[FFmpeg-devel] [PATCH] lavc/vvc: Prevent OOB write to slice_top_left_ctu_x in PPS CBS (PR #21317)

[FFmpeg-devel] [PATCH] lavc/vvc: Prevent OOB write to slice_top_left_ctu_x in PPS CBS (PR #21317)

[frankplow via ffmpeg-devel]

PR #21317 opened by frankplow
URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/21317
Patch URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/21317.patch

Prior to the fix, in the case of a tile containing multiple slices
(pps_num_exp_slices_in_tile != 0) the number of slices was temporarily
allowed to exceed pps_num_slices_in_pic_minus1+1 and therefore
VVC_MAX_SLICES.  The number of slices was later verified, but while the
current slice index was higher than expected it was used to write to a
array of size VVC_MAX_SLICES, leading to an OOB write.

To rectify this, the patch adds some checks at an earlier stage, to
ensure that the slice index i + j at no point exceeds
pps_num_slices_in_pic_minus1.

Fixes #YWH-PGM40646-30


>From 65574e151d92613cda0f8861f71de891c0e060b9 Mon Sep 17 00:00:00 2001
From: Frank Plowman <post@frankplowman.com>
Date: Mon, 29 Dec 2025 22:14:53 +0000
Subject: [PATCH] lavc/vvc: Prevent OOB write to slice_top_left_ctu_x in PPS
 CBS

Prior to the fix, in the case of a tile containing multiple slices
(pps_num_exp_slices_in_tile != 0) the number of slices was temporarily
allowed to exceed pps_num_slices_in_pic_minus1+1 and therefore
VVC_MAX_SLICES.  The number of slices was later verified, but while the
current slice index was higher than expected it was used to write to a
array of size VVC_MAX_SLICES, leading to an OOB write.

To rectify this, the patch adds some checks at an earlier stage, to
ensure that the slice index i + j at no point exceeds
pps_num_slices_in_pic_minus1.

Fixes #YWH-PGM40646-30
---
 libavcodec/cbs_h266_syntax_template.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/libavcodec/cbs_h266_syntax_template.c b/libavcodec/cbs_h266_syntax_template.c
index d9a65a9c19..4f6ae76e27 100644
--- a/libavcodec/cbs_h266_syntax_template.c
+++ b/libavcodec/cbs_h266_syntax_template.c
@@ -2011,6 +2011,12 @@ static int FUNC(pps) (CodedBitstreamContext *ctx, RWContext *rw,
                         slice_top_left_ctu_y[i] = ctu_y;
                     } else {
                         uint16_t slice_height_in_ctus;
+                        int num_uniform_slices;
+
+                        if (i + current->pps_num_exp_slices_in_tile[i] >
+                            current->pps_num_slices_in_pic_minus1 + 1)
+                            return AVERROR_INVALIDDATA;
+
                         for (j = 0; j < current->pps_num_exp_slices_in_tile[i];
                              j++) {
                             ues(pps_exp_slice_height_in_ctus_minus1[i][j], 0,
@@ -2031,6 +2037,13 @@ static int FUNC(pps) (CodedBitstreamContext *ctx, RWContext *rw,
                         uniform_slice_height = 1 +
                             (j == 0 ? current->row_height_val[tile_y] - 1:
                             current->pps_exp_slice_height_in_ctus_minus1[i][j-1]);
+
+                        num_uniform_slices = (remaining_height_in_ctbs_y + uniform_slice_height - 1)
+                                           / uniform_slice_height;
+                        if (i + current->pps_num_exp_slices_in_tile[i] + num_uniform_slices >
+                            current->pps_num_slices_in_pic_minus1 + 1)
+                            return AVERROR_INVALIDDATA;
+
                         while (remaining_height_in_ctbs_y > uniform_slice_height) {
                             current->slice_height_in_ctus[i + j] =
                                                           uniform_slice_height;
-- 
2.49.1

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