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XNU - Remote Double-Free via Data Race in IPComp Input Path


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=== Summary ===
This report describes a bug in the XNU implementation of the IPComp protocol
(https://tools.ietf.org/html/rfc3173). This bug can be remotely triggered by an
attacker who is able to send traffic to a macOS system (iOS AFAIK isn't
affected) *over two network interfaces at the same time*.

=== Some basics to provide context ===
IPComp is a protocol for compressing the payload of IP packets.

The XNU implementation of IPComp is (going by the last public XNU release)
enabled only on X86-64; ARM64 doesn't seem to have the feature enabled at all
(look for ipcomp_zlib in config/MASTER.x86_64 and config/MASTER.arm64). In other
words, it's enabled on macOS and disabled on iOS.

While IPComp is related to IPsec, the IPComp input path processes input even
when the user has not configured any IPsec stuff on the system.

zlib requires fairly large buffers for decompression and especially for
compression. In order to avoid allocating such buffers for each packet, IPComp
uses two global z_stream instances "deflate_stream" and "inflate_stream".
If IPComp isn't used, the buffer pointers in these z_stream instances remain
NULL; only when IPComp is actually used, the kernel will attempt to initialize
the buffer pointers.

As far as I can tell, the IPComp implementation of XNU has been completely
broken for years, which makes it impossible to actually reach the decompression
code. ipcomp_algorithm_lookup() is responsible for allocating global buffers for
the compression and decompression code; however, all of these allocations go
through deflate_alloc(), which (since xnu-1228, which corresponds to macOS 10.5
from 2007) calls _MALLOC() with M_NOWAIT. _MALLOC() leads to kalloc_canblock(),
which, if the M_NOWAIT flag was set and the allocation is too big for a kalloc
zone (size >= kalloc_max_prerounded), immediately returns NULL. On X86-64,
kalloc_max_prerounded is 0x2001; both deflateInit2() and inflateInit2() attempt
allocations bigger than that, causing them to fail with Z_MEM_ERROR, as is
visible with dtrace when observing the system's reaction to a single incoming
IPComp packet [empty lines removed]:

bash-3.2# ./inflate_test.dtrace
dtrace: script './inflate_test.dtrace' matched 11 probes
CPU     ID                    FUNCTION:NAME
  0 243037              deflateInit2_:entry deflate init (thread=ffffff802db84a40)
  0 224285            kalloc_canblock:entry kalloc_canblock(size=0x1738, canblock=0, site=ffffff8018e787e8)
  0 224286           kalloc_canblock:return kalloc_canblock()=0xffffff80496b9800
  0 224285            kalloc_canblock:entry kalloc_canblock(size=0x2000, canblock=0, site=ffffff8018e787e8)
  0 224286           kalloc_canblock:return kalloc_canblock()=0xffffff802f42f000
  0 224285            kalloc_canblock:entry kalloc_canblock(size=0x2000, canblock=0, site=ffffff8018e787e8)
  0 224286           kalloc_canblock:return kalloc_canblock()=0x0
  0 224285            kalloc_canblock:entry kalloc_canblock(size=0x20000, canblock=0, site=ffffff8018e787e8)
  0 224286           kalloc_canblock:return kalloc_canblock()=0x0
  0 224285            kalloc_canblock:entry kalloc_canblock(size=0x20000, canblock=0, site=ffffff8018e787e8)
  0 224286           kalloc_canblock:return kalloc_canblock()=0x0
  0 243038             deflateInit2_:return rval=0xfffffffc
  0 243073              inflateInit2_:entry inflate init (thread=ffffff802db84a40)
  0 224285            kalloc_canblock:entry kalloc_canblock(size=0x2550, canblock=0, site=ffffff8018e787e8)
  0 224286           kalloc_canblock:return kalloc_canblock()=0x0
  0 243074             inflateInit2_:return rval=0xfffffffc

(On iOS, the kalloc() limit seems to be higher, so if IPComp was built there,
the input path might actually work?)

=== main bug description ===
IPComp uses a single global `static z_stream inflate_stream` for decompressing
all incoming packets. This global is used without any locking. While processing
of packets from a single interface seems to be single-threaded, packets arriving
on multiple ethernet interfaces at the same time (or on an ethernet interface
and a non-ethernet interface) can be processed in parallel (see
dlil_create_input_thread() and its caller for the precise threading rules).
Since zlib isn't designed for concurrent use of a z_stream, this leads to memory

If IPComp actually worked, I believe that this bug would lead to
things like out-of-bounds reads, out-of-bounds writes and use-after-frees.
However, since IPComp never actually manages to set up the compression and
decompression state, the bug instead manifests in the code that, for every
incoming IPComp packet, attempts to set up the deflate buffers and tears down
the successfully allocated buffers because some of the allocations failed:

deflateInit2_(z_streamp strm, int  level, int  method, int  windowBits,
              int  memLevel, int  strategy, const char *version,
              int stream_size)
    if (memLevel < 1 || memLevel > MAX_MEM_LEVEL || method != Z_DEFLATED ||
        windowBits < 8 || windowBits > 15 || level < 0 || level > 9 ||
        strategy < 0 || strategy > Z_FIXED) {
        return Z_STREAM_ERROR;
    if (windowBits == 8) windowBits = 9;  /* until 256-byte window bug fixed */
    s = (deflate_state *) ZALLOC(strm, 1, sizeof(deflate_state));
    if (s == Z_NULL) return Z_MEM_ERROR;
    strm->state = (struct internal_state FAR *)s;
    s->window = (Bytef *) ZALLOC(strm, s->w_size, 2*sizeof(Byte));
    s->prev   = (Posf *)  ZALLOC(strm, s->w_size, sizeof(Pos));
    s->head   = (Posf *)  ZALLOC(strm, s->hash_size, sizeof(Pos));

    s->lit_bufsize = 1 << (memLevel + 6); /* 16K elements by default */

    overlay = (ushf *) ZALLOC(strm, s->lit_bufsize, sizeof(ush)+2);
    s->pending_buf = (uchf *) overlay;
    if (s->window == Z_NULL || s->prev == Z_NULL || s->head == Z_NULL ||
        s->pending_buf == Z_NULL) {
        deflateEnd (strm);
        return Z_MEM_ERROR;
deflateEnd(z_streamp strm)
    /* Deallocate in reverse order of allocations: */
    TRY_FREE(strm, strm->state->pending_buf);
    TRY_FREE(strm, strm->state->head);
    TRY_FREE(strm, strm->state->prev);
    TRY_FREE(strm, strm->state->window);

    ZFREE(strm, strm->state);
    strm->state = Z_NULL;

    return status == BUSY_STATE ? Z_DATA_ERROR : Z_OK;

When multiple executions of this code race, it is possible for two threads to
free the same buffer, causing a double-free:

*** Panic Report ***
panic(cpu 2 caller 0xffffff8012802df5): "zfree: double free of 0xffffff80285d9000 to zone kalloc.8192\n"@/BuildRoot/Library/Caches/com.apple.xbs/Sources/xnu/xnu-4903.261.4/osfmk/kern/zalloc.c:1304
Backtrace (CPU 2), Frame : Return Address
0xffffff912141b420 : 0xffffff80127aea2d mach_kernel : _handle_debugger_trap + 0x47d
0xffffff912141b470 : 0xffffff80128e9e95 mach_kernel : _kdp_i386_trap + 0x155
0xffffff912141b4b0 : 0xffffff80128db70a mach_kernel : _kernel_trap + 0x50a
0xffffff912141b520 : 0xffffff801275bb40 mach_kernel : _return_from_trap + 0xe0
0xffffff912141b540 : 0xffffff80127ae447 mach_kernel : _panic_trap_to_debugger + 0x197
0xffffff912141b660 : 0xffffff80127ae293 mach_kernel : _panic + 0x63
0xffffff912141b6d0 : 0xffffff8012802df5 mach_kernel : _zcram + 0xa15
0xffffff912141b710 : 0xffffff8012804d4a mach_kernel : _zfree + 0x67a
0xffffff912141b7f0 : 0xffffff80127bac58 mach_kernel : _kfree_addr + 0x68
0xffffff912141b850 : 0xffffff8012dfc837 mach_kernel : _deflateEnd + 0x87
0xffffff912141b870 : 0xffffff8012dfc793 mach_kernel : _deflateInit2_ + 0x253
0xffffff912141b8c0 : 0xffffff8012c164a3 mach_kernel : _ipcomp_algorithm_lookup + 0x63
0xffffff912141b8f0 : 0xffffff8012c16fb2 mach_kernel : _ipcomp4_input + 0x112
0xffffff912141b990 : 0xffffff8012b89907 mach_kernel : _ip_proto_dispatch_in_wrapper + 0x1a7
0xffffff912141b9e0 : 0xffffff8012b8bfa6 mach_kernel : _ip_input + 0x18b6
0xffffff912141ba40 : 0xffffff8012b8a5a9 mach_kernel : _ip_input_process_list + 0xc69
0xffffff912141bcb0 : 0xffffff8012aac3ed mach_kernel : _proto_input + 0x9d
0xffffff912141bce0 : 0xffffff8012a76c41 mach_kernel : _ether_attach_inet + 0x471
0xffffff912141bd70 : 0xffffff8012a6b036 mach_kernel : _dlil_rxpoll_set_params + 0x1b36
0xffffff912141bda0 : 0xffffff8012a6aedc mach_kernel : _dlil_rxpoll_set_params + 0x19dc
0xffffff912141bf10 : 0xffffff8012a692e9 mach_kernel : _ifp_if_ioctl + 0x10d9
0xffffff912141bfa0 : 0xffffff801275b0ce mach_kernel : _call_continuation + 0x2e

BSD process name corresponding to current thread: kernel_task
Boot args: -zp -v keepsyms=1

Mac OS version:

Kernel version:
Darwin Kernel Version 18.6.0: Thu Apr 25 23:16:27 PDT 2019; root:xnu-4903.261.4~2/RELEASE_X86_64
Kernel UUID: 7C8BB636-E593-3CE4-8528-9BD24A688851
Kernel slide:     0x0000000012400000
Kernel text base: 0xffffff8012600000
__HIB  text base: 0xffffff8012500000
System model name: Macmini7,1 (Mac-XXXXXXXXXXXXXXXX)

=== Repro steps ===
You'll need a Mac (I used a Mac mini) and a Linux workstation.
Stick two USB ethernet adapters into the Mac.
Make sure that your Linux workstation has two free ethernet ports; if it
doesn't, also stick USB ethernet adapters into your workstation.
Take two ethernet cables; for both of them, stick one end into the Linux
workstation and the other end into the Mac.
Set up static IP addresses for both interfaces on the Linux box and the Mac. I'm

 - Linux, first connection:
 - Mac, first connection:
 - Linux, second connection:
 - Mac, second connection:

On the Linux workstation, ping both IP addresses of the Mac, then dump the
relevant ARP table entries:

$ ping -c1
PING ( 56(84) bytes of data.
64 bytes from icmp_seq=1 ttl=64 time=0.794 ms
$ ping -c1
PING ( 56(84) bytes of data.
64 bytes from icmp_seq=1 ttl=64 time=0.762 ms
$ arp -n | egrep '192\.168\.25[01]'            ether   aa:aa:aa:aa:aa:aa   C                     eth0            ether   bb:bb:bb:bb:bb:bb   C                     eth1

On the Linux workstation, build the attached ipcomp_uaf.c and run it:

$ gcc -o ipcomp_recursion ipcomp_recursion.c -Wall
$ sudo bash
# ./ipcomp_uaf
usage: ./ipcomp_uaf <if1> <target_mac1> <src_ip1> <dst_ip1> <if2> <target_mac2> <src_ip2> <dst_ip2>
# ./ipcomp_uaf eth0 aa:aa:aa:aa:aa:aa eth1 bb:bb:bb:bb:bb:bb

After something like a second, you should be able to observe that the Mac panics.
I have observed panics via double-free and via null deref triggered by the PoC.
(Stop the PoC afterwards, otherwise it'll panic again as soon as the network
interfaces are up.)

(The PoC also works if you use broadcast addresses as follows: ```
# ./ipcomp_uaf eth0 ff:ff:ff:ff:ff:ff eth1 ff:ff:ff:ff:ff:ff

=== Fixing the bug ===
I believe that by far the best way to fix this issue is to rip out the entire
feature. Unless I'm missing some way for the initialization to succeed, it looks
like nobody can have successfully used this feature in the last few years; and
apparently nobody felt strongly enough about that to get the feature fixed.
At the same time, this thing is remote attack surface in the IP stack, and it
looks like it has already led to a remote DoS bug in the past - the first search
result on bing.com for both "ipcomp macos" and "ipcomp xnu" is

In case you decide to fix the bug in a different way, please note:

 - I believe that this can *NOT* be fixed by removing the PR_PROTOLOCK flag from
   the entries in `inetsw` and `inet6sw`. While removal of that flag would cause
   the input code to take the domain mutex before invoking the protocol handler,
   IPv4 and IPv6 are different domains, and so concurrent processing of
   IPv4+IPComp and IPv6+IPComp packets would probably still trigger the bug.
 - If you decide to fix the memory allocation of IPComp so that the input path
   works again (please don't - you'll never again have such a great way to prove
   that nobody is using that code), I think another bug will become reachable:
   I don't see anything that prevents unbounded recursion between
   ip_proto_dispatch_in() and ipcomp4_input() using an IP packet with a series
   of IPComp headers, which would be usable to cause a kernel panic via stack
   overflow with a single IP packet.
   In case you want to play with that, I wrote a PoC that generates packets with
   100 such headers and attached it as ipcomp_recursion.c.
   (The other IPv6 handlers for pseudo-protocols like IPPROTO_FRAGMENT seem to
   avoid this problem by having the )

Proof of Concept:
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