libav: invalid memory access in put_no_rnd_pixels8_xy2_mmx (rnd_template.c)

Description:
Libav is an open source set of tools for audio and video processing.

A fuzzing with an mp3 file as input discovered an invalid memory access in put_no_rnd_pixels8_xy2_mmx.

The complete ASan output:

# avconv -i $FILE -f null -
avconv version 11.7, Copyright (c) 2000-2016 the Libav developers
  built on Aug 16 2016 15:34:42 with clang version 3.8.1 (tags/RELEASE_381/final)
[h263 @ 0x61a00001f280] Format detected only with low score of 25, misdetection possible!
[IMGUTILS @ 0x7ff589955420] Picture size 0x0 is invalid
[h263 @ 0x619000000580] header damaged
[h263 @ 0x619000000580] Syntax-based Arithmetic Coding (SAC) not supported
[h263 @ 0x619000000580] Independent Segment Decoding not supported
[h263 @ 0x619000000580] warning: first frame is no keyframe
[h263 @ 0x619000000580] cbpc damaged at 0 0
[h263 @ 0x619000000580] Error at MB: 0
[h263 @ 0x619000000580] concealing 1584 DC, 1584 AC, 1584 MV errors
[h263 @ 0x61a00001f280] Estimating duration from bitrate, this may be inaccurate
Input #0, h263, from '9.crashes':
  Duration: N/A, bitrate: N/A
    Stream #0.0: Video: h263, yuv420p, 704x576 [PAR 12:11 DAR 4:3], 25 fps, 25 tbn, 18.73 tbc
Output #0, null, to 'pipe:':
  Metadata:
    encoder         : Lavf56.1.0
    Stream #0.0: Video: rawvideo, yuv420p, 704x576 [PAR 12:11 DAR 4:3], q=2-31, 200 kb/s, 25 tbn, 25 tbc
    Metadata:
      encoder         : Lavc56.1.0 rawvideo
Stream mapping:
  Stream #0:0 -> #0:0 (h263 (native) -> rawvideo (native))
Press ctrl-c to stop encoding
[h263 @ 0x61900001ea80] warning: first frame is no keyframe
ASAN:DEADLYSIGNAL
=================================================================
==26790==ERROR: AddressSanitizer: SEGV on unknown address 0x7ff584ddb77f (pc 0x7ff5910cdeee bp 0x7ffdc464d7f0 sp 0x7ffdc464d780 T0)
    #0 0x7ff5910cdeed in put_no_rnd_pixels8_xy2_mmx /var/tmp/portage/media-video/libav-11.7/work/libav-11.7/libavcodec/x86/rnd_template.c:37:5
    #1 0x7ff590209de0 in hpel_motion /var/tmp/portage/media-video/libav-11.7/work/libav-11.7/libavcodec/mpegvideo_motion.c:224:5
    #2 0x7ff590209de0 in apply_8x8 /var/tmp/portage/media-video/libav-11.7/work/libav-11.7/libavcodec/mpegvideo_motion.c:798
    #3 0x7ff590209de0 in mpv_motion_internal /var/tmp/portage/media-video/libav-11.7/work/libav-11.7/libavcodec/mpegvideo_motion.c:877
    #4 0x7ff590209de0 in ff_mpv_motion /var/tmp/portage/media-video/libav-11.7/work/libav-11.7/libavcodec/mpegvideo_motion.c:981
    #5 0x7ff59013659b in mpv_decode_mb_internal /var/tmp/portage/media-video/libav-11.7/work/libav-11.7/libavcodec/mpegvideo.c:2223:21
    #6 0x7ff59013659b in ff_mpv_decode_mb /var/tmp/portage/media-video/libav-11.7/work/libav-11.7/libavcodec/mpegvideo.c:2358
    #7 0x7ff58f048c95 in decode_slice /var/tmp/portage/media-video/libav-11.7/work/libav-11.7/libavcodec/h263dec.c:273:13
    #8 0x7ff58f0442cd in ff_h263_decode_frame /var/tmp/portage/media-video/libav-11.7/work/libav-11.7/libavcodec/h263dec.c:575:11
    #9 0x7ff5909cf906 in avcodec_decode_video2 /var/tmp/portage/media-video/libav-11.7/work/libav-11.7/libavcodec/utils.c:1600:19
    #10 0x5647eb in decode_video /var/tmp/portage/media-video/libav-11.7/work/libav-11.7/avconv.c:1259:11
    #11 0x5647eb in process_input_packet /var/tmp/portage/media-video/libav-11.7/work/libav-11.7/avconv.c:1398
    #12 0x550e63 in process_input /var/tmp/portage/media-video/libav-11.7/work/libav-11.7/avconv.c:2440:11
    #13 0x550e63 in transcode /var/tmp/portage/media-video/libav-11.7/work/libav-11.7/avconv.c:2488
    #14 0x550e63 in main /var/tmp/portage/media-video/libav-11.7/work/libav-11.7/avconv.c:2647
    #15 0x7ff58cd6461f in __libc_start_main /var/tmp/portage/sys-libs/glibc-2.22-r4/work/glibc-2.22/csu/libc-start.c:289
    #16 0x41d098 in _init (/usr/bin/avconv+0x41d098)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV /var/tmp/portage/media-video/libav-11.7/work/libav-11.7/libavcodec/x86/rnd_template.c:37:5 in put_no_rnd_pixels8_xy2_mmx
==26790==ABORTING

Affected version:
11.7

Fixed version:
N/A

Commit fix:
https://git.libav.org/?p=libav.git;a=commit;h=136f55207521f0b03194ef5b55ba70f1635d6aee

Credit:
This bug was discovered by Agostino Sarubbo of Gentoo.

CVE:
CVE-2016-7424

Timeline:
2016-08-15: bug discovered
2016-08-16: bug reported to upstream
2016-09-16: upstream released a patch
2016-09-17: blog post about the issue
2016-09-17: CVE Assigned

Note:
This bug was found with American Fuzzy Lop.
This bug was reported F4B3CD@STARLAB on 2016-09-12 via libav-security while it was already public since
2016-08-15 on the upstream bugtracker.
After an investigation it looks like an invalid memory access (read), so I’m changing a bit the description but I’m keeping the permalink as-is.

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libav: invalid memory access in put_no_rnd_pixels8_xy2_mmx (rnd_template.c)