[FFmpeg-devel] [PATCH] fix j2k cdef out of array (PR #20131)

[michaelni <code@ffmpeg.org>]

PR #20131 opened by michaelni
URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/20131
Patch URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/20131.patch


From c7c16002ea3f9d6f284b61c7e5d4dc3fc9d1410a Mon Sep 17 00:00:00 2001
From: Michael Niedermayer <michael@niedermayer.cc>
Date: Tue, 5 Aug 2025 23:18:47 +0200
Subject: [PATCH 1/2] avcodec/jpeg2000dec: move cdef default check into
 get_siz()

This way cdef is at its final value earlier

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
---
 libavcodec/jpeg2000dec.c | 22 +++++++++++-----------
 1 file changed, 11 insertions(+), 11 deletions(-)

diff --git a/libavcodec/jpeg2000dec.c b/libavcodec/jpeg2000dec.c
index 9a4afd37d0..aab48532ef 100644
--- a/libavcodec/jpeg2000dec.c
+++ b/libavcodec/jpeg2000dec.c
@@ -271,6 +271,17 @@ static int get_siz(Jpeg2000DecoderContext *s)
         return AVERROR_INVALIDDATA;
     }
 
+    for (i = 0; i < s->ncomponents; i++) {
+        if (s->cdef[i] < 0) {
+            for (i = 0; i < s->ncomponents; i++) {
+                s->cdef[i] = i + 1;
+            }
+            if ((s->ncomponents & 1) == 0)
+                s->cdef[s->ncomponents-1] = 0;
+        }
+    }
+    // after here we no longer have to consider negative cdef
+
     for (i = 0; i < s->ncomponents; i++) { // Ssiz_i XRsiz_i, YRsiz_i
         uint8_t x    = bytestream2_get_byteu(&s->g);
         s->cbps[i]   = (x & 0x7f) + 1;
@@ -2885,17 +2896,6 @@ static int jpeg2000_decode_frame(AVCodecContext *avctx, AVFrame *picture,
     if (ret = jpeg2000_read_bitstream_packets(s))
         goto end;
 
-    for (int x = 0; x < s->ncomponents; x++) {
-        if (s->cdef[x] < 0) {
-            for (x = 0; x < s->ncomponents; x++) {
-                s->cdef[x] = x + 1;
-            }
-            if ((s->ncomponents & 1) == 0)
-                s->cdef[s->ncomponents-1] = 0;
-            break;
-        }
-    }
-
     for (int x = 0; x < s->ncomponents && s->codsty[x].transform == FF_DWT53;)
         if (++x == s->ncomponents)
             picture->flags |= AV_FRAME_FLAG_LOSSLESS;
-- 
2.49.1


From b01cfe3d2b5e7e02e0ae0c079c31ee8a56201f30 Mon Sep 17 00:00:00 2001
From: Michael Niedermayer <michael@niedermayer.cc>
Date: Tue, 5 Aug 2025 23:42:23 +0200
Subject: [PATCH 2/2] avcodec/jpeg2000dec: implement cdef remapping during
 pixel format matching

Fixes: out of array access
Fixes: poc.jp2

Found-by: Andy Nguyen <theflow@google.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
---
 libavcodec/jpeg2000dec.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/libavcodec/jpeg2000dec.c b/libavcodec/jpeg2000dec.c
index aab48532ef..59f3133d1f 100644
--- a/libavcodec/jpeg2000dec.c
+++ b/libavcodec/jpeg2000dec.c
@@ -282,6 +282,14 @@ static int get_siz(Jpeg2000DecoderContext *s)
     }
     // after here we no longer have to consider negative cdef
 
+    int cdef_used = 0;
+    for (i = 0; i < s->ncomponents; i++)
+        cdef_used |= 1<<s->cdef[i];
+
+    // Check that the channels we have are what we expect for the number of components
+    if (cdef_used != ((int[]){0,2,3,14,15})[s->ncomponents])
+        return AVERROR_INVALIDDATA;
+
     for (i = 0; i < s->ncomponents; i++) { // Ssiz_i XRsiz_i, YRsiz_i
         uint8_t x    = bytestream2_get_byteu(&s->g);
         s->cbps[i]   = (x & 0x7f) + 1;
@@ -294,7 +302,9 @@ static int get_siz(Jpeg2000DecoderContext *s)
             av_log(s->avctx, AV_LOG_ERROR, "Invalid sample separation %d/%d\n", s->cdx[i], s->cdy[i]);
             return AVERROR_INVALIDDATA;
         }
-        log2_chroma_wh |= s->cdy[i] >> 1 << i * 4 | s->cdx[i] >> 1 << i * 4 + 2;
+        int i_remapped = s->cdef[i] ? s->cdef[i]-1 : (s->ncomponents-1);
+
+        log2_chroma_wh |= s->cdy[i] >> 1 << i_remapped * 4 | s->cdx[i] >> 1 << i_remapped * 4 + 2;
     }
 
     s->numXtiles = ff_jpeg2000_ceildiv(s->width  - s->tile_offset_x, s->tile_width);
-- 
2.49.1

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