Late in 2015 I was looking for a way to create an instance of an IOKit user client with a visible NULL pointer dereference when I discovered something intriguing: the default implementation of IOService::newUserClient checks the IOUserClientClass property on the service when determining what user client class to allocate. This caught my attention because IOKit provides an API to set arbitrary properties on an IOService from user space. If any IOService allowed setting the IOUserClientClass property, that would create an opportunity for kernel code execution.

I immediately started looking for setProperty calls with attacker-controlled keys and values. Amazingly, I found that IOHIDevice would iterate the attacker-supplied properties dictionary and indiscriminately add each key-value pair to its own set of properties. This post is about how I leveraged this vulnerability to gain read/write access to physical memory from user space, and how this awesome primitive can be used to get fully reliable kernel code execution.

I reported this issue to Apple in January of 2016, and it was assigned CVE-2016-1825. It was fixed in OS X El Capitan 10.11.5. A proof-of-concept exploit for this vulnerability (and the variant CVE-2016-7617) is available in my physmem repository on GitHub. This vulnerability is not present on iOS.

The vulnerability: CVE-2016-1825

Arbitrary properties can be associated with an IORegistryEntry using the methods setProperty and setProperties. The default implementation of setProperties just returns an error. However, subclasses of IORegistryEntry can override setProperties to allow user programs to set properties via the io_registry_entry_set_properties Mach trap.

CVE-2016-1825 is a vulnerability in the method IOHIDevice::setParamProperties from IOHIDFamily.kext. This method iterates a dictionary of key-value pairs and calls setProperty with each pair:

The issue is that the keys and values in the dictionary are entirely attacker-controlled, which means a user program can set arbitrary IOKit registry properties on an instance of IOHIDevice.

This is dangerous because several IOKit properties are used to store privileged state that should not be modifiable from user space. For example, some IOServices store the name of their user client class in the registry under a property called IOUserClientClass. The default implementation of IOService::newUserClient checks this property when allocating a new user client:

Thus, a user space application can set the IOUserClientClass property on an instance of IOHIDevice and then allocate an instance of any subclass of IOUserClient within the kernel. This is clearly a serious issue. However, finding the best exploitation strategy requires a little more digging.

Finding the right user client

Many user clients override IOUserClient::initWithTask to perform additional checks on the user task that is creating the connection. For instance, IOHIDEventSystemUserClient::initWithTask checks that the user task has administrator privileges, and fails initialization if not. Thus, the most interesting user clients will be those that don’t perform these types of security checks within initWithTask.

My first thought was to find a user client that accesses its provider (which is passed to the client through the IOUserClient::start method) without dynamically checking the provider’s type. For instance, the IOFramebufferUserClient class performs a C-style cast of its provider in its start method and then sets a field in the provider:

This write could be problematic in a few different ways. If serverConnect is at a sufficiently large offset in IOFramebuffer, it might overwrite memory in the subsequent object on the heap. Furthermore, even if serverConnect is within the bounds of the IOHIDevice object, it may overlap with a sensitive field, such that the write corrupts the object and leads to an exploitable condition.

After identifying several promising type confusions, I eventually happened upon a user client called IOPCIDiagnosticsClient, in IOPCIFamily.kext. I suspect IOPCIDiagnosticsClient is meant to be used to debug the PCI bridge. You can see examples of its usage in a tool called pcidump.

IOPCIDiagnosticsClient is implemented in IOPCIBridge.cpp. As you can see below, its externalMethod will read and write physical memory if the spaceType parameter is kIOPCI64BitMemorySpace.

There are checks in place to ensure that IOPCIDiagnosticsClient can only be used by an authorized user. Specifically, IOPCIBridge::newUserClient checks that the caller has administrator privileges and that the debug boot argument is set before instantiating the client. However, these checks are not performed in IOPCIDiagnosticsClient::initWithTask, so they are completely bypassed when allocating this user client via the method above.

Accessing physical memory

My next step was to write a proof-of-concept exploit that would leverage the ability to set arbitrary properties in IOHIDevice instances to directly access physical memory. Creating the connection to IOPCIDiagnosticsClient is quite simple:

Physical memory can then be read or written by calling the appropriate method on the user client:

The initial version of physmem provided a simple command-line tool to read or write words of physical memory. However, with such a powerful (and reliable) read/write primitive, I knew it would be quite easy to get kernel code execution.

Reliable kernel code execution

The first thing I needed to do is figure out how to turn my access to physical memory into access to kernel virtual memory. Fortunately, there’s a lot of information on physical memory analysis online. According to a paper by Matthieu Suiche on BlackHat, virtual addresses within the kernel image map directly to physical addresses, at least for 32-bit systems. After looking at the ID_MAP_VTOP macro in XNU, I concluded that on 64-bit systems, a virtual address within the kernel image can be translated to its corresponding physical address by discarding the upper 32 bits:

This is great for exploitability, since we can parse the kernel image on disk to look up the addresses of kernel symbols, then convert those virtual addresses to their corresponding physical addresses using the formula above. The one issue is bypassing OS X’s implementation of kernel ASLR, which causes the kernel to be loaded at a randomized base address each boot.

As it turns out, we don’t need a separate vulnerability to find the kernel slide. The amount of entropy in the kernel slide is quite low: on the order of 8 or 9 bits, not much more. This means we can simply guess-and-check until we find the right kernel slide. Specifically, for each possible kernel slide in increasing order, we will try to read the kern.bootsessionuuid sysctl variable, and compare the returned data to its known value. It’s highly unlikely that reading random physical memory will return that exact UUID string, so if we find a kernel slide that works, it is overwhelmingly likely that it is correct.1

Thus, we know the kernel slide and we have a read/write primitive within the kernel image (but not the heap or anywhere else). The next step is figuring out how to execute arbitrary code in the kernel.

The customary approach in this situation is patching the system call table (called sysent in the code) to add a new system call. By overwriting an unused sysent and pointing it to a dispatch stub we inject into the kernel, we can call any function in the kernel with up to five arguments and get the result back in user space.2

Apple has defended against patching the system call table by not exporting the _sysent symbol and by placing the table in a readonly region of kernel memory. As far as defenses go, hiding the sysent symbol doesn’t do much: it is quite easy to scan the kernel image to find it. (Just calculate what the first several bytes must be given the first few system calls, then look for that data within the kernel image.) On the other hand, placing the sysent table in readonly memory does make patching more difficult. On iOS, Kernel Patch Protection makes this defense doubly effective, since modifications to the sysent table will trigger a panic. Fortunately we don’t have to deal with KPP on macOS.

Unfortunately for Apple, we can completely bypass the memory protections on the sysent table without doing anything: IOPCIDiagnosticsClient establishes a read-write mapping of the physical address when writing, so we are actually writing to a writable virtual page that happens to map to the same physical page as the readonly sysent table. Memory protections are associated with virtual addresses, not physical addresses, so writing to the writable mapping of the same page doesn’t trigger any sort of memory protection error. Thus, our kernel image read/write primitive can also write to readonly memory for free, rendering all virtual memory protections useless.

To establish an execute primitive, I overwrite the function bsd_init (which is not called after boot) with a dispatch stub that calls the kernel function specified in the first syscall argument with the parameters given by the remaining syscall arguments. I then overwrite an unused sysent to point to bsd_init, at which point I can then call any function in the kernel with up to five arguments.

What’s great about this technique is that it is fully reliable. There’s no memory corruption, no racing, no hardcoded offsets, and no uncertainty. The same exploit strategy works without modification across many different versions of OS X.

Safe privilege escalation

It is worth mentioning the method I use for safe privilege escalation, which I alluded to in my last post but didn’t discuss in detail.

In order to elevate privileges, we need to set the cr_uid, cr_ruid, and cr_svuid fields of the current process’s ucred structure to 0. However, doing this directly is dangerous, because multiple processes can share the same ucred. This means that directly setting these fields can cause other processes to also elevate to root, which is undesirable from a usability perspective and messes up the accounting done in chgproccnt, leading to a panic on reboot.3

Instead, I elect to use the kauth_cred_setsvuidgid kernel API to set the saved UID and GID of the current process to 0. Since this API is not designed to be conveniently called from user space, we need to perform some setup and cleanup to avoid leaking memory.

The first step is obtaining the pointer to the current process’s proc structure using current_proc. Then, we can call kauth_cred_proc_ref with the proc pointer as an argument to add a reference to the current process’s ucred and return a pointer to it. Next, we call kauth_cred_setsvuidgid on the ucred to set the saved UID and GID. This consumes a reference on the supplied ucred, and if the ucred is shared with another process, a new ucred with the saved UID and GID set to 0 is returned.

At this point we have a pointer to a ucred with a saved UID of 0, but the current process’s ucred still has the same number of references as before. In order to ensure that we don’t leak memory, we need to remove a reference on the current process’s ucred. However, the function to do this, kauth_cred_unref, accepts a kauth_cred_t *, which is a pointer to a pointer to a ucred struct. We can’t pass the address of the ucred pointer in the proc struct since if that ucred is released, any references to the current process’s ucred will be a use-after-free. Instead, we need to store a pointer to the old ucred somewhere in memory and then pass the address of that memory to kauth_cred_unref.

We can call IOMalloc (or kalloc, or any other kernel allocator) to allocate a pointer-size region of memory, and then copyin to write the pointer to the old ucred to the allocated memory. We call copyin again to set the ucred pointer in the current proc struct to the new privileged credentials. Finally, we call kauth_cred_unref on the heap-allocated ucred pointer to remove the reference, and then IOFree to free the allocation.

Now, the current process has a saved UID of 0, which means we can call seteuid to set the effective UID to 0.

Variant: CVE-2016-7617

In macOS Sierra 10.12.2, Apple patched a variant of this vulnerability known as CVE-2016-7617. This was a nearly identical issue in AppleBroadcomBluetoothHostController. You can see details of this vulnerability at Project Zero.

Mitigations as of macOS 10.12.2

Starting in macOS Sierra 10.12.2, allocating instances of IOPCIDiagnosticsClient in order to directly access physical memory from user space is more difficult. Apple has placed checks in IOPCIDiagnosticsClient::initWithTask to ensure that the user task initiating the connection is running as root and that the debug boot argument is set. Thus, this technique is largely dead on new systems.

Conclusion

In this post I discussed how I leveraged a logic error in IOKit that allowed writing to privileged IOKit registry properties to establish a kernel execute primitive, and how to use reliable kernel execution to safely elevate privileges. This bug was kind of magical: the stars aligned to make exploitability almost as easy as possible (thank you, IOPCIDiagnosticsClient!). Nowadays, elevating from user to kernel code execution usually takes multiple bugs.

A proof-of-concept privilege escalation is available in my physmem repository. Even though CVE-2016-1825 was patched way back in 10.11.5, the exploit still works up to 10.12.1 due to the variant CVE-2016-7617. Unfortunately, it’s not as easy to exploit the variant in a virtual machine, since AppleBroadcomBluetoothHostController is not loaded unless Bluetooth is present.

Footnotes

1. In theory, there is a chance that any of the physical reads we do before determining the correct kernel slide could trigger a panic, because we are effectively reading random pages of physical memory. However, in practice I’ve never once experienced a panic while using this technique. I have not investigated exactly why this appears safe in practice, but my best guess is that physical memory is mapped contiguously from 0, meaning all physical pages we read will be valid as long as we test kernel slides in increasing order. Furthermore, memory-mapped registers and other dicey regions of virtual memory all seem to reside at different physical addresses than the ones we probe while determining the kernel slide.

This technique does not work as well with a virtual memory read primitive, as compared to a physical memory read primitive, because we are likely to touch an unmapped page during the search, triggering a panic.

2. Each system call can accept up to six 64-bit values passed from user space in the registers rdi, rsi, rdx, r10 (not rcx), r8, and r9. The simple approach is for the dispatch stub to treat the first value as the function to call and the remaining five values as its arguments. We could theoretically pass an arbitrary number of parameters by having the dispatch stub copy in more arguments from user space, but calling arbitrary kernel functions with five arguments is sufficient for our purposes.

3. The tpwn exploit tries to get around this by directly calling chgproccnt to adjust the number of processes. However, tpwn only changes the process count by one, effectively assuming that the current process’s ucred struct is not shared. If the exploit is run under tmux, tpwn’s ucred will be shared with tmux’s, causing the system to panic on reboot.