[FFmpeg-devel] [PATCH] avcodec/mjpegdec: ignore APPx stubs unless AV_EF_EXPLODE is set (PR #20422)

[Ramiro Polla via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>]

PR #20422 opened by Ramiro Polla (ramiro)
URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/20422
Patch URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/20422.patch


>From 073afa4be351bf99897033e5299cb680109727a1 Mon Sep 17 00:00:00 2001
From: Ramiro Polla <ramiro.polla@gmail.com>
Date: Thu, 17 Oct 2024 13:00:11 +0200
Subject: [PATCH 1/2] avcodec/mjpegdec: fix skipping of bytes for unknown APPx
 markers

The loop to skip the remaining bytes was off by one for all markers
except for Adob.

This patch uses post-decrement instead of pre-decrement in the while
loop to make the len value easier to understand, and updates the len
value to reflect this change for the Adob marker.
---
 libavcodec/mjpegdec.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/libavcodec/mjpegdec.c b/libavcodec/mjpegdec.c
index 4d6379805c..c44d7f8181 100644
--- a/libavcodec/mjpegdec.c
+++ b/libavcodec/mjpegdec.c
@@ -1934,7 +1934,7 @@ static int mjpeg_decode_app(MJpegDecodeContext *s)
     }
 
     if (   id == AV_RB32("Adob")
-        && len >= 7
+        && len >= 8
         && show_bits(&s->gb, 8) == 'e'
         && show_bits_long(&s->gb, 32) != AV_RB32("e_CM")) {
         skip_bits(&s->gb,  8); /* 'e' */
@@ -1944,7 +1944,7 @@ static int mjpeg_decode_app(MJpegDecodeContext *s)
         s->adobe_transform = get_bits(&s->gb,  8);
         if (s->avctx->debug & FF_DEBUG_PICT_INFO)
             av_log(s->avctx, AV_LOG_INFO, "mjpeg: Adobe header found, transform=%d\n", s->adobe_transform);
-        len -= 7;
+        len -= 8;
         goto out;
     }
 
@@ -2153,7 +2153,7 @@ out:
     if (len < 0)
         av_log(s->avctx, AV_LOG_ERROR,
                "mjpeg: error, decode_app parser read over the end\n");
-    while (--len > 0)
+    while (len-- > 0)
         skip_bits(&s->gb, 8);
 
     return 0;
-- 
2.49.1


>From 1bee4a6ec5ad5cdfd15ccf09fe7d45d59b00a060 Mon Sep 17 00:00:00 2001
From: Ramiro Polla <ramiro.polla@gmail.com>
Date: Tue, 1 Oct 2024 20:50:05 +0200
Subject: [PATCH 2/2] avcodec/mjpegdec: ignore APPx stubs unless AV_EF_EXPLODE
 is set

Consider APPx fields that are too short to contain an id field (32-bit)
as stubs, and ignore them if AV_EF_EXPLODE is not set.

This has been seen in the MJPEG output from some webcams (such as the
Logitech C270 and C920) and the JPEG images embedded in DNG images
from the Pentax K-1 camera.
---
 libavcodec/mjpegdec.c | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

diff --git a/libavcodec/mjpegdec.c b/libavcodec/mjpegdec.c
index c44d7f8181..3dde759fea 100644
--- a/libavcodec/mjpegdec.c
+++ b/libavcodec/mjpegdec.c
@@ -1859,20 +1859,22 @@ static int mjpeg_decode_app(MJpegDecodeContext *s)
     int len, id, i;
 
     len = get_bits(&s->gb, 16);
-    if (len < 6) {
-        if (s->bayer) {
-            // Pentax K-1 (digital camera) JPEG images embedded in DNG images contain unknown APP0 markers
-            av_log(s->avctx, AV_LOG_WARNING, "skipping APPx (len=%"PRId32") for bayer-encoded image\n", len);
-            skip_bits(&s->gb, len);
-            return 0;
-        } else
+    if (len < 2)
+        return AVERROR_INVALIDDATA;
+    len -= 2;
+
+    if (len < 4) {
+        if (s->avctx->err_recognition & AV_EF_EXPLODE)
             return AVERROR_INVALIDDATA;
+        av_log(s->avctx, AV_LOG_VERBOSE, "skipping APPx stub (len=%" PRId32 ")\n", len);
+        goto out;
     }
+
     if (8 * len > get_bits_left(&s->gb))
         return AVERROR_INVALIDDATA;
 
     id   = get_bits_long(&s->gb, 32);
-    len -= 6;
+    len -= 4;
 
     if (s->avctx->debug & FF_DEBUG_STARTCODE)
         av_log(s->avctx, AV_LOG_DEBUG, "APPx (%s / %8X) len=%d\n",
-- 
2.49.1

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