RFC-0080: Detecting VMO Immutability

RFC-0080: Detecting VMO Immutability
  • Kernel

This RFC proposes a new flag `ZX_INFO_VMO_IMMUTABLE` that makes it possible to determine if a VMO is immutable and has been since creation.

Date submitted (year-month-day)2021-02-18
Date reviewed (year-month-day)2021-03-29


This RFC proposes a new flag ZX_INFO_VMO_IMMUTABLE that makes it possible to determine if a VMO is immutable and has been since creation.


There are three main motivations for adding a mechanism to check for immutability.

Improving the safety and security of using shared memory

It is difficult to write secure and correct programs that use shared memory. Some programs don't need fully mutable VMOs - they might write to a VMO once and then only read from it.

If it is possible to test for restrictions like immutability on the VMO, then it is easier to reduce the security surface and verify correctness.

Large message support in FIDL

An explicit choice was made to limit channel messages to 64k and make it the responsibility of applications that need larger sizes to find workarounds.

In the future, FIDL might provide support for large messages. One potential way to handle this is to introduce boxing of messages as out-of-line VMOs. A FIDL definition that includes Box<Type> would have that section of the message copied to a VMO automatically by FIDL.

Another way to handle it might be to have all FIDL messages above 64k automatically be sent in a VMO. This is the approach taken in a withdrawn RFC.

In both cases, the recipient of the message has no way to verify that the message won't be mutated as they are reading it. If there was a way to confirm that the VMO wouldn't be modified, it is easier to verify that the VMO is used safely and may also lead to optimization opportunities.


Overnet is a channel-level proxy that transports messages over the network.

Overnet diagram

Overnet is a transparent proxy. Overnet clients send channel messages the same way they would to a local process. The messages are then serialized by Overnet, sent over the network, and reconstructed on the other side. The receiving process receives the message the same way they would any other channel message.

The application using Overnet sees the following:

View from the application perspective

Overnet doesn't currently support sending messages containing VMOs. This has been a pain point for several Overnet clients that have needed to send messages larger than the existing 64k channel limit and would otherwise send large chunks of data in VMOs (wrapped in the fuchsia.mem.Buffer or fuchsia.mem.Data FIDL type).

Within a single machine, VMOs are passed by reference regardless of whether they are mutable or immutable -- that is, sending a VMO in a channel will not cause Zircon to implicitly copy it. This is simple and efficient to implement. However, pass by reference between multiple machines over the network would be inefficient and impractical. Instead, pass by copy is suitable for these scenarios.

The challenge is that pass by copy is not always safe to use. Because Overnet is a transparent proxy, remote processes expect references to the same VMO to be in sync. Pass by copy changes the semantics of VMOs and can subtly break applications. For example, if a hypothetical media player process sends a VMO to a remote speaker process and then writes a song to the VMO, the speaker will never receive the song or be able to play it because the song wasn't present in the VMO when it was first sent.

Fortunately, there are some cases where pass by copy is safe. One such case is when the VMO arriving at the Overnet proxy is immutable. The application has already given up its ability to mutate the data so it is guaranteed that any copies that are made of the data will remain consistent.

If Overnet is able to detect immutable VMOs, then it can fail fast when it encounters a mutable VMO. In doing so, it can avoid subtle bugs that would cause applications to silently break.


Add a new ZX_INFO_VMO_IMMUTABLE flag to the flags field of zx_info_vmo_t, indicating if the VMO is immutable in a way that can be guaranteed and verified.

Today, a VMO is immutable iff it was created with the zx_vmo_create_child syscall with the ZX_VMO_CHILD_SNAPSHOT and ZX_VMO_CHILD_NO_WRITE flags set and the ZX_VMO_RESIZABLE and ZX_VMO_DISCARDABLE flags unset. However, the formula for determining if a VMO is immutable could change in the future if new ways of creating immutable VMOs are introduced.

Determining when to make a VMO immutable is left to the application.


The implementation is relatively straightforward. Most of the state needed to compute the value of ZX_INFO_VMO_IMMUTABLE is already stored in the kernel.

The exception to this is a bit indicating that the VMO was not writable at creation time. This can be stored in VmObjectDispatcher.


There is no direct performance impact from adding the flag.

However, creating an immutable VMO currently has a non-insignificant cost and should be used only where strictly needed. In the case of Overnet, the relative overhead of the immutable clone may be less significant because Overnet is already doing significant work to send a message, including network communication.

Two benchmarks were run and compared to further understand the cost of immutable clones (CL):

  • Create VMO, Write, Read, Close
  • Create VMO, Write, Immutable Clone, Read Cloned VMO, Close

It is important to emphasize that this benchmark is reflective of performance today, but there is room to optimize clones - particularly for the immutable clone case.

This benchmark was recorded on an Intel NUC NUC7i5DNHE.

Effect of snapshot clone

Up until 64 KiB, there is a 2 us cost for performing an immutable clone, after which it starts to increase. After 2 MiB, the cost increases significantly.

It is worth noting, however, that the relative cost of clone compared to the overall create-write-clone-read-destroy flow decreases as the size increases:

Relative effect of snapshot clone

For a 64k VMO, the immutuable clone adds an extra 20% to the time - a fairly significant overhead, but with larger sizes the overhead gradually decreases towards zero.

Security considerations

It is important to verify that VMOs claimed to be immutable through this technique are actually immutable. It would be a security bug if a VMO is falsely identified as immutable.

Privacy considerations

No impact.


The flag implementations will be verified through unit tests.


Today, documentation is missing on the ZX_INFO_VMO_* flags and the reader is referred to the repo. As part of this change, documentation will be written to describe ZX_INFO_VMO_* options.

Drawbacks, alternatives, and unknowns

Multiple Flags for Different Properties that Determine Immutability

This RFC proposes using a single ZX_INFO_VMO_IMMUTABLE flag to determine immutability. Instead, several separate flags could be used on the user's end to determine this:


The benefit of having multiple flags based on underlying state is that a specialized flag isn't needed for immutability and only base properties are exposed that can be used in different combinations.

However, it is relatively complex to determine immutability through these flags and immutability can be thought of as a fundamental property on its own. By having a dedicated flag for immutability, it is easier to avoid security bugs. Further, a single flag makes it easier to introduce new methods of creating immutable VMOs.

Checking for last-remaining reference

It may be tempting to use ZX_INFO_HANDLE_COUNT to check whether the process holds the last handle to a VMO and assume that if it does, no other process can mutate the VMO. However, that assumption would not be correct, because handles are only one type of reference in Zircon. For example, a memory mapping of a VMO does not contribute towards the ZX_INFO_HANDLE_COUNT value, so a process cannot use this value to determine whether it has exclusive access to a VMO.

Communicating safety to Overnet via FIDL

A change could be made to the FIDL language making it possible to mark that a VMO is immutable or "Overnet-able". If Overnet is able to tell if a given VMO has this property, then it can decide whether to send the VMO or reject it.

There are a number of options for communicating this information to Overnet:

  • Send it in an unused bit on the handle
  • Add a mechanism to store bits on the VMO that Overnet can read
  • Reserve 64 bits in the FIDL message header to mark VMO handles as immutable
  • Have Overnet communicate with the client via a protocol

Unfortunately, these all have downsides that make them complex and not preferable to the proposed solution.

Prior art and references

VMO Sealing Abandoned RFC proposed a mechanism to restrict actions that can be performed on VMOs. This could be used instead of cloning to create and test for an immutable VMO. However, VMO Sealing is much more invasive to the VMO implementation than a simple check for immutability.

Recently, there has been work on safety features for zx_vmar_map, for instance ZX_VM_ALLOW_FAULTS. It may be desirable to add features dealing with immutability to zx_vmar_map in the future.