[PATCH] media: meson: vdec: fix use-after-free of prev_frame in codec_vp9_rm_noshow_frame()

[Doruk Tan Ozturk]

codec_vp9_rm_noshow_frame() frees the first non-shown reference frame on
ref_frames_list without excluding vp9->prev_frame. When the previously
decoded frame was a non-show (alt-ref) frame and the current frame is a
non-show inter frame, the freed object is the one vp9->prev_frame still
points to; codec_vp9_set_mpred_mv() then dereferences the stale pointer
(use_prev_frame_mvs and codec_vp9_get_frame_mv_paddr()), a use-after-free.

The sibling cleanup codec_vp9_show_frame() already guards this pointer
(tmp == vp9->prev_frame); rm_noshow_frame() simply omits the same check.
Add it.

The fields that drive this path (show_frame, frame_type, intra_only) are
parsed from the VP9 bitstream, so a crafted stream fed to the stateless
decoder can trigger the free-then-use.

Found by static analysis; not yet runtime-reproduced (Amlogic Meson
hardware required).

Found by 0sec's autonomous vulnerability analysis (https://0sec.ai).

Signed-off-by: Doruk Tan Ozturk <doruk at 0sec.ai>
---
 drivers/staging/media/meson/vdec/codec_vp9.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/staging/media/meson/vdec/codec_vp9.c b/drivers/staging/media/meson/vdec/codec_vp9.c
index 8e80ecf84..572f418c9 100644
--- a/drivers/staging/media/meson/vdec/codec_vp9.c
+++ b/drivers/staging/media/meson/vdec/codec_vp9.c
@@ -1247,6 +1247,15 @@ static void codec_vp9_rm_noshow_frame(struct amvdec_session *sess)
 		if (tmp->show)
 			continue;
 
+		/*
+		 * prev_frame is still referenced by the MV predictor in
+		 * codec_vp9_set_mpred_mv(); the sibling codec_vp9_show_frame()
+		 * already excludes it before freeing. Do the same here to avoid
+		 * a use-after-free of vp9->prev_frame.
+		 */
+		if (tmp == vp9->prev_frame)
+			continue;
+
 		pr_debug("rm noshow: %u\n", tmp->index);
 		v4l2_m2m_buf_queue(sess->m2m_ctx, tmp->vbuf);
 		list_del(&tmp->list);
-- 
2.43.0