On 10/2/23 16:40, Andreas Rheinhardt wrote:
Leo Izen:
The spec caps the prefix alphabet size to 32768 (i.e. 1 << 15) so we
need to check for that and reject alphabets that are too large.
No, we don't "need to", we can. FFmpeg is not a validator tool.
We need to because we risk over-allocating otherwise. If the signalled
value is far too large, we consume a pointlessly large amount of memory.
Additionally, there's no need to allocate buffers that are as large as
the maximum alphabet size as these aren't stack-allocated, they're heap
allocated and thus can be variable size.
Added an overflow check as well, which fixes leaking the buffer, and
capping the alphabet size fixes two potential overruns as well.
Fixes: out of array access
Fixes: 62089/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-
5437089094959104.fuzz
Found-by: continuous fuzzing process
https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Found-by: Hardik Shah of Vehere (Dawn Treaders team)
Co-authored-by: Michael Niedermayer <mich...@niedermayer.cc>
Signed-off-by: Leo Izen <leo.i...@gmail.com>
---
libavcodec/jpegxl_parser.c | 23 +++++++++++++++++------
1 file changed, 17 insertions(+), 6 deletions(-)
diff --git a/libavcodec/jpegxl_parser.c b/libavcodec/jpegxl_parser.c
index d25a1b6e1d..51af0f4ed1 100644
--- a/libavcodec/jpegxl_parser.c
+++ b/libavcodec/jpegxl_parser.c
@@ -46,6 +46,8 @@
#define JXL_FLAG_USE_LF_FRAME 32
#define JXL_FLAG_SKIP_ADAPTIVE_LF_SMOOTH 128
+#define MAX_PREFIX_ALPHABET_SIZE (1u << 15)
+
#define clog1p(x) (ff_log2(x) + !!(x))
#define unpack_signed(x) (((x) & 1 ? -(x)-1 : (x))/2)
#define div_ceil(x, y) (((x) - 1) / (y) + 1)
@@ -724,16 +726,17 @@ static int read_vlc_prefix(GetBitContext *gb,
JXLEntropyDecoder *dec, JXLSymbolD
if (ret < 0)
goto end;
- buf = av_calloc(1, 262148); // 32768 * 8 + 4
+ buf = av_calloc(1, dist->alphabet_size * (2 * sizeof(int8_t) +
sizeof(int16_t) + sizeof(uint32_t))
+ + sizeof(uint32_t));
You can avoid the multiplication by using av_calloc((2 * sizeof(int8_t)
+ sizeof(int16_t) + sizeof(uint32_t)) + sizeof(uint32_t),
dist->alphabet_size).
That's not the same thing. This will cause us to overallocate by
dist-alphabet_size - 4 bytes. Is that okay?
if (!buf) {
ret = AVERROR(ENOMEM);
goto end;
}
level2_lens = (int8_t *)buf;
- level2_lens_s = (int8_t *)(buf + 32768);
- level2_syms = (int16_t *)(buf + 65536);
- level2_codecounts = (uint32_t *)(buf + 131072);
+ 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)));
total_code = 0;
for (int i = 0; i < dist->alphabet_size; i++) {
@@ -742,6 +745,10 @@ static int read_vlc_prefix(GetBitContext *gb,
JXLEntropyDecoder *dec, JXLSymbolD
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;
+ }
for (int j = 0; j < extra; j++)
level2_lens[i + j] = prev;
total_code += (32768 >> prev) * extra;
@@ -772,8 +779,10 @@ static int read_vlc_prefix(GetBitContext *gb,
JXLEntropyDecoder *dec, JXLSymbolD
}
}
- if (total_code != 32768 && level2_codecounts[0] < dist->alphabet_size - 1)
- return AVERROR_INVALIDDATA;
+ if (total_code != 32768 && level2_codecounts[0] < dist->alphabet_size - 1)
{
+ ret = AVERROR_INVALIDDATA;
+ goto end;
+ }
for (int i = 1; i < dist->alphabet_size + 1; i++)
level2_codecounts[i] += level2_codecounts[i - 1];
@@ -848,6 +857,8 @@ static int read_distribution_bundle(GetBitContext *gb,
JXLEntropyDecoder *dec,
if (get_bits1(gb)) {
int n = get_bits(gb, 4);
dist->alphabet_size = 1 + (1 << n) + get_bitsz(gb, n);
+ if (dist->alphabet_size > MAX_PREFIX_ALPHABET_SIZE)
+ return AVERROR_INVALIDDATA;
} else {
dist->alphabet_size = 1;
}
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