[PATCH 2/2] media: meson: vdec: guard against NULL prev_frame in codec_vp9_set_mpred_mv()

[Doruk Tan Ozturk <doruk@0sec.ai>]

codec_vp9_set_mpred_mv() dereferences vp9->prev_frame unconditionally,
both when computing use_prev_frame_mvs (prev_frame->width, ->height,
->intra_only, ->show, ->type) and when programming the previous-frame
MV read registers via codec_vp9_get_frame_mv_paddr(vp9, vp9->prev_frame)
(HEVC_MPRED_MV_RD_START_ADDR, HEVC_MPRED_MV_RPTR and the RD_END_ADDR
computation).

vp9->prev_frame is only assigned (= vp9->cur_frame) after a frame has
been processed, and is NULL after allocation and after a flush. The
caller, codec_vp9_process_frame(), reaches codec_vp9_set_mpred_mv()
whenever the frame is a non-key, non-intra-only frame, without checking
that a previous frame exists. A stream whose first decoded frame is an
inter frame (malformed/adversarial input, or the first frame after a
drain) therefore triggers a NULL pointer dereference.

Disable previous-frame MV use (clear HEVC_MPRED_CTRL4 BIT(6), which the
function already does up front) and return early when prev_frame is
NULL, before any dereference. There are no previous-frame motion
vectors to consume in that case, so this is the correct behaviour as
well as the safe one.

Found by 0sec's autonomous vulnerability analysis (https://0sec.ai).
Found by static analysis; not yet runtime-reproduced (Amlogic Meson
hardware required).

Fixes: 00c43088aa68 ("media: meson: vdec: add VP9 decoder support")
Signed-off-by: Doruk Tan Ozturk <doruk@0sec.ai>
---
 drivers/staging/media/meson/vdec/codec_vp9.c | 27 +++++++++++++++-----
 1 file changed, 20 insertions(+), 7 deletions(-)

diff --git a/drivers/staging/media/meson/vdec/codec_vp9.c b/drivers/staging/media/meson/vdec/codec_vp9.c
index 5ca27930239f..1df641202687 100644
--- a/drivers/staging/media/meson/vdec/codec_vp9.c
+++ b/drivers/staging/media/meson/vdec/codec_vp9.c
@@ -993,19 +993,32 @@ static void codec_vp9_set_mpred_mv(struct amvdec_core *core,
 				   struct codec_vp9 *vp9)
 {
 	int mpred_mv_rd_end_addr;
-	int use_prev_frame_mvs = vp9->prev_frame->width ==
-					vp9->cur_frame->width &&
-				 vp9->prev_frame->height ==
-					vp9->cur_frame->height &&
-				 !vp9->prev_frame->intra_only &&
-				 vp9->prev_frame->show &&
-				 vp9->prev_frame->type != KEY_FRAME;
+	int use_prev_frame_mvs;
 
 	amvdec_write_dos(core, HEVC_MPRED_CTRL3, 0x24122412);
 	amvdec_write_dos(core, HEVC_MPRED_ABV_START_ADDR,
 			 vp9->workspace_paddr + MPRED_ABV_OFFSET);
 
 	amvdec_clear_dos_bits(core, HEVC_MPRED_CTRL4, BIT(6));
+
+	/*
+	 * prev_frame is NULL when an inter frame is the first frame decoded
+	 * (e.g. a stream starting on a non-key frame, or the first frame
+	 * after a flush). There are no previous-frame motion vectors to use
+	 * and every read below would dereference a NULL pointer, so leave
+	 * prev-MV use disabled (BIT(6) already cleared) and bail out.
+	 */
+	if (!vp9->prev_frame)
+		return;
+
+	use_prev_frame_mvs = vp9->prev_frame->width ==
+				vp9->cur_frame->width &&
+			     vp9->prev_frame->height ==
+				vp9->cur_frame->height &&
+			     !vp9->prev_frame->intra_only &&
+			     vp9->prev_frame->show &&
+			     vp9->prev_frame->type != KEY_FRAME;
+
 	if (use_prev_frame_mvs)
 		amvdec_write_dos_bits(core, HEVC_MPRED_CTRL4, BIT(6));
 
-- 
2.53.0