[PATCH][next] drm/kmb: fix array out-of-bounds writes to kmb->plane_status[]

[Chrisanthus, Anitha]

Hi Sam and Colin,

> -----Original Message-----
> From: Sam Ravnborg <sam at ravnborg.org>
> Sent: Friday, November 13, 2020 10:02 AM
> To: Colin Ian King <colin.king at canonical.com>
> Cc: Chrisanthus, Anitha <anitha.chrisanthus at intel.com>; Dea, Edmund J
> <edmund.j.dea at intel.com>; David Airlie <airlied at linux.ie>; Daniel Vetter
> <daniel at ffwll.ch>; dri-devel at lists.freedesktop.org; kernel-
> janitors at vger.kernel.org; linux-kernel at vger.kernel.org
> Subject: Re: [PATCH][next] drm/kmb: fix array out-of-bounds writes to kmb-
> >plane_status[]
> 
> Hi Colin.
> On Fri, Nov 13, 2020 at 03:04:34PM +0000, Colin Ian King wrote:
> > On 13/11/2020 14:55, Sam Ravnborg wrote:
> > > Hi Colin.
> > >
> > > On Fri, Nov 13, 2020 at 12:01:21PM +0000, Colin King wrote:
> > >> From: Colin Ian King <colin.king at canonical.com>
> > >>
> > >> Writes to elements in the kmb->plane_status array in function
> > >> kmb_plane_atomic_disable are overrunning the array when plane_id is
> > >> more than 1 because currently the array is KMB_MAX_PLANES elements
> > >> in size and this is currently #defined as 1.  Fix this by defining
> > >> KMB_MAX_PLANES to 4.
> > >
> > > I fail to follow you here.
> > > In kmb_plane_init() only one plane is allocated - with id set to 0.
> > > So for now only one plane is allocated thus kmb_plane_atomic_disable()
> > > is only called for this plane.
> > >
> > > With your change we will start allocating four planes, something that is
> > > not tested.
> > >
> > > Do I miss something?
> > >
> > > 	Sam
> > >
> >
> > The static analysis from coverity on linux-next suggested that there was
> > an array overflow as follows:
> >
> > 108 static void kmb_plane_atomic_disable(struct drm_plane *plane,
> > 109                                     struct drm_plane_state *state)
> > 110 {
> >
> >    1. Condition 0 /* !!(!__builtin_types_compatible_p() &&
> > !__builtin_types_compatible_p()) */, taking false branch.
> >
> > 111        struct kmb_plane *kmb_plane = to_kmb_plane(plane);
> >
> >    2. assignment: Assigning: plane_id = kmb_plane->id.
> >
> > 112        int plane_id = kmb_plane->id;
> > 113        struct kmb_drm_private *kmb;
> > 114
> > 115        kmb = to_kmb(plane->dev);
> > 116
> >
> >    3. Switch case value LAYER_3.
> >
> > 117        switch (plane_id) {
> > 118        case LAYER_0:
> > 119                kmb->plane_status[plane_id].ctrl = LCD_CTRL_VL1_ENABLE;
> > 120                break;
> 
> With the current code this is the only case that hits.
> So coverity is right that if we hit other cases that would result in a
> bug. But kmb_plane->id will for now not have other values than 0.
> 
> So it is a subtle false positive.
> There is some "dead" code here - but this is in preparation for more
> than one layer and we will keep the code for now, unless Anitha chimes
> in and says otherwise.

Thanks Sam, I was out on Friday. Agree with Sam, let's keep the current code for now. Kmb->plane_id will not have non-zero values now.
Only one plane is supported and tested currently, the extra code is in preparation for multiple planes.

Thanks,
Anitha
> 
> 	Sam
> 
> > 121        case LAYER_1:
> >
> >    (#2 of 4): Out-of-bounds write (OVERRUN)
> >
> > 122                kmb->plane_status[plane_id].ctrl = LCD_CTRL_VL2_ENABLE;
> > 123                break;
> > 124        case LAYER_2:
> >
> >    (#3 of 4): Out-of-bounds write (OVERRUN)
> >
> > 125                kmb->plane_status[plane_id].ctrl = LCD_CTRL_GL1_ENABLE;
> > 126                break;
> >
> >    4. equality_cond: Jumping to case LAYER_3.
> >
> > 127        case LAYER_3:
> >
> >    (#1 of 4): Out-of-bounds write (OVERRUN)
> >    5. overrun-local: Overrunning array kmb->plane_status of 1 8-byte
> > elements at element index 3 (byte offset 31) using index plane_id (which
> > evaluates to 3).
> >
> > 128                kmb->plane_status[plane_id].ctrl = LCD_CTRL_GL2_ENABLE;
> > 129                break;
> > 130        }
> > 131
> >
> >    (#4 of 4): Out-of-bounds write (OVERRUN)
> >
> > 132        kmb->plane_status[plane_id].disable = true;
> > 133 }
> > 134
> >
> > So it seems the assignments to  kmb->plane_status[plane_id] are
> > overrunning the array since plane_status is allocated as 1 element and
> > yet plane_id can be 0..3
> >
> > I could be misunderstanding this, or it may be a false positive.
> >
> > Colin