[FFmpeg-devel] [PATCH v2] avcodec/jpegxl_parser: fix OOB read regression

[Leo Izen]

In f7ac3512f5b5cb8eb149f37300b43461d8e93af3 the size of the dynamically
allocated buffer was shrunk, but it was made too small for very small
alphabet sizes. This patch restores the size to prevent an OOB read.

Reported-by: Cole Dilorenzo <coolkingcole at gmail.com>
Signed-off-by: Leo Izen <leo.izen at gmail.com>
---
 libavcodec/jpegxl_parser.c | 30 +++++++++++++++++-------------
 1 file changed, 17 insertions(+), 13 deletions(-)

diff --git a/libavcodec/jpegxl_parser.c b/libavcodec/jpegxl_parser.c
index dde36b0d6e..630fc8a60b 100644
--- a/libavcodec/jpegxl_parser.c
+++ b/libavcodec/jpegxl_parser.c
@@ -683,7 +683,7 @@ static int read_vlc_prefix(GetBitContext *gb, JXLEntropyDecoder *dec, JXLSymbolD
     int repeat_count_prev = 0, repeat_count_zero = 0, prev = 8;
     int total_code = 0, len, hskip, num_codes = 0, ret;
 
-    VLC level1_vlc;
+    VLC level1_vlc = { 0 };
 
     if (dist->alphabet_size == 1) {
         dist->vlc.bits = 0;
@@ -709,8 +709,10 @@ static int read_vlc_prefix(GetBitContext *gb, JXLEntropyDecoder *dec, JXLSymbolD
         }
     }
 
-    if (total_code != 32 && num_codes >= 2 || num_codes < 1)
-        return AVERROR_INVALIDDATA;
+    if (total_code != 32 && num_codes >= 2 || num_codes < 1) {
+        ret = AVERROR_INVALIDDATA;
+        goto end;
+    }
 
     for (int i = 1; i < 19; i++)
          level1_codecounts[i] += level1_codecounts[i - 1];
@@ -726,7 +728,7 @@ static int read_vlc_prefix(GetBitContext *gb, JXLEntropyDecoder *dec, JXLSymbolD
     if (ret < 0)
         goto end;
 
-    buf = av_mallocz(dist->alphabet_size * (2 * sizeof(int8_t) + sizeof(int16_t) + sizeof(uint32_t))
+    buf = av_mallocz(MAX_PREFIX_ALPHABET_SIZE * (2 * sizeof(int8_t) + sizeof(int16_t) + sizeof(uint32_t))
                      + sizeof(uint32_t));
     if (!buf) {
         ret = AVERROR(ENOMEM);
@@ -734,21 +736,22 @@ static int read_vlc_prefix(GetBitContext *gb, JXLEntropyDecoder *dec, JXLSymbolD
     }
 
     level2_lens = (int8_t *)buf;
-    level2_lens_s = (int8_t *)(buf + dist->alphabet_size * sizeof(int8_t));
-    level2_syms = (int16_t *)(buf + dist->alphabet_size * (2 * sizeof(int8_t)));
-    level2_codecounts = (uint32_t *)(buf + dist->alphabet_size * (2 * sizeof(int8_t) + sizeof(int16_t)));
+    level2_lens_s = (int8_t *)(buf + MAX_PREFIX_ALPHABET_SIZE * sizeof(int8_t));
+    level2_syms = (int16_t *)(buf + MAX_PREFIX_ALPHABET_SIZE * (2 * sizeof(int8_t)));
+    level2_codecounts = (uint32_t *)(buf + MAX_PREFIX_ALPHABET_SIZE * (2 * sizeof(int8_t) + sizeof(int16_t)));
 
     total_code = 0;
     for (int i = 0; i < dist->alphabet_size; i++) {
         len = get_vlc2(gb, level1_vlc.table, 5, 1);
+        if (get_bits_left(gb) < 0) {
+            ret = AVERROR_BUFFER_TOO_SMALL;
+            goto end;
+        }
         if (len == 16) {
             int extra = 3 + get_bits(gb, 2);
             if (repeat_count_prev)
-                extra = 4 * (repeat_count_prev - 2) - repeat_count_prev + extra;
-            if (i + extra > dist->alphabet_size) {
-                ret = AVERROR_INVALIDDATA;
-                goto end;
-            }
+                extra += 4 * (repeat_count_prev - 2) - repeat_count_prev;
+            extra = FFMIN(extra, dist->alphabet_size - i);
             for (int j = 0; j < extra; j++)
                 level2_lens[i + j] = prev;
             total_code += (32768 >> prev) * extra;
@@ -759,7 +762,8 @@ static int read_vlc_prefix(GetBitContext *gb, JXLEntropyDecoder *dec, JXLSymbolD
         } else if (len == 17) {
             int extra = 3 + get_bits(gb, 3);
             if (repeat_count_zero > 0)
-                extra = 8 * (repeat_count_zero - 2) - repeat_count_zero + extra;
+                extra += 8 * (repeat_count_zero - 2) - repeat_count_zero;
+            extra = FFMIN(extra, dist->alphabet_size - i);
             i += extra - 1;
             repeat_count_prev = 0;
             repeat_count_zero += extra;
-- 
2.42.0