path: root/chromium/docs/security/mojo.md

# Mojo "Style" Guide

Mojo is Chrome's new IPC system and provides lots of useful abstractions. These
abstractions can make it easier to write code that makes interprocess calls,
but can also add significant complexity. Below are some recommendation from
Mojo and IPC reviewers for best practices.

For questions, concerns, or suggestions, reach out to
[chromium-mojo@chromium.org](https://groups.google.com/a/chromium.org/forum/#!forum/chromium-mojo).

> For legacy IPC, please see [security tips for IPC][security-tips-for-ipc].

[TOC]


## Simplicity

Strive to write simple interfaces. Minimize the amount of cross-process state
that needs to be maintained in sync.

**_Good_**

```c++
interface TeleporterFactory {
  Create(Location start, Location end) => (pending_remote<Teleporter>);
};

interface Teleporter {
  TeleportGoat(Goat) = ();
};
```

**_Bad_**

```c++
interface Teleporter {
  // Bad: comments will need to explicitly call out that both locations need to
  // be bound before calling TeleportGoat()!
  //
  // In addition, if untrustworthy processes can talk to trustworthy processes,
  // the Teleporter implementation will need to also handle the case where the
  // Location objects are not yet bound.
  SetStart(Location);
  SetEnd(Location);
  TeleportGoat(Goat) = ();
};
```

Similarly, strive to make methods focused. Do not overuse optional types.

**_Good_**

```c++
struct TeleporterStats {
  AnimalStats animal_stats;
  FungiStats fungi_stats;
  GoatStats goat_stats;
  PlantStats plant_stats;
};

interface Teleporter {
  TeleportAnimal(Animal) => ();
  TeleportFungi(Fungi) => ();
  TeleportGoat(Goat) = ();
  TeleportPlant(Plant) => ();

  // TeleporterStats will be have a value if and only if the call was
  // successful.
  GetStats() => (TeleporterStats?);
};
```

**_Bad_**

```c++
interface Teleporter {
  // The intent of four optional arguments is unclear: can this call teleport
  // multiple objects of different types at once, or is the caller only
  // supposed to only pass one non-null argument per call?
  Teleport(Animal?, Fungi?, Goat?, Plant?) => ();

  // Does this return all stats if success is true? Or just the categories that
  // the teleporter already has stats for? The intent is uncertain, so wrapping
  // the disparate values into a result struct would be cleaner.
  GetStats() =>
      (bool success, AnimalStats?, FungiStats?, PlantStats?, FungiStats?);
};
```


## Documentation

Mojo structs, interfaces, and methods should all have comments. Make sure the
comments cover the "how" and the "why" of using an interface and its methods,
and not just the "what". Document preconditions, postconditions, and trust: if
an interface is implemented in the browser process and handles requests from
the renderer process, this should be mentioned in the comments. Complex features
should also have an external `README.md` that covers the high-level flow of
information through interfaces and how they interact to implement the feature.

**_Good_**

```c++
// Interface for controlling a teleporter. Lives in the browser process, and
// used to implement the Teleportation over Mojo IPC RFC.
interface Teleporter {
  // Teleportation helpers for different taxonomic kingdoms. Teleportation is
  // not complete until the reply callback is invoked. The caller must NOT
  // release the sender side resources until the reply callback runs; releasing
  // the resources early will cause splinching.
  TeleportAnimal(Animal) => ();
  TeleportFungi(Fungi) => ();
  // Goats require a specialized teleportation channel distinct from
  // TeleportAnimal to ensure goatiness isolation.
  TeleportGoat(Goat) => ();
  TeleportPlant(Plant) => ();

  // Returns current teleporter stats. On failure (e.g. a teleportation
  // operation is currently in progress) a null stats object will be returned.
  GetStats() => (TeleporterStats?);
};
```


## Security

Policy should be controlled solely by the browser process. "Policy" can mean
any number of things, such as sizes, addresses, permissions, URLs, origins,
etc. In an ideal world:

1.  Unprivileged process asks for a capability from the privileged process that
    owns the resource.
1.  Privileged process applies policy to find an implementation for the
    capability.
1.  Unprivileged process performs operations on the capability, constrained in
    scope.

The privileged process must own the capability lifecycle.


### Do not trust less privileged processes

This is the overriding principle for all guidelines in this section. When
receiving data from a less trusted process, treat the data as if it were
generated by a malicious adversary. Message handlers cannot assume that offsets
are valid, calculations won't overflow, et cetera.

In general:

*   the browser process is the most privileged process type and therefore, must
    be maximally suspicious of its IPC inputs
*   the renderer and the ARC++ processes are the least privileged and least
    trustworthy process types
*   other process types, such as GPU and plugin, fall in between

When passing objects up a privilege gradient (from less → more privileged), the
callee must validate the inputs before acting on them. When passing objects
down a privilege gradient, such as from browser → renderer, it is OK for the
callee to trust the caller.

> See also: [Do not Handle Impossible Situations](#Do-not-handle-impossible-situations)


### Do not send unnecessary or privilege-presuming data

> Each `BrowserInterfaceBroker` for frames and workers is strongly associated with an
> origin. Where possible, prefer to use this associated origin rather than
> sending it over IPC. (See <https://crbug.com/734210> and
> <https://crbug.com/775792/>).

For example, the browser process must not (fully) trust the renderer's claims
about origins. The browser process should already know what origin the renderer
is evaluating, and thus should already have this data (for example, see
`RenderFrameHost::GetLastCommittedOrigin()`). Thus, a method that requires
passing an origin from the renderer to the browser process has a conceptual
error, and quite possibly, a vulnerability.

> Note: there are currently subtle races when using `GetLastCommittedOrigin()`
> that will be resolved by fixing <https://crbug.com/729021>.

Similarly, the browser process must not trust the renderer's claims about file
pathnames. It would be unsafe for the browser process to save a downloaded file
to `~/.bashrc` just because the renderer asked. Instead, it would be better for
the browser process to:

1.  Kill the renderer if `basename(pathname) != pathname`, since the renderer is
    obviously compromised if it makes this mistake.
1.  Defang the basename, by removing leading dots, et cetera. Note that the
    definition of proper defanging varies per platform.
1.  Prepend its own parent directory to the basename, e.g. ~/Downloads.

> TODO(https://crbug.com/779196): Even better would be to implement a C++ type
> performs the appropriate sanitizations and recommend its usage directly here.


### Validate privilege-presuming data received over IPC

If it is not possible to avoid sending privilege-presuming data over IPC (see
the previous section), then such data should be verified before being used.

* Browser process:
    - Use `ChildProcessSecurityPolicy`'s methods like
      `CanAccessDataForOrigin` or `CanReadFile` to verify IPC messages
      received from less privileged processes.
    - When verification fails, ignore the IPC and terminate the renderer process
      using `mojo::ReportBadMessage` (or using `mojo::GetBadMessageCallback` for
      messages handled asynchronously).  For legacy IPC, the renderer process
      may be terminated by calling the `ReceivedBadMessage` function (separate
      implementations exist for `//content`, `//chrome` and other layers).


### Do not define unused or unimplemented things

Mojo interfaces often cross privilege boundaries. Having well-defined interfaces
that don't contain stubbed out methods or unused parameters makes it easier to
understand and evaluate the implications of crossing these boundaries. Several
common areas to watch out for:


#### Do use EnableIf to guard platform-specific constructs

Platform-specific functionality should only be defined on the platforms where
it is implemented. Use the Mojo `EnableIf` annotation to guard definitions that
should only be visible in certain build configurations.

**_Good_**
```c++
// GN file:
mojom("view_bindings") {
  // ...

  enabled_features = []
  if (is_android) {
    enabled_features += [ "is_android" ]
  }
}

// mojom definition:
interface View {
  // ...

  [EnableIf=is_android]
  UpdateBrowserControlsState(bool enable_hiding, bool enable_showing,
                             bool animate);
};

// C++ implementation:
class View : public mojom::View {
 public:
  // ...

#if defined(OS_ANDROID)
  void UpdateBrowserControlsState(bool enable_hiding, bool enable_showing,
                                  bool animate);
#endif
};
```

**_Bad_**
```c++
// mojom definition:
interface View {
  // ...

  UpdateBrowserControlsState(bool enable_hiding, bool enable_showing,
                             bool animate);
};

// C++ implementation:
class View : public mojom::View {
 public:
  // ...

#if defined(OS_ANDROID)
  void UpdateBrowserControlsState(bool enable_hiding, bool enable_showing,
                                  bool animate) override;
#else
  void UpdateBrowserControlsState(bool enable_hiding, bool enable_showing,
                                  bool animate) override {
    NOTREACHED();
  }
#endif
};
```

The `EnableIf` annotation can be applied to almost anything: imports,
interfaces, methods, arguments, constants, structs, struct members, enums,
enumerator values, et cetera.


#### Do not define unimplemented methods

Reviewing IPC requires reviewing a concrete implementation of the Mojo
interface, to evaluate how the (possibly untrustworthy) inputs are used, what
outputs are produced, et cetera. If a method is not yet implemented, do not
define it in the interface.

**_Bad_**
```c++
// mojom definition:
interface Spaceship {
  EnterHyperspace();
  ExitHyperspace();
};

// C++ implementation:
class SpaceshipPrototype : public mojom::Spaceship {
  void EnterHyperspace() { /* TODO(dcheng): Implement. */ }
  void ExitHyperspace() { /* TODO(dcheng): Implement. */ }
};
```


#### Do not define placeholder enumerator values

Do not define placeholder enumerator values like `kLast`, `kMax`, `kCount`, et
cetera. Instead, rely on the autogenerated `kMaxValue` enumerator emitted for
Mojo C++ bindings.

For UMA histograms, logging a Mojo enum is simple: simply use the two argument
version of `UMA_HISTOGRAM_ENUMERATION`:

**_Good_**
```c++
// mojom definition:
enum GoatStatus {
  kHappy,
  kSad,
  kHungry,
  kGoaty,
};

// C++:
UMA_HISTOGRAM_ENUMERATION("Goat.Status", status);
```

Using a `kCount` sentinel complicates `switch` statements and makes it harder to
enforce invariants: code needs to actively enforce that the otherwise invalid
`kCount` sentinel value is not incorrectly passed around.

**_Bad_**
```c++
// mojom definition:
enum CatStatus {
  kAloof,
  kCount,
};

// C++
switch (cat_status) {
  case CatStatus::kAloof:
    IgnoreHuman();
    break;
  case CatStatus::kCount:
    // this should never happen
}
```

Defining `kLast` manually results in ugly casts to perform arithmetic:

**_Bad_**
```c++
// mojom definition:
enum WhaleStatus {
  kFail,
  kNotFail,
  kLast = kNotFail,
};

// C++:
UMA_HISTOGRAM_ENUMERATION("Whale.Status", status,
                          static_cast<int>(WhaleStatus::kLast) + 1);
```

For interoperation with legacy IPC, also use `kMaxValue` rather than defining a
custom `kLast`:

**_Good_**
```c++
IPC_ENUM_TRAITS_MAX_VALUE(GoatStatus, GoatStatus::kMaxValue);
```


### Use structured types

Where possible, use structured types: this allows the type system to help
enforce that the input data is valid. Common ones to watch out for:

*   Files: use `mojo_base.mojom.File`, not raw descriptor types like `HANDLE`
    and `int`.
*   File paths: use `mojo_base.mojom.FilePath`, not `string`.
*   JSON: use `mojo_base.mojom.Value`, not `string`.
*   Mojo interfaces: use `Interface` or `Interface&`, not `handle` or
    `handle<message_pipe>`.
*   Nonces: use `mojo_base.mojom.UnguessableToken`, not `string`.
*   Origins: use `url.mojom.Origin`, not `url.mojom.Url` and certainly not
    `string`.
*   Time types: use `mojo_base.mojom.TimeDelta` /
    `mojo_base.mojom.TimeTicks` / `mojo_base.mojom.Time`, not `int64` /
    `uint64` / `double` / et cetera.
*   URLs: use `url.mojom.Url`, not `string`.
*   `array<uint8>` or `string` and `memcpy()`: use a Mojo struct and statically
    define the serialized fields. While `memcpy()` may be tempting for its
    simplicity, it can leak info in padding. Even worse, `memcpy()` can easily
    copy [undocumented fields][serialize-struct-tm-safely] or newly introduced
    fields that were never evaluated for safety by the developer or reviewer.

**_Good_**

```c++
interface ReportingService {
  ReportDeprecation(mojo_base.mojom.TimeTicks time,
                    url.mojom.Url resource,
                    uint32 line_number);
};
```

**_Bad_**

```c++
interface ReportingService {
  // Bad: unclear what units |time| is or what |data| contains.
  ReportDeprecation(double time, mojo_base.mojom.Value data);
};
```

Avoid parallel arrays of data that require the receiver to validate that the
arrays have matching lengths. Instead, bundle the data together in a struct so
it is impossible to have a mismatch:

**_Good_**

```c++
struct Pixel {
  int8 reds;
  int8 greens;
  int8 blues;
  int8 alphas;
};

struct Bitmap {
  // Good: it is impossible for there to be mismatched data.
  array<Pixel> pixels;
};
```

**_Bad_**

```c++
// Bad: code using this struct will need to validate that all the arrays have
// matching sizes.
struct Bitmap {
  array<int8> reds;
  array<int8> greens;
  array<int8> blues;
  array<int8> alphas;
};
```


### Beware of arithmetic overflow

> TODO(dcheng): Import the guidance from the legacy IPC doc.

Signed overflow is undefined in C++. If unsure about whether or not something
will overflow, use the safe numeric helpers from `//base/numerics`!

**_Good_**

```c++
base::CheckedNumeric<int32_t> size = mojo_rect->width();
size *= mojo_rect.height();
if (!size.IsValid()) {
  mojo::ReportBadMessage("Bad size from renderer!");
}
```

**_Bad_**

```c++
// Bad: Signed overflow is undefined in C++!
int32_t size = mojo_rect->width() * mojo_rect.height();
```

Note that even if the types have defined overflow semantics, it is almost always
a good idea to check for overflow.

**_Good_**

```c++
uint32_t alloc_size;
if (!CheckMul(request->elements(), request->element_size())
         .AssignIfValid(&alloc_size)) {
  // Safe: avoids allocating with a bogus size that overflowed to a smaller than
  // expected value.
  mojo::ReportBadMessage("Invalid allocation size");
}

Element* array = CreateArray(alloc_size);
for (size_t i = 0; i < request->element_size(); ++i) {
  array[i] = PopulateArray(i);
}
```

**_Bad_**

```c++
uint32_t alloc_size = request->elements() * request->element_size();

// Dangerous: alloc_size can overflow so that CreateArray allocates too little
// memory. Subsequent assignments will turn into an out-of-bound write!
Element* array = CreateArray(alloc_size);
for (size_t i = 0; i < request->element_size(); ++i) {
  array[i] = PopulateArray(i);
}
```


### All possible message values are semantically valid

When possible, messages should be defined in such a way that all possible values
are semantically valid. As a corollary, avoid having the value of one field
dictate the validity of other fields.

**_Good_**

```c++
union CreateTokenResult {
  // Implies success.
  string token;

  // Implies failure.
  string error_message;
};

struct TokenManager {
  CreateToken() => (CreateTokenResult result);
};
```

**_Bad_**
```c++
struct TokenManager {
  // Requires caller to handle edge case where |success| is set to true, but
  // |token| is null.
  CreateToken() => (bool success, string? token, string? error_message);

  // Requires caller to handle edge case where both |token| and |error_message|
  // are set, or both are null.
  CreateToken() => (string? token, string? error_message);
};
```

There are some known exceptions to this rule because mojo does not handle
optional primitives.

**_Allowed because mojo has no support for optional primitives_**
```c++
  struct Foo {
    int32 x;
    bool has_x;  // does the value of `x` have meaning?
    int32 y;
    bool has_y;  // does the value of `y` have meaning?
  };
```

Another common case where we tolerate imperfect message semantics is
with weakly typed integer [bitfields](#handling-bitfields).

### Handling bitfields

Mojom has no native support for bitfields. There are two common approaches: a
type-safe struct of bools which is a bit clunky (preferred) and an integer-based
approach (allowed but not preferred).

**_Type-safe bitfields_**
```c++
struct VehicleBits {
  bool has_car;
  bool has_bicycle;
  bool has_boat;
};

struct Person {
  VehicleBits bits;
};
```

**_Integer based approach_**
```c++
struct Person {
  const uint64 kHasCar = 1;
  const uint64 kHasBicycle = 2;
  const uint64 kHasGoat= 4;

  uint32 vehicle_bitfield;
};
```

## C++ Best Practices


### Use mojo::WrapCallbackWithDefaultInvokeIfNotRun And mojo::WrapCallbackWithDropHandler sparingly

Mojo provides several convenience helpers to automatically invoke a callback if
the callback has not already been invoked in some other way when the callback is
destroyed, e.g.:

```c++
  {
    base::OnceCallback<int> cb = mojo::WrapCallbackWithDefaultInvokeIfNotRun(
        base::BindOnce([](int) { ... }), -1);
  }  // |cb| is automatically invoked with an argument of -1.
```

This can be useful for detecting interface errors:

```c++
  process->GetMemoryStatistics(
      mojo::WrapCallbackWithDefaultInvokeIfNotRun(
          base::BindOnce(&MemoryProfiler::OnReplyFromRenderer), <failure args>));
  // If the remote process dies, &MemoryProfiler::OnReplyFromRenderer will be
  // invoked with <failure args> when Mojo drops outstanding callbacks due to
  // a connection error on |process|.
```

However, due to limitations of the current implementation, it's difficult to
tell if a callback object has invoke-on-destroy behavior. In general:

1.  Prefer error connection handlers where possible.
1.  Only use the callback helpers for detecting interface errors. These
    callbacks may be invoked during destruction and must carefully consider
    receiver object lifetime. For more information, please see the
    [Mojo documentation][mojo-doc-process-crashes].

> Note that using the callback wrappers in the renderer is often unnecessary.
> Message pipes are typically closed as part of a Document shutting down; since
> many Blink objects already inherit `blink::ContextLifecycleObserver`, it is
> usually more idiomatic to use this signal to perform any needed cleanup work.

### Use StructTraits

Creating a typemap and defining a `StructTraits` specialization moves the
complexity of serialization, deserialization, and validation into a central
location. We universally recommend this over defining `TypeConverter`
specializations: when a value fails deserialization, the receiver method will
never even be invoked. As a bonus, it also reduces the number of copies during
serialization and deserialization. 😄

**_Good_**

```c++
// In url_gurl_mojom_traits.h:
template <>
struct StructTraits<url::mojom::UrlDataView, GURL> {
  static base::StringPiece url(const GURL& r);

  // If Read() returns false, Mojo will discard the message.
  static bool Read(url::mojom::UrlDataView data, GURL* out);
};

// In url_gurl_mojom_traits.cc:
// Note that methods that aren't simple getters should be defined
// out-of-line to avoid code bloat.
base::StringPiece StructTraits<url::mojom::UrlDataView, GURL>::url(
    const GURL& r) {
  if (r.possibly_invalid_spec().length() > url::kMaxURLChars ||
      !r.is_valid()) {
    return base::StringPiece();
  }
  return base::StringPiece(r.possibly_invalid_spec().c_str(),
                           r.possibly_invalid_spec().length());
}

bool StructTraits<url::mojom::UrlDataView, GURL>::Read(
    url::mojom::UrlDataView data, GURL* out) {
  base::StringPiece url_string;
  if (!data.ReadUrl(&url_string))
    return false;
  if (url_string.length() > url::kMaxURLChars)
    return false;
  *out = GURL(url_string);
  if (!url_string.empty() && !out->is_valid())
    return false;
  return true;
}
```

**_Bad_**

```c++
template <>
struct TypeConverter<url::mojom::UrlPtr, GURL> {
  // Inefficient: this copies data once off the wire to create a
  // url.mojom.Url object, then copies it again to create a GURL.
  static GURL Convert(const url::mojom::UrlPtr url) {
    if (url.url.is_empty()) return GURL();
    // Not good: no way to signal errors, so any code that converts the
    // Mojo struct to a GURL will somehow need to check for errors…
    // but it can't even be distinguished from the empty URL case!
    if (url.url.size() > url::kMaxURLChars) return GURL();
    return GURL(url.url);
  }
};
```

There are also corresponding `EnumTraits` and `UnionTraits` specializations for
mojo enums and unions respectively.


### StructTraits getters should be simple

Where possible, `StructTraits` should be returning const references or simple
read-only views of the data. Having to create temporary data structures during
serialization should be rare, and it should be even rarer to mutate the input
argument.


### Out-of-line complex serialization/deserialization logic

A `StructTraits` specialization is almost always fully specialized. Only define
`StructTraits` methods inline in the header if the method is a simple getter
that returns a reference, pointer, or other simple POD. Define all other
methods out-of-line to avoid code bloat.


### Do not write one-off functions to convert to/from Mojo types

There are some instances where it is simply not possible to define a
`StructTraits` for type mapping: this commonly occurs with Blink IDL and Oilpan
types. In these instances, add a `TypeConverter` specialization rather than
defining a one-off conversion function. This makes it easier to search for and
audit code that does potentially risky type conversions.

> The use of `TypeConverter` should be limited as much as possible: ideally,
> only use it in renderers.

**_Good_**

```c++
template <>
struct TypeConverter<IDLDictionary, mojom::blink::DictionaryPtr> {
  static IDLDictionary* Convert(const mojom::blink::DictionaryPtr& in) {
    // Note that unlike StructTraits, there is no out-of-band way to signal
    // failure.
    IDLDictionary* out = new IDLDictionary;
    out->int_value = in->int_value;
    out->str_value = in->str_value;
    return out;
  }
};
```

**_Bad_**

```c++
// Using a custom one-off function makes this hard to discover in security
// audits.
IDLDictionary* FromMojo(const mojom::blink::DictionaryPtr& in) {
  IDLDictionary* out = new IDLDictionary;
  out->int_value = in->int_value;
  out->str_value = in->str_value;
  return out;
}
```


### Use the proper abstractions

`mojo::ReceiverSet` implies multiple clients may connect. If this actually isn't
the case, please do not use it. For example, if an interface can be rebound,
then use the singular `mojo::Receiver` and simply `reset()` the existing receiver
before reusing it.


### Explicitly reject bad input

While validation should be done inside `StructTraits` specializations when
possible, there are situations where additional checks, e.g. overflow checks,
are needed outside of `StructTraits` specializations. Use
`mojo::ReportBadMessage()` or `mojo::GetBadMessageCallback()` to reject bad
input in these situations. Under the hood, this may record UMAs, kill the
process sending bad input, et cetera.

*   `mojo::ReportBadMessage()`: use to report bad IPC input while a message is
    being dispatched on the stack.
*   `mojo::GetBadMessageCallback()`: use to generate a callback to report bad
    IPC input. The callback must be generated while a message is being
    dispatched on the stack; however, the returned callback may be invoked be
    freely invoked in asynchronously posted callbacks.


## Java Best Practices

Unfortunately, there are no strongly established conventions here. Most code
tends to write manual conversion helpers and throw an exception on conversion
failure. See [NfcTypeConverter.java] as one example of how to write conversion
code.


## General Code Health


### Naming Conventions

Place mojo types in `<top-level namespace>.mojom`. Directories for Mojo traits
should be named `mojom` (preferable) or `ipc`. Legacy names that are also
encountered are `public/interfaces`, `interfaces`, or just `mojo`.

`mojom` is preferred for consistency between the directory name and the nested
namespace name.


### Do not handle impossible situations

Do not clutter the code by handling impossible situations. Omitting it makes
the invariants clear. This takes two different forms:

*   A less trustworthy process can be compromised by an adversary and send
    arbitrary data. When processing data from a less trustworthy process, do
    not attempt to handle this invalid data: just call
    `mojo::ReportBadMessage()`. When invoked in the context of processing an
    IPC from the renderer, this will kill the renderer process.
*   A more trustworthy process must be trusted, by definition. Do not write
    code to handle impossible situations "just in case" there's a bug. For
    example, the renderer class `content::RenderFrameImpl` must always be
    connected to certain control interfaces in the browser. It does not makes
    sense to handle a Mojo connection error and try to reconnect: a connection
    error signals that the browser process is in the process of deleting the
    frame, and any attempt at reconnecting will never succeed.


### Using mojo enums directly when possible

`EnumTraits` generally do not add much value: incoming Mojo enum values are
already validated before typemapping, so it is guaranteed that the input value
to `EnumTraits<T>::FromMojom()` is already a valid enum value, so the method
itself is just a bunch of boilerplate to map between two very similarly named,
yet slightly different, enums.

To avoid this, prefer to use the Mojo enum directly when possible.


### Consider the cost of setting up message pipes

Message pipes are fairly inexpensive, but they are not free either: it takes 6
control messages to establish a message pipe. Keep this in mind: if the
interface is used relatively frequently, connecting once and reusing the
interface pointer is probably a good idea.


## Ensure An Explicit Grant For WebUI Bindings

WebUI renderers sometimes need to call special, powerful IPC endpoints in a
privileged process. It is important to enforce the constraint that the
privileged callee previously created and blessed the calling process as a WebUI
process, and not as a (potentially compromised) web renderer or other
low-privilege process.

* Use the standard pattern for instantiating `MojoWebUIController`. WebUI
Mojo interfaces must only be exposed through a `MojoWebUIController` subclass.
* If there is external functionality that the WebUI needs, make sure to route
it through the Mojo interfaces implemented by the `MojoWebUIController`, to
avoid circumventing access checks.


## Not-Yet-Shipped Features Should Be Feature-Checked On The Privileged Side

Sometimes, there will be powerful new features that are not yet turned on by
default, such as behind a flag, Finch trial, or [origin
trial](https://www.chromium.org/blink/origin-trials). It is not safe to check
for the feature's availability on the renderer side (or in another low-privilege
process type). Instead, ensure that the check is done in the process that has
power to actually enact the feature. Otherwise, a compromised renderer could opt
itself in to the feature! If the feature might not yet be fully developed and
safe, vulnerabilities could arise.


[security-tips-for-ipc]: https://www.chromium.org/Home/chromium-security/education/security-tips-for-ipc
[NfcTypeConverter.java]: https://chromium.googlesource.com/chromium/src/+/e97442ee6e8c4cf6bcf7f5623c6fb2cc8cce92ac/services/device/nfc/android/java/src/org/chromium/device/nfc/NfcTypeConverter.java
[mojo-doc-process-crashes]: https://chromium.googlesource.com/chromium/src/+/main/mojo/public/cpp/bindings#Best-practices-for-dealing-with-process-crashes-and-callbacks
[serialize-struct-tm-safely]: https://chromium-review.googlesource.com/c/chromium/src/+/679441