drm/vc4: hvs: Fix buffer overflow with the dlist handling

Commit 0a038c1c ("drm/vc4: Move LBM creation out of
vc4_plane_mode_set()") changed the LBM allocation logic from first
allocating the LBM memory for the plane to running mode_set,
adding a gap in the LBM, and then running the dlist allocation filling
that gap.

The gap was introduced by incrementing the dlist array index, but was
never checking whether or not we were over the array length, leading
eventually to memory corruptions if we ever crossed this limit.

vc4_dlist_write had that logic though, and was reallocating a larger
dlist array when reaching the end of the buffer. Let's share the logic
between both functions.

Cc: Boris Brezillon <boris.brezillon@collabora.com>
Cc: Eric Anholt <eric@anholt.net>
Fixes: 0a038c1c ("drm/vc4: Move LBM creation out of vc4_plane_mode_set()")
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210129160647.128373-1-maxime@cerno.tech

drivers/gpu/drm/vc4/vc4_plane.c

@@ -220,7 +220,7 @@ static void vc4_plane_reset(struct drm_plane *plane)
 	__drm_atomic_helper_plane_reset(plane, &vc4_state->base);
 }
 
-static void vc4_dlist_write(struct vc4_plane_state *vc4_state, u32 val)
+static void vc4_dlist_counter_increment(struct vc4_plane_state *vc4_state)
 {
 	if (vc4_state->dlist_count == vc4_state->dlist_size) {
 		u32 new_size = max(4u, vc4_state->dlist_count * 2);
@@ -235,7 +235,15 @@ static void vc4_dlist_write(struct vc4_plane_state *vc4_state, u32 val)
 		vc4_state->dlist_size = new_size;
 	}
 
-	vc4_state->dlist[vc4_state->dlist_count++] = val;
+	vc4_state->dlist_count++;
+}
+
+static void vc4_dlist_write(struct vc4_plane_state *vc4_state, u32 val)
+{
+	unsigned int idx = vc4_state->dlist_count;
+
+	vc4_dlist_counter_increment(vc4_state);
+	vc4_state->dlist[idx] = val;
 }
 
 /* Returns the scl0/scl1 field based on whether the dimensions need to
@@ -978,8 +986,10 @@ static int vc4_plane_mode_set(struct drm_plane *plane,
 		 * be set when calling vc4_plane_allocate_lbm().
 		 */
 		if (vc4_state->y_scaling[0] != VC4_SCALING_NONE ||
-		    vc4_state->y_scaling[1] != VC4_SCALING_NONE)
-			vc4_state->lbm_offset = vc4_state->dlist_count++;
+		    vc4_state->y_scaling[1] != VC4_SCALING_NONE) {
+			vc4_state->lbm_offset = vc4_state->dlist_count;
+			vc4_dlist_counter_increment(vc4_state);
+		}
 
 		if (num_planes > 1) {
 			/* Emit Cb/Cr as channel 0 and Y as channel