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Date: Fri, 15 Dec 2017 05:28:45 -0500
From: Stiepan <stie@....swiss>
To: "oss-security@...ts.openwall.com" <oss-security@...ts.openwall.com>
Cc: "fulldisclosure@...lists.org" <fulldisclosure@...lists.org>,
 "bugtraq@...urityfocus.com" <bugtraq@...urityfocus.com>
Subject: Re: [FD] [oss-security] CVE-2017-17670: vlc: type conversion
	vulnerability

Nice job! By the way, when is back-porting of the fix to the current stable version(s) envisioned? (I doubt most oss OS distributions use the "HEAD of the VLC master branch", nor that most Windows or Mac users use the latest bleeding-edge build, leaving a potentially large window for exploitation if former versions don't get fixed; knowing VLC's popularity, I think that the question should be seriously considered)
And is there a standalone patch or workaround that could be used for older versions (besides not opening mp4 videos anymore)?

> -------- Original Message --------
> Subject: [oss-security] CVE-2017-17670: vlc: type conversion vulnerability
> Local Time: December 15, 2017 1:32 AM
> UTC Time: December 15, 2017 1:32 AM
> From: hji@...topia.com
> To: oss-security@...ts.openwall.com, bugtraq@...urityfocus.com, fulldisclosure@...lists.org
>
> About
>
> A type conversion vulnerability exist in the MP4 demux module in VLC
> <=2.2.8. This issue has been assigned CVE-2017-17670 and it could be
> used to cause an arbitrary free.
>
> Details
>
> MP4 is a container format for video, audio, subtitles and images. The
> various parts of an .mp4 are organized as hierarchical boxes/atoms in
> big-endian byte ordering [1].
>
> VLC processes these boxes by using a lookup table:
>
> vlc-2.2.8/modules/demux/mp4/libmp4.c
> ,----
> | 3297 static const struct
> | 3298 {
> | 3299 uint32_t i_type;
> | 3300 int (*MP4_ReadBox_function )( stream_t *p_stream, MP4_Box_t *p_box );
> | 3301 void (*MP4_FreeBox_function )( MP4_Box_t p_box );
> | 3302 uint32_t i_parent; / set parent to restrict, duplicating if needed; 0 for any /
> | 3303 } MP4_Box_Function [] =
> | 3304 {
> | 3305 / Containers /
> | 3306 { ATOM_moov, MP4_ReadBoxContainer, MP4_FreeBox_Common, 0 },
> | 3307 { ATOM_trak, MP4_ReadBoxContainer, MP4_FreeBox_Common, ATOM_moov },
> | ....
> | 3565 / Last entry */
> | 3566 { 0, MP4_ReadBox_default, NULL, 0 }
> | 3567 };
> ---- vlc-2.2.8/modules/demux/mp4/libmp4.c ,---- | 3574 static MP4_Box_t *MP4_ReadBox( stream_t *p_stream, MP4_Box_t *p_father ) | 3575 { | 3576 MP4_Box_t *p_box = calloc( 1, sizeof( MP4_Box_t ) ); /* Needed to ensure simple on error handler */ | 3577 unsigned int i_index; | .... | 3582 if( !MP4_ReadBoxCommon( p_stream, p_box ) ) | 3583 { | .... | 3587 } | .... | 3605 /* Now search function to call */ | 3606 for( i_index = 0; ; i_index++ ) | 3607 { | .... | 3613 if( ( MP4_Box_Function[i_index].i_type == p_box->i_type )|| | 3614 ( MP4_Box_Function[i_index].i_type == 0 ) ) | 3615 { | 3616 break; | 3617 } | 3618 } | 3619 | 3620 if( !(MP4_Box_Function[i_index].MP4_ReadBox_function)( p_stream, p_box ) ) | 3621 { | 3622 MP4_BoxFree( p_stream, p_box ); | 3623 return NULL; | 3624 } | 3625 | 3626 return p_box; | 3627 }----
>
> MP4_ReadBox() allocates a MP4_Box_t structure and invokes
> MP4_ReadBoxCommon() to read the properties common to all mp4 boxes;
> i_size' andi_type' (and optionally an extended size). Afterwards,
> MP4_Box_Function is used to dispatch further parsing to a suitable
> function based on its i_type'. When VLC is done with the boxes, they are freed with MP4_BoxFree(): vlc-2.2.8/modules/demux/mp4/libmp4.c ,---- | 3633 void MP4_BoxFree( stream_t *s, MP4_Box_t *p_box ) | 3634 { | 3635 unsigned int i_index; | .... | 3650 /* Now search function to call */ | 3651 if( p_box->data.p_payload ) | 3652 { | 3653 for( i_index = 0; ; i_index++ ) | 3654 { | .... | 3660 if( ( MP4_Box_Function[i_index].i_type == p_box->i_type )|| | 3661 ( MP4_Box_Function[i_index].i_type == 0 ) ) | 3662 { | 3663 break; | 3664 } | 3665 } | 3666 if( MP4_Box_Function[i_index].MP4_FreeBox_function == NULL ) | 3667 { | .... | 3677 } | 3678 else | 3679 { | 3680 MP4_Box_Function[i_index].MP4_FreeBox_function( p_box ); | 3681 } | .... | 3685 }----
>
> Again, i_type' is used to find a suitable free-function. The reason this may be problematic is thati_type' could be changed
> when VLC handles sinf' andfrma' boxes in TrackCreateES() -- meaning
> that a box may be read as one type, and freed as another.
>
> sinf' is the "Protection Scheme Information Box" and it's used for protected/encrypted media.frma' is the "Original Format Box" and it's
> used to declare the format of the unprotected media.
>
> If a sinf/frma is found underneath a sample box, the i_type' of that sample is replaced with the original format declared in thefrma':
>
> vlc-2.2.8/modules/demux/mp4/mp4.c
> ,----
> | 2180 static int TrackCreateES( demux_t *p_demux, mp4_track_t *p_track,
> | 2181 unsigned int i_chunk, es_out_id_t **pp_es )
> | 2182 {
> | ....
> | 2208 p_sample = MP4_BoxGet( p_track->p_stsd, "[%d]",
> | 2209 i_sample_description_index - 1 );
> | ....
> | 2219 p_track->p_sample = p_sample;
> | 2220
> | 2221 if( ( p_frma = MP4_BoxGet( p_track->p_sample, "sinf/frma" ) ) && p_frma->data.p_frma )
> | 2222 {
> | 2223 msg_Warn( p_demux, "Original Format Box: %4.4s", (char *)&p_frma->data.p_frma->i_type );
> | 2224
> | 2225 p_sample->i_type = p_frma->data.p_frma->i_type;
> | 2226 }
> | ....
> ---- No sanity check is done to make sure that the MP4_FreeBox_function associated with the newi_type' is compatible with the old
> MP4_ReadBox_function.
>
> Example
>
> One way to abuse the type change is to have asoun' changed to avide'. This results in a 72-byte allocation (x86-64) for the
> p_sample_soun' member of the p_box->data union when the box is read: vlc-2.2.8/modules/demux/mp4/libmp4.c ,---- | 1614 static int MP4_ReadBox_sample_soun( stream_t *p_stream, MP4_Box_t *p_box ) | 1615 { | 1616 p_box->i_handler = ATOM_soun; | 1617 MP4_READBOX_ENTER( MP4_Box_data_sample_soun_t ); | ....----
>
> vlc-2.2.8/modules/demux/mp4/libmp4.h
> ,----
> | 1351 #define MP4_READBOX_ENTER( MP4_Box_data_TYPE_t )
> | ....
> | 1369 if( !( p_box->data.p_payload = calloc( 1, sizeof( MP4_Box_data_TYPE_t ) ) ) )
> | 1370 {
> | ....
> | 1373 }
> ---- wherep_box' is MP4_Box_t:
>
> vlc-2.2.8/modules/demux/mp4/libmp4.h
> ,----
> | 1284 typedef struct MP4_Box_s
> | 1285 {
> | ....
> | 1296 MP4_Box_data_t data; /* union of pointers on extended data depending
> | 1297 on i_type (or i_usertype) */
> | ....
> | 1306 } MP4_Box_t;
> ---- and MP4_Box_data_t: vlc-2.2.8/modules/demux/mp4/libmp4.h ,---- | 1200 typedef union MP4_Box_data_s | 1201 { | .... | 1220 MP4_Box_data_sample_vide_t *p_sample_vide; | 1221 MP4_Box_data_sample_soun_t *p_sample_soun; | .... | 1278 void *p_payload; /* for unknow type */ | 1279 } MP4_Box_data_t;----
>
> ,----
> | (gdb) p sizeof(MP4_Box_data_sample_soun_t)
> | $1 = 72
> ---- After the box has had its type changed tovide' and it's later freed,
> the p_sample_vide' member of the p_box->data union is used: vlc-2.2.8/modules/demux/mp4/libmp4.c ,---- | 1861 void MP4_FreeBox_sample_vide( MP4_Box_t *p_box ) | 1862 { | 1863 FREENULL( p_box->data.p_sample_vide->p_qt_image_description ); | 1864 }----
>
> ,----
> | (gdb) p sizeof(MP4_Box_data_sample_vide_t)
> | $2 = 96
> | (gdb)
> ---- vlc-2.2.8/modules/demux/mp4/libmp4.h ,---- | 529 typedef struct MP4_Box_data_sample_vide_s | 530 { | ... | 557 uint8_t *p_qt_image_description; | 558 | 559 } MP4_Box_data_sample_vide_t;----
>
> p_sample_vide' is 24 bytes larger thanp_sample_soun', and
> p_qt_image_description' is at the end of the vide struct; i.e. the pointer to be free()d is read out-of-bounds from potentially user-controlled memory. mkmp4.py' at [2]
>
> ,----
> | $ uname -imrs
> | FreeBSD 11.1-RELEASE-p4 amd64 GENERIC
> | $ ./mkmp4.py file.mp4
> | $ vlc --version
> | VLC media player 2.2.8 Weatherwax (revision 2.2.7-14-g3cc1d8cba9)
> | $ gdb -q --args vlc file.mp4
> | (gdb) set breakpoint pending on
> | (gdb) b libmp4.c:1618
> | No source file named libmp4.c.
> | Breakpoint 1 (libmp4.c:1618) pending.
> | (gdb) b libmp4.c:1863
> | No source file named libmp4.c.
> | Breakpoint 2 (libmp4.c:1863) pending.
> | (gdb) r
> | [...]
> | Breakpoint 3, MP4_ReadBox_sample_soun (p_stream=0x802ab2710, p_box=0x802a85000) at demux/mp4/libmp4.c:1618
> | 1618 p_box->data.p_sample_soun->p_qt_description = NULL;
> | (gdb) p p_box->data.p_sample_soun
> | $1 = (MP4_Box_data_sample_soun_t *) 0x802a79810
> | (gdb) c
> | Continuing.
> |
> | Breakpoint 4, MP4_FreeBox_sample_vide (p_box=0x802a85000) at demux/mp4/libmp4.c:1863
> | 1863 FREENULL( p_box->data.p_sample_vide->p_qt_image_description );
> | (gdb) p p_box->data.p_sample_vide
> | $2 = (MP4_Box_data_sample_vide_t *) 0x802a79810
> | (gdb) p p_box->data.p_sample_vide->p_qt_image_description
> | $3 = (uint8_t ) 0x1122334455667788 <Error reading address 0x1122334455667788: Bad address>
> | (gdb) b free
> | Breakpoint 5 at 0x8019d3ce4
> | (gdb) c
> | Continuing.
> |
> | Breakpoint 5, 0x00000008019d3ce4 in free () from /lib/libc.so.7
> | (gdb) p/x $rdi
> | $4 = 0x1122334455667788
> | (gdb) c
> | Continuing.
> |
> | Program received signal SIGBUS, Bus error.
> | 0x00000008019d36f3 in realloc () from /lib/libc.so.7
> | (gdb) x/i $rip
> | 0x8019d36f3 <realloc+3939>: mov rbx,QWORD PTR [rax+rcx8+0x68]
> | (gdb) i r
> | rax 0x1122334455600000 1234605616436084736
> | rbx 0x1122334455667788 1234605616436508552
> | rcx 0x5a 90
> | [...]
> | (gdb) bt 4
> | #0 0x00000008019d36f3 in realloc () from /lib/libc.so.7
> | #1 0x00000008019d3d51 in free () from /lib/libc.so.7
> | #2 0x0000000806d7fafd in MP4_FreeBox_sample_vide (p_box=0x802a85000) at demux/mp4/libmp4.c:1863
> | #3 0x0000000806d7fcfd in MP4_BoxFree (s=0x802ab2710, p_box=0x802a85000) at demux/mp4/libmp4.c:3680
> `----
>
> Solution
>
> This issue does not affect the HEAD of the VLC master branch.
>
> Footnotes
> ---------------------------------------------------------------
>
> [1] [http://xhelmboyx.tripod.com/formats/mp4-layout.txt]
>
> [2] [https://gist.github.com/dyntopia/194d912287656f66dd502158b0cd2e68]
>
> hji

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