[PATCH v2 3/3] media: rkvdec: hevc: guard INTER_REF_PIC_SET_PRED index underflow

[Michael Bommarito <michael.bommarito@gmail.com>]

st_ref_pic_set_prediction() computes the reference RPS index as
st_rps_idx - (delta_idx_minus1 + 1) per HEVC spec equation 7-59.
Both operands are u8, so when delta_idx_minus1 + 1 exceeds the
current index the subtraction wraps and the subsequent array access
at calculated_rps_st_sets[ref_rps_idx] reads far out of bounds.

A userspace V4L2 client that can open the RKVDEC m2m decoder can
submit an EXT_SPS_ST_RPS control with INTER_REF_PIC_SET_PRED set
and delta_idx_minus1 crafted to trigger the underflow.

Reject the entry early when the reference index would underflow.

Fixes: c9a59dc2acc7 ("media: rkvdec: Add HEVC support for the VDPU381 variant")
Cc: stable@vger.kernel.org
Suggested-by: Detlev Casanova <detlev.casanova@collabora.com>
Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
---
 drivers/media/platform/rockchip/rkvdec/rkvdec-hevc-common.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/media/platform/rockchip/rkvdec/rkvdec-hevc-common.c b/drivers/media/platform/rockchip/rkvdec/rkvdec-hevc-common.c
index 3119f3bc9f98b..898d1ce74f38a 100644
--- a/drivers/media/platform/rockchip/rkvdec/rkvdec-hevc-common.c
+++ b/drivers/media/platform/rockchip/rkvdec/rkvdec-hevc-common.c
@@ -268,6 +268,9 @@ static void st_ref_pic_set_prediction(struct rkvdec_hevc_run *run, int idx,
 	int i, j;
 	int dPoc;
 
+	if ((unsigned int)rps_data->delta_idx_minus1 + 1 > idx)
+		return;
+
 	ref_rps_idx = st_rps_idx - (rps_data->delta_idx_minus1 + 1); /* 7-59 */
 	delta_rps = (1 - 2 * rps_data->delta_rps_sign) *
 		   (rps_data->abs_delta_rps_minus1 + 1); /* 7-60 */
-- 
2.53.0

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