appreciated.

Cheers,
Emil
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