Help with disabling _syscall

[Aswag1 8.5k 8]

Well after laying dormant for quite awhile, I thought I might add a couple of pointers....

 

It seems that for 64-bit devices, there are certain additional checks in the binary in most GameLoft games. Most notably, the addition of a __sysctl. What this call does is check whether the __syscall flag is checked. If it sees that it has been manipulated in some fashion (such as NOPing the BLX or NOP the MOV) it will immediately give a EXEC_BAD_ADDRESS error.

 

Now here are some of my findings to combat this...

 

I have seen that making any alterations in a binary patch have led to the game crashing once I attach to it in lldb. So, logically I just patch the memory of the game rather than make crude adjustments. It seems to work, though it is a pain in the ass and you have to keep stepping through the memory stack to make sure that all checks are thoroughly disabled. I had all the checks for Brothers In Arms 3 before, I am not sure if I have them still. I would like to help, but I'm not really into iOS hacking anymore so there's that. Hopefully one of the young bloods can help you out and resolve your problem.

[Guest]

Well after laying dormant for quite awhile, I thought I might add a couple of pointers....

 

It seems that for 64-bit devices, there are certain additional checks in the binary in most GameLoft games. Most notably, the addition of a __sysctl. What this call does is check whether the __syscall flag is checked. If it sees that it has been manipulated in some fashion (such as NOPing the BLX or NOP the MOV) it will immediately give a EXEC_BAD_ADDRESS error.

 

Now here are some of my findings to combat this...

 

I have seen that making any alterations in a binary patch have led to the game crashing once I attach to it in lldb. So, logically I just patch the memory of the game rather than make crude adjustments. It seems to work, though it is a pain in the ass and you have to keep stepping through the memory stack to make sure that all checks are thoroughly disabled. I had all the checks for Brothers In Arms 3 before, I am not sure if I have them still. I would like to help, but I'm not really into iOS hacking anymore so there's that. Hopefully one of the young bloods can help you out and resolve your problem.

 

Thank you for a great explanation. So basically, I cannot use my 5S to make hacks for Gameloft games (except for patching the memory, which would be temporary because the addresses would change every time) because it is 64-bit, but I can use a device that is not 64-bit, for example my old ipod touch 5g. So if I load up the binary from my iPod, I might be able to patch __syscall so that I can attach it to GDB?

[Aswag1 8.5k 8]

That is correct, however I doubt you would be able to debug on a iPod Touch 5g considering the low RAM it has.

[Gh0stByte 3.3k 4 6.1.2]

Find the syscal method, and BX LR the first instruction.

[jx1992 0]

Find the syscal method, and BX LR the first instruction.

That's right. If calls to the functions can easily be disabled, you could simply make the actual syscall function to return and do nothing. Can you post the disassembly for the syscall function?

[Guest]

@@Aswag1 yeah, it has been jailbroken four times and freezes regularly... i don't think i can use it, and out of curiosity, is android hacking easier than ios?

 

@@Gh0stByte i feel like if i BX LR the first instruction, it will crash because of what aswag1 said

 

@jx1992 i can post the disassembly here later, just not now, i'm going to a movie in 5 mins

Updated January 24, 2015 by Guest

[Guest]

@@jx1992 here are the things that i think are the _syscall function.

 

1.

 

__text:0000C5C8   sub_C5C8                                ; CODE XREF: -[sendInfoInterface IGPSendGameInfo:::::]+536p

__text:0000C5C8                 PUSH            {R7,LR}

__text:0000C5CA                 MOV             R7, SP

__text:0000C5CC                 SUB             SP, SP, #0x6C

__text:0000C5CE                 MOVW            R1, #(:lower16:(aLibraryMobiles - 0xC5DE)) ; "/Library/MobileSubstrate/MobileSubstrat"...

__text:0000C5D2                 MOV             R2, SP

__text:0000C5D4                 MOVT.W          R1, #(:upper16:(aLibraryMobiles - 0xC5DE)) ; "/Library/MobileSubstrate/MobileSubstrat"...

__text:0000C5D8                 MOVS            R0, #0xBC ; int

__text:0000C5DA                 ADD             R1, PC  ; "/Library/MobileSubstrate/MobileSubstrat"...

__text:0000C5DC                 BLX.W           _syscall

__text:0000C5E0                 CMP             R0, #0

__text:0000C5E2                 BEQ             loc_C66C

__text:0000C5E4                 MOVW            R1, #(:lower16:(aApplicationsCy - 0xC5F4)) ; "/Applications/Cydia.app"

__text:0000C5E8                 MOV             R2, SP

__text:0000C5EA                 MOVT.W          R1, #(:upper16:(aApplicationsCy - 0xC5F4)) ; "/Applications/Cydia.app"

__text:0000C5EE                 MOVS            R0, #0xBC ; int

__text:0000C5F0                 ADD             R1, PC  ; "/Applications/Cydia.app"

__text:0000C5F2                 BLX.W           _syscall

__text:0000C5F6                 CMP             R0, #0

__text:0000C5F8                 BEQ             loc_C66C

__text:0000C5FA                 MOVW            R1, #(:lower16:(aVarLibApt - 0xC60A)) ; "/var/lib/apt"

__text:0000C5FE                 MOV             R2, SP

__text:0000C600                 MOVT.W          R1, #(:upper16:(aVarLibApt - 0xC60A)) ; "/var/lib/apt"

__text:0000C604                 MOVS            R0, #0xBC ; int

__text:0000C606                 ADD             R1, PC  ; "/var/lib/apt"

__text:0000C608                 BLX.W           _syscall

__text:0000C60C                 CMP             R0, #0

__text:0000C60E                 BEQ             loc_C66C

__text:0000C610                 MOVW            R1, #(:lower16:(aVarLibCydia - 0xC620)) ; "/var/lib/cydia"

__text:0000C614                 MOV             R2, SP

__text:0000C616                 MOVT.W          R1, #(:upper16:(aVarLibCydia - 0xC620)) ; "/var/lib/cydia"

__text:0000C61A                 MOVS            R0, #0xBC ; int

__text:0000C61C                 ADD             R1, PC  ; "/var/lib/cydia"

__text:0000C61E                 BLX.W           _syscall

__text:0000C622                 CMP             R0, #0

__text:0000C624                 BEQ             loc_C66C

__text:0000C626                 MOVW            R1, #(:lower16:(aVarTmpCydia_lo - 0xC636)) ; "/var/tmp/cydia.log"

__text:0000C62A                 MOV             R2, SP

__text:0000C62C                 MOVT.W          R1, #(:upper16:(aVarTmpCydia_lo - 0xC636)) ; "/var/tmp/cydia.log"

__text:0000C630                 MOVS            R0, #0xBC ; int

__text:0000C632                 ADD             R1, PC  ; "/var/tmp/cydia.log"

__text:0000C634                 BLX.W           _syscall

__text:0000C638                 CMP             R0, #0

__text:0000C63A                 BEQ             loc_C66C

__text:0000C63C                 MOVW            R1, #(:lower16:(aEtcApt - 0xC64C)) ; "/etc/apt"

__text:0000C640                 MOV             R2, SP

__text:0000C642                 MOVT.W          R1, #(:upper16:(aEtcApt - 0xC64C)) ; "/etc/apt"

__text:0000C646                 MOVS            R0, #0xBC ; int

__text:0000C648                 ADD             R1, PC  ; "/etc/apt"

__text:0000C64A                 BLX.W           _syscall

__text:0000C64E                 CMP             R0, #0

__text:0000C650                 BEQ             loc_C66C

__text:0000C652                 MOVS            R0, #2  ; int

__text:0000C654                 BLX.W           _syscall

__text:0000C658                 MOV             R1, R0

__text:0000C65A                 MOVS            R0, #0  ; int

__text:0000C65C                 CMP             R1, #0

__text:0000C65E                 BEQ             loc_C672

__text:0000C660                 CMP.W           R1, #0xFFFFFFFF

__text:0000C664                 IT GT

__text:0000C666                 MOVGT           R0, #1

__text:0000C668                 ADD             SP, SP, #0x6C

__text:0000C66A                 POP             {R7,PC}

__text:0000C66C ; ---------------------------------------------------------------------------

__text:0000C66C

__text:0000C66C loc_C66C                                ; CODE XREF: sub_C5C8+1Aj

__text:0000C66C                                         ; sub_C5C8+30j ...

__text:0000C66C                 MOVS            R0, #1

__text:0000C66E                 ADD             SP, SP, #0x6C

__text:0000C670                 POP             {R7,PC}

__text:0000C672 ; ---------------------------------------------------------------------------

__text:0000C672

__text:0000C672 loc_C672                                ; CODE XREF: sub_C5C8+96j

__text:0000C672                 BLX.W           _exit

__text:0000C676                 NOP

__text:0000C676 ; End of function sub_C5C8

 

2.

 

__picsymbolstub4:0095AEE8 _syscall                                ; CODE XREF: sub_C5C8+14p

__picsymbolstub4:0095AEE8                                         ; sub_C5C8+2Ap ...

__picsymbolstub4:0095AEE8                 LDR             R12, =(_syscall_ptr - 0x95AEF4)

__picsymbolstub4:0095AEEC                 ADD             R12, PC, R12 ; _syscall_ptr

__picsymbolstub4:0095AEF0                 LDR             PC, [R12] ; __imp__syscall

__picsymbolstub4:0095AEF0 ; End of function _syscall

 

Again, I'm not used to doing all of this just to debug a game. I was surprised that there where almost no answers from when I first searched this, or no guide on any website. I am also surprised that there are so many people that are willing to help me, it really means alot

[jx1992 0]

@@Shmoo419

 

In the disassembly posted on page 1, the syscall is made to ptrace with the argument "PT_DENY_ATTACH".

In linux, this would be used to deny debuggers from attaching to the current process (in this case, Nova 3).

 

Since the disassembly shows that the program doesn't check for the return value of the ptrace syscall, what you can do is one of these 2.

 

1) Set r1 to -1

 

Because -1 is not a valid ptrace request argument, the ptrace syscall will return with an error and the call will not succeed.

 

2) Nop the branch to syscall

 

This simply stops the function from making a syscall. Simple as that.

 

From what I can infer, I suppose you are trying to attach a debugger to the Nova 3 app. You can try the above 2 methods to see if you can get your debugger to attach.

 

For more details you can reference this site : http://www.coredump.gr/articles/ios-anti-debugging-protections-part-1/

 

Good luck

[Guest]

@@jx1992 I know that PT_DENY_ATTACH keeps the debugger from attaching, but I never tried -1, but instead anything other than 31 (0x1F) or I NOP'ed it. And I usually stay away from NOP'ing branches because it usually results in a crash (for me at least), but I'll give it a go. That website was really useful, too.

[jx1992 0]

@@Shmoo419 I noticed theres a sysctl call in the function as well. That function is usually used to detect the presence of a debugger as well. You can visit this site http://www.coredump.gr/articles/ios-anti-debugging-protections-part-2/ to read more about bypassing it or you can post the whole disassembly of the first function and we can figure it out