OS X privilege escalation due to XPC type confusion in sysmond (with exploit)

[Nytro]

Reported by ianb.. @google.com, Oct 7, 2014 tested on OS X 10.9.5 - uses some hard-coded offsets which will have to be fixed-up for other versions!

this poc uses liblorgnette to resolve some private symbols; grab the code from github:

git clone https://github.com/rodionovd/liblorgnette.git

build this PoC with:

clang -o sysmond_exploit_writeup sysmond_exploit_writeup.c liblorgnette/lorgnette.c -framework CoreFoundation

sysmond is a daemon running as root. You can interact with sysmond via XPC ("com.apple.sysmond".)

sub_100001AAF calls sub_100003120 passing the xpc dictionary received from the attacker. This function

allocates a sysmond_request object and fills in fields from the attacker-controlled xpc request dictionary:

;read a uint64 with the key "Type"

__text:0000000100003144 mov rax, cs:_SYSMON_XPC_KEY_TYPE_ptr

__text:000000010000314B mov rsi, [rax]

__text:000000010000314E mov rdi, r14

__text:0000000100003151 call _xpc_dictionary_get_uint64

__text:0000000100003156 mov [rbx+20h], rax ;rbx points to sysmond_request

;read anything with the key "Attributes"

__text:000000010000315A mov rax, cs:_SYSMON_XPC_KEY_ATTRIBUTES_ptr

__text:0000000100003161 mov rsi, [rax]

__text:0000000100003164 mov rdi, r14

__text:0000000100003167 call _xpc_dictionary_get_value

__text:000000010000316C mov [rbx+28h], rax

... continues parsing more fields

The sysmond_request is returned from this function and passed as the first argument to sub_10000337D:

__text:000000010000337D sub_10000337D proc near ; CODE XREF: sub_100001AAF+4Bp

__text:000000010000337D

__text:000000010000337D var_38 = qword ptr -38h

__text:000000010000337D var_30 = dword ptr -30h

__text:000000010000337D var_2C = dword ptr -2Ch

__text:000000010000337D var_28 = qword ptr -28h

__text:000000010000337D var_20 = qword ptr -20h

__text:000000010000337D var_18 = qword ptr -18h

__text:000000010000337D

__text:000000010000337D push rbp

__text:000000010000337E mov rbp, rsp

__text:0000000100003381 push r14

__text:0000000100003383 push rbx

__text:0000000100003384 sub rsp, 30h

__text:0000000100003388 mov rbx, rdi ; sysmond_request pointer

__text:000000010000338B mov rdi, [rbx+20h] ; "Type" uint64 value in the xpc request dictionary

__text:000000010000338F mov rsi, [rbx+28h] ; "Attributes" value in the xpc request dictionary

__text:0000000100003393 call sub_100003454

this function extracts the Type and Attribute values and passes them to sub_100003454:

__text:0000000100003454 sub_100003454 proc near ; CODE XREF: sub_10000337D+16p

__text:0000000100003454 ; handler+C0 p

__text:0000000100003454 push rbp

__text:0000000100003455 mov rbp, rsp

__text:0000000100003458 push r15

__text:000000010000345A push r14

__text:000000010000345C push r12

__text:000000010000345E push rbx

__text:000000010000345F mov r12, rsi ; this is "Attributes" value

__text:0000000100003462 mov r14, rdi ; which was read from the dictionary with xpc_dictionary_get_value

__text:0000000100003465 mov rdi, r12 ; meaning it could be any xpc type

__text:0000000100003468 call _xpc_data_get_length ; use "Attributes" value as an xpc_data object

__text:000000010000346D mov r15, rax

__text:0000000100003470 mov rdi, r15 ; size_t

__text:0000000100003473 call _malloc

__text:0000000100003478 mov rbx, rax

__text:000000010000347B mov rdi, r12

__text:000000010000347E mov rsi, rbx

__text:0000000100003481 xor edx, edx

__text:0000000100003483 mov rcx, r15

__text:0000000100003486 call _xpc_data_get_bytes ; use "Attributes" value again interpreted as an xpc_data

the xpc_data_get_bytes call is the interesting one:

__text:00000000000114BE _xpc_data_get_bytes proc near

__text:00000000000114BE push rbp

__text:00000000000114BF mov rbp, rsp

...

__text:00000000000114D2 mov r14, rsi

__text:00000000000114D5 mov r13, rdi

__text:00000000000114D8 cmp qword ptr [r13+28h], 0FFFFFFFFFFFFFFFFh

__text:00000000000114DD jnz short loc_11515

...

__text:0000000000011515 lea rdi, [r13+28h] ; predicate

__text:0000000000011519 lea rdx, __xpc_data_map_once ; function

__text:0000000000011520 mov rsi, r13 ; context

__text:0000000000011523 call _dispatch_once_f

here, if the value at +28h isn't -1 then our xpc object will be passed as the context to __xpc_data_map_once:

__text:00000000000028E9 __xpc_data_map_once proc near ; DATA XREF: _xpc_data_get_bytes_ptr+1Fo

__text:00000000000028E9 ; __xpc_data_equal+46ao ...

__text:00000000000028E9 push rbp

__text:00000000000028EA mov rbp, rsp

__text:00000000000028ED push r14

__text:00000000000028EF push rbx

__text:00000000000028F0 mov rbx, rdi ; controlled xpc object

__text:00000000000028F3 cmp byte ptr [rbx+48h], 0 ; if the byte at +48h is 0

__text:00000000000028F7 jnz short loc_291E

__text:00000000000028F9 mov rdi, [rbx+30h] ; then pass the pointer at +30h

__text:00000000000028FD lea rsi, [rbx+38h]

__text:0000000000002901 lea rdx, [rbx+40h]

__text:0000000000002905 call _dispatch_data_create_map ; to dispatch_data_create_map

__text:000000000000290A mov r14, rax

__text:000000000000290D mov rdi, [rbx+30h] ; object

__text:0000000000002911 call _dispatch_release ; and then to dispatch_release

we can return early from dispatch_data_create_map by setting the value at +28h from the pointer passed as the first arg to 0:

__text:00000000000012B6 _dispatch_data_create_map proc near ; CODE XREF: __dispatch_data_subrange_map+34p

__text:00000000000012B6 ; __dispatch_operation_perform+DEap

__text:00000000000012B6

__text:00000000000012B6 push rbp

__text:00000000000012B7 mov rbp, rsp

__text:00000000000012BA push r15

__text:00000000000012BC push r14

__text:00000000000012BE push r13

__text:00000000000012C0 push r12

__text:00000000000012C2 push rbx

__text:00000000000012C3 sub rsp, 38h

__text:00000000000012C7 mov [rbp+var_58], rdx

__text:00000000000012CB mov r15, rsi

__text:00000000000012CE mov r14, rdi

__text:00000000000012D1 mov r12, [r14+28h] ; if this is 0

__text:00000000000012D5 test r12, r12

__text:00000000000012D8 jz short loc_131C ; jumps to early return without disturbing anything else

we then reach the call to dispatch_release which is passing the pointer at +30h of the xpc object we control (the API believes this is an xpc_data object)

this ends up calling _dispatch_objc_release which sends the objective c "release" message to the object.

We'll come back to how to get code code execution from that later.

The crux of the bug is that the value of the "Attributes" key in the request dictionary is never validated to actually be an xpc_data object and the gets

passed to functions expecting an xpc_data. In order to exploit this we need to have a value of a type other than xpc_data as the "Attributes" value

in the request dictionary - specifically one where the offsets outlined above have suitably controlled values:

+28h qword 0

+30h pointer to controlled data

+48h byte 0

the xpc_uuid type comes the closest to fulfilling these requirements. We completely control the 16 bytes from +28h so the first two constraints are easily

satisfied. Heap spraying is very reliable and fast in xpc, we can easily map a gigabyte of data into sysmond at a predicable address so we can point the

pointer at +30h to that.

The xpc_uuid object is only 40h bytes though, so we have no control over the byte at +48h which must be 0...

OS X uses magazine malloc which is a heap-based allocator. It has three broad size classes (x<1k = tiny; 1k<x<15k = small; x>15k = large) and within these

it will allocate approximately contiguously (using size-based free-lists to speed things up) with no inline-metadata which means there's a

reasonable expectation that sequential allocations of similar sizes will be contiguous.

Our xpc_uuid object is allocated when the request dictionary is received, so what's the next thing which is allocated?

xpc_dictionaries have 6 hash buckets which store the heads of linked-lists for each bucket. As the dictionary is being deserialized first the value of a

key is deserialized (allocating in this case the xpc_uuid) object then the entry is added to the linked-list (allocting a new linked-list entry struct.)

The structure of a linked-list entry is approximately:

struct ll {

struct ll* forward;

struct ll* backward;

xpc_object_t* object;

uint64_t flags;

char key[0];

}

This is a variable-size struct - the key is allocated inline. If the xpc_uuid is immediately followed in memory by its linked-list entry the the value at +48

will be the least-significant byte of the backward linked-list pointer. Our only requirement is that this byte be 0, which is easily achieved by ensuring that

the previous linked-list entry struct in the list (which this linked-list entry points to) was allocated with an alignment of at least 256 bytes.

The magazine malloc "small" size class heap chunks all have an alignment

of 512 bytes meaning that we just need the linked-list entry prior to the xpc_uuid to be between 1k and 15k. In order for the key to end up in the right linked-list

when it's deserialized we also need to make sure that the long key hashes to the same hash as "Attributes" - since there are only 6 possible hash values this is trivial.

Finally, we can add another xpc_data object to the reqest dictionary with a gigabyte of heapspray as the value - this will be mapped into sysmond at a suitably

predictable address meaning we can set the high 8 bytes of the uuid value to point to this.

At this point we control a pointer to an objective-c object and the code will call objc_msgSend to "send a message" to our controlled object, which is the

objective-c paradigm for calling methods. Let's look at the implementation of this to see how we can turn that into instruction pointer control:

__text:000000000000117F __dispatch_objc_release proc near ; CODE XREF: _dispatch_release:loc_117Aj

__text:000000000000117F ; _dispatch_data_create_subrange+183_p ...

__text:000000000000117F mov rax, rdi

__text:0000000000001182 cmp cs:__os_object_have_gc, 0

__text:0000000000001189 jnz short loc_119E

__text:000000000000118B mov rcx, cs:msgRef_release__objc_msgSend_fixup

__text:0000000000001192 lea rsi, msgRef_release__objc_msgSend_fixup

__text:0000000000001199 mov rdi, rax

__text:000000000000119C jmp rcx

rdi points to our heap sprayed fake objective-c object. This code sets rsi to point to the msgRef_release__objc_msgSend_fixup structure then calls the value at that address

which is objc_msgSend_fixup. msgRef_release__objc_msgSend_fixup is in the __objc_msgrefs section of the data segment and in lldb we can see that at runtime is has the following

contents:

{ /usr/lib/libobjc.A.dylib`objc_msgSend_fixedup, "release" }

and the implementation of objc_msgSend_fixedup is:

(lldb) disassemble --name objc_msgSend_fixedup

libobjc.A.dylib`objc_msgSend_fixedup:

0x7fff91d5d1c4: mov RSI, QWORD PTR [RSI + 8]

0x7fff91d5d1c8: jmpq 0x7fff91d5d080 ; objc_msgSend

which just calls through to objc_msgSend passing the address of the "release" string as the second argument:

(lldb) disassemble --name objc_msgSend

libobjc.A.dylib`objc_msgSend:

0x7fff91d5d080: test RDI, RDI

0x7fff91d5d083: je 0x7fff91d5d0f8

0x7fff91d5d086: test DIL, 1

0x7fff91d5d08a: jne 0x7fff91d5d10f

0x7fff91d5d091: mov R11, QWORD PTR [RDI] ; rdi points to controlled fake objective-c object - read pointer to objective-c class

0x7fff91d5d094: mov R10, RSI ; copy selector (pointer to string of method to call) to r10

0x7fff91d5d097: and R10D, DWORD PTR [R11 + 24] ; mask off n upper bits of the pointer according to value of fake_class+18h

0x7fff91d5d09b: shl R10, 4 ;

0x7fff91d5d09f: add R10, QWORD PTR [R11 + 16] ; use that masked off value as an index into a cache array pointed to by fake_class+10h

0x7fff91d5d0a3: cmp RSI, QWORD PTR [R10] ; does the cache entry selector match the selector passed as the second arg?

0x7fff91d5d0a6: jne 0x7fff91d5d0ac

0x7fff91d5d0a8: jmp QWORD PTR [R10 + 8] ; if so, then call the cached function implementation address

Objective-c classses cache the addresses of the selector strings, not the contents of the strings so in order to exploit this we need to be able

to find the address of the "release" selector passed by _dispatch_objc_release so we can construct a fake selector cache.

All these libraries are loaded at the same address in all processes so we can just find the selector address in this process and it'll be valid for sysmond.

Having done this we get instruction pointer control. At this point rax and rdi point to the heap spray so this PoC uses a pivot gadget in CoreFoundation

to move the stack to point into the heap spray and ROP to a system() call with controlled string

(the poc does "touch /tmp/hello_root" as root )

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[TD] sysmond_exploit_writeup.c

19.6 KB Download[/TD]

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Sursa: https://code.google.com/p/google-security-research/issues/detail?id=121