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Date: Thu, 16 Jul 2015 12:34:13 +0200
From: Agostino Sarubbo <>
Subject: libav: divide-by-zero in ff_h263_decode_mba()

Libav is an open source set of tools for audio and video processing.                                                                                                                                                                                                           
After talking with Luca Barbato which is both a Gentoo and Libav 
developer, I spent a bit of my time fuzzing libav and in particular I 
fuzzed libavcodec though avplay.                                                                                                        
I hit a crash and after I reported it to upstream, they confirmed the 
issue as a divide-by-zero.                                                                                                                                                                               
The complete gdb output:                                                                                                                                                                                                                                                       
ago@...loughby $ gdb --args /usr/bin/avplay avplay.crash                                                                                                                                                                                                                       
GNU gdb (Gentoo 7.7.1 p1) 7.7.1                                                                                                                                                                                                                                                
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Reading symbols from /usr/bin/avplay...Reading symbols from 
(gdb) run                                                                                                                                                                                                                                                                      
Starting program: /usr/bin/avplay avplay.crash                                                                                                                                                                                                                                 
warning: Could not load shared library symbols for                                                                                                                                                                                                            
Do you need "set solib-search-path" or "set sysroot"?                                                                                                                                                                                                                          
[Thread debugging using libthread_db enabled]                                                                                                                                                                                                                                  
Using host libthread_db library "/lib64/".                                                                                                                                                                                                                    
avplay version 11.3, Copyright (c) 2003-2014 the Libav developers                                                                                                                                                                                                              
  built on Jun 19 2015 09:50:59 with gcc 4.8.4 (Gentoo 4.8.4 p1.6, 
[New Thread 0x7fffec4c7700 (LWP 7016)]                                                                                                                                                                                                                                         
[New Thread 0x7fffeb166700 (LWP 7017)]                                                                                                                                                                                                                                         
INFO: AddressSanitizer ignores mlock/mlockall/munlock/munlockall                                                                                                                                                                                                               
[New Thread 0x7fffe9e28700 (LWP 7018)]                                                                                                                                                                                                                                         
[h263 @ 0x60480000f680] Format detected only with low score of 25, 
misdetection possible!                                                                                                                                                                                      
[h263 @ 0x60440001f980] Syntax-based Arithmetic Coding (SAC) not 
[h263 @ 0x60440001f980] Reference Picture Selection not supported                                                                                                                                                                                                              
[h263 @ 0x60440001f980] Independent Segment Decoding not supported                                                                                                                                                                                                             
[h263 @ 0x60440001f980] header damaged                                                                                                                                                                                                                                         
Program received signal SIGFPE, Arithmetic exception.                                                                                                                                                                                                                          
[Switching to Thread 0x7fffe9e28700 (LWP 7018)]                                                                                                                                                                                                                                
0x00007ffff21e3313 in ff_h263_decode_mba (s=s@...ry=0x60720005a100) at 
142     /tmp/portage/media-
video/libav-11.3/work/libav-11.3/libavcodec/ituh263dec.c: No such file 
or directory.                                                                                                                                                                
(gdb) bt                                                                                                                                                                                                                                                                       
#0  0x00007ffff21e3313 in ff_h263_decode_mba (s=s@...ry=0x60720005a100) 
at /tmp/portage/media-
#1  0x00007ffff21f3c2d in ff_h263_decode_picture_header 
(s=0x60720005a100) at /tmp/portage/media-
#2  0x00007ffff1ae16ed in ff_h263_decode_frame (avctx=0x60440001f980, 
data=0x60380002f480, got_frame=0x7fffe9e272f0, avpkt=) at 
#3  0x00007ffff2cd963e in avcodec_decode_video2 (avctx=0x60440001f980, 
avpkt=avpkt@...ry=0x7fffe9e273b0) at /tmp/portage/media-
#4  0x00007ffff44d4fb4 in try_decode_frame (st=st@...ry=0x60340002fb00, 
avpkt=avpkt@...ry=0x601c00037b00, options=) at /tmp/portage/media-
#5  0x00007ffff44ebd89 in avformat_find_stream_info (ic=0x60480000f680, 
options=0x600a00009e80) at /tmp/portage/media-
#6  0x0000000000431834 in decode_thread (arg=0x7ffff7e0b800) at 
#7  0x00007ffff0284b08 in ?? () from /usr/lib64/
#8  0x00007ffff02b4be9 in ?? () from /usr/lib64/
#9  0x00007ffff4e65aa8 in ?? () from /usr/lib/gcc/x86_64-pc-linux-
#10 0x00007ffff0062204 in start_thread () from /lib64/
#11 0x00007fffefda957d in clone () from /lib64/

Affected version:
11.3 (and maybe past versions)

Fixed version:
11.5 and 12.0

Commit fix:;a=commitdiff;h=0a49a62f998747cfa564d98d36a459fe70d3299b;hp=6f4cd33efb5a9ec75db1677d5f7846c60337129f

This bug was discovered by Agostino Sarubbo of Gentoo.


2015-06-21: bug discovered
2015-06-22: bug reported privately to upstream
2015-06-30: upstream commit the fix
2015-07-14: CVE assigned
2015-07-16: advisory release

This bug was found with American Fuzzy Lop.
This bug does not affect ffmpeg.


Agostino Sarubbo
Gentoo Linux Developer

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