[FFmpeg-cvslog] avcodec/h2645_parse: Only trim RBSP trailing padding if it exists

[Andreas Rheinhardt]

ffmpeg | branch: master | Andreas Rheinhardt <andreas.rheinhardt at outlook.com> | Wed Jun 22 09:19:01 2022 +0200| [a217c1472d3bb2dc949ac45f241336ae65c3a7b7] | committer: Andreas Rheinhardt

avcodec/h2645_parse: Only trim RBSP trailing padding if it exists

It does not exist for NALUs for which the SODB is empty;
it also does not exist for NALUs for which not even
the complete header is present. The former category contains
end of sequence and end of bitstream units. The latter category
consists of one-byte HEVC units (the ordinary H.264 header is only
one byte long).
This commit therefore stops stripping RBSP trailing padding
from the former type of unit and discards the latter type of unit
altogether.

This also fixes an assertion failure: Before this commit, a one-byte
HEVC NALU from an ISOBMFF packet could pass all the checks in
hevc_parse_nal_header() (because the first byte of the size field
of the next unit is mistaken as containing the temporal_id);
yet because the trailing padding bits were stripped, its actually
had a size of less than eight bits; because h2645_parse.c uses
the checked bitstream reader, the get_bits_count() of the GetBitContext
is not 16 in this case; it is not even a multiple of eight
and this can trigger an assert in ff_hevc_decode_nal_sei().

Fixes: Assertion failure
Fixes: 46662/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_HEVC_fuzzer-4947860854013952

Reviewed-by: Michael Niedermayer <michael at niedermayer.cc>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at outlook.com>

> http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=a217c1472d3bb2dc949ac45f241336ae65c3a7b7
---

 libavcodec/h2645_parse.c | 26 ++++++++++++++++----------
 1 file changed, 16 insertions(+), 10 deletions(-)

diff --git a/libavcodec/h2645_parse.c b/libavcodec/h2645_parse.c
index 03780680c6..90944177c7 100644
--- a/libavcodec/h2645_parse.c
+++ b/libavcodec/h2645_parse.c
@@ -259,10 +259,10 @@ static const char *h264_nal_unit_name(int nal_type)
     return h264_nal_type_name[nal_type];
 }
 
-static int get_bit_length(H2645NAL *nal, int skip_trailing_zeros)
+static int get_bit_length(H2645NAL *nal, int min_size, int skip_trailing_zeros)
 {
     int size = nal->size;
-    int v;
+    int trailing_padding = 0;
 
     while (skip_trailing_zeros && size > 0 && nal->data[size - 1] == 0)
         size--;
@@ -270,18 +270,23 @@ static int get_bit_length(H2645NAL *nal, int skip_trailing_zeros)
     if (!size)
         return 0;
 
-    v = nal->data[size - 1];
+    if (size <= min_size) {
+        if (nal->size < min_size)
+            return AVERROR_INVALIDDATA;
+        size = min_size;
+    } else {
+        int v = nal->data[size - 1];
+        /* remove the stop bit and following trailing zeros,
+         * or nothing for damaged bitstreams */
+        if (v)
+            trailing_padding = ff_ctz(v) + 1;
+    }
 
     if (size > INT_MAX / 8)
         return AVERROR(ERANGE);
     size *= 8;
 
-    /* remove the stop bit and following trailing zeros,
-     * or nothing for damaged bitstreams */
-    if (v)
-        size -= ff_ctz(v) + 1;
-
-    return size;
+    return size - trailing_padding;
 }
 
 /**
@@ -491,7 +496,8 @@ int ff_h2645_packet_split(H2645Packet *pkt, const uint8_t *buf, int length,
             bytestream2_peek_be32(&bc) == 0x000001E0)
             skip_trailing_zeros = 0;
 
-        nal->size_bits = get_bit_length(nal, skip_trailing_zeros);
+        nal->size_bits = get_bit_length(nal, 1 + (codec_id == AV_CODEC_ID_HEVC),
+                                        skip_trailing_zeros);
 
         if (nal->size <= 0 || nal->size_bits <= 0)
             continue;