[go: nahoru, domu]
Chromium Docs

Preventing OOB through Unsafe Buffers errors (aka Spanification)

Out-of-bounds (OOB) security bugs commonly happen through C-style pointers which have no bounds information associated with them. We can prevent such bugs by always using C++ containers. Furthermore, the Clang compiler can warn about unsafe pointer usage that should be converted to containers. When an unsafe usage is detected, Clang prints a warning similar to

error: unsafe buffer access [-Werror,-Wunsafe-buffer-usage]

and directs developers to this file for more information.

Suppressions

Our compiler enables the -Wunsafe-buffer-usage warning on all files by default. Because the Chromium codebase is not yet compliant with these warnings, there are mechanisms to opt out code on a directory, file, or per-occurence basis.

Entire directories are opted out of unsafe pointer usage warnings through the //build/config/unsafe_buffers_paths.txt file. As work progresses, directories will be removed from this list, and non-compliant files marked on a per-file basis as below. Early results indicate that often 85%+ of files in a directory already happen to be compliant, so file-by-file suppression allows this code to be subject to enforcement.

Individual files are opted out of unsafe pointer usage warnings though the use of the following snippet, which is to be placed immediately following the copyright header in a source file.

#ifdef UNSAFE_BUFFERS_BUILD
// TODO(crbug.com/ABC): Remove this and convert code to safer constructs.
#pragma allow_unsafe_buffers
#endif

Individual expressions or blocks of code are opted out by using the UNSAFE_BUFFERS() macro as defined in [//base/compiler_specific.h[(../base/compiler_specific.h) file. These should be rare once a project is fully converted, except perhaps when working with C-style external APIs. These must always be accompanied by a // SAFETY: comment explaining in detail how the code has been evaluated to be safe for all possible input.

To allow for incremental conversion, the use of a safety comment with a TODO() is permitted, along the lines of // SAFETY: TODO(crbug.com/xxxxxx): resolve safety issues.

Code introducing UNSAFE_BUFFERS() macro invocations without corresponding // SAFETY: comment should be summarily rejected during code review.

Container-based ecosystem

Containers may be owning types or view types. The common owning containers that us contiguous storage are std::vector, std::string, base::HeapArray, std::array. Their common view types are base::span, std::string_view, base::cstring_view.

Other owning containers include maps, sets, deques, etc. These are not compatible with base::span as they are not contiguous and generally do not have an associated view type at this time.

We are using base::span instead of std::span in order to provide a type that can do more than the standard type. We also have other types and functions to work with ranges and spans instead of unbounded pointers and iterators.

The common conversions to spans are:

  • base::span<T> replaces T* ptr, size_t size.
  • base::span<T, N> replaces T (&ptr)[N] (a reference to a compile-time-sized array).
  • base::raw_span<T> replaces base::span<T> (and T* ptr, size_t size) for class fields.

Span construction

  • base::span() constructor can make a span, and deduce the type and size, from:
    • a T[N] array
    • std::array<T, N>
    • std::vector
    • std::string
    • any contiguous range with begin() and end() methods.
    • any type with T* data() and size_t size() methods.
  • base::make_span<N>() can make a fixed-size span from any range.
  • base::as_bytes() and base::as_chars() convert a span’s inner type to uint8_t or char respectively, making a byte-span or char-span.
  • base::span_from_ref() and base::byte_span_from_ref() make a span, or byte-span, from a single object.
  • base::as_byte_span() and base::as_writable_byte_span() to make a byte-span (const or mutable) from any container that can convert to a base::span<T>, such as std::string or std::vector<Stuff>.

Padding bytes

Note that if the type contains padding bytes that were not somehow explicitly initialized, this can create reads of uninitialized memory. Conversion to a byte-span is most commonly used for spans of primitive types, such as going from char (such as in std::string) or uint32_t (in a std::vector) to unit8_t.

Dynamic read/write of a span

  • base::SpanReader reads heterogeneous values from a (typically, byte-) span in a dynamic manner.
  • base::SpanWriter writes heterogeneous values into a (typically, byte-) span in a dynamic manner.

Values to/from byte spans

In //base/numerics/byte_conversions.h we have conversions between byte-arrays and big/little endian integers or floats. For example (and there are many other variations):

  • base::U32FromBigEndian converts from a big-endian byte-span to an unsigned 32-bit integer.
  • base::U32FromLittleEndian converts from a little-endian byte-span to an unsigned
  • base::U32ToBigEndian converts from an integer to a big-endian-encoded 4-byte-array.
  • base::U32ToLittleEndian converts from an integer to a little-endian-encoded 4-byte-array.

Heap-allocated arrays

  • base::HeapArray<T> replaces std::unique_ptr<T[]> and places the bounds of the array inside the HeapArray which makes it a bounds-safe range.

Copying and filling arrays

  • base::span::copy_from(span) replaces memcpy and memmove, and verifies that the source and destination spans have the same size instead of writing out of bounds. It lowers to the same code as memmove when possible.
    • Note std::ranges::copy is not bounds-safe (though its name sounds like it should be).
  • std::ranges::fill replaces memset and works with a range so you don't need explicit bounds.

String pointers

A common form of pointer is const char* which is used (sometimes) to represent a NUL-terminated string. The standard library gives us two types to replace char*, which allow us to know the bounds of the character array and work with the string as a range:

  • std::string owns a NUL-terminated string.
  • std::string_view is a view of a non-NUL-terminated string.

What’s missing is a view of a string that is guaranteed to be NUL-terminated so that you can call .c_str() to generate a const char* suitable for C APIs.

  • base::cstring_view is a view of a NUL-terminated string. This avoids the need to construct a std::string in order to ensure a terminating NUL is present. Use this as a view type whenever your code bottoms out in a C API that needs NUL-terminated string pointer.

Use of std::array.

The clang plugin is very particular about indexing a C-style array (e.g. int arr[100]) with a variable index. Often these issues can be resolved by replacing this with std::array<int, 100> arr, which provides safe indexed operations. In particular, new code should prefer to use the std::array<T, N> mechanism.

For arrays where the size is determined by the compiler (e.g. int arr[] = { 1, 3, 5 };), the std::to_array<T>() helper function should be used along with the auto keyword: auto arr = std::to_array<int>({1, 3, 5});

Avoid reinterpret_cast

Writing to a byte span

A common idiom in older code is to write into a byte array by casting the array into a pointer to a larger type (such as uint32_t or float) and then writing through that pointer. This an result in Undefined Behaviour and violates the rules of the C++ abstract machine.

Instead, keep the byte array as a base::span<uint8_t>, and write to it directly by chunking it up into pieces of the size you want to write.

Using first():

void write_floats(base::span<uint8_t> out, float f1, float f2) {
  out.first<4>().copy_from(base::byte_span_from_ref(f1));
  out = out.subspan(4u);  // Advance the span past what we wrote.
  out.first<4>().copy_from(base::byte_span_from_ref(f2));
}

Using split_at():

void write_floats(base::span<uint8_t> out, float f1, float f2) {
  auto [write_f1, rem] = out.split_at<4>();
  auto [write_f2, rem2] = rem.split_at<4>();
  write_f1.copy_from(base::byte_span_from_ref(f1));
  write_f2.copy_from(base::byte_span_from_ref(f2));
}

Using SpanWriter and endian-aware FloatToLittleEndian():

void write_floats(base::span<uint8_t> out, float f1, float f2) {
  auto writer = base::SpanWriter(out);
  CHECK(writer.Write(base::FloatToLittleEndian(f1)));
  CHECK(writer.Write(base::FloatToLittleEndian(f2)));
}

Writing big-endian, with SpanWriter and endian-aware U32ToBigEndian():

void write_values(base::span<uint8_t> out, uint32_t i1, uint32_t i2) {
  auto writer = base::SpanWriter(out);
  CHECK(writer.Write(base::U32ToBigEndian(i1)));
  // SpanWriter has a built-in shortcut to do the same thing.
  CHECK(writer.WriteU32BigEndian(i2));
  // Verify we wrote to the whole output. We can put a size parameter in the
  // `out` span to push this check to compile-time when it's a constant.
  CHECK_EQ(writer.remaining(), 0u);
}

Writing an array to a byte span with copy_from():

void write_floats(base::span<uint8_t> out, std::vector<const float> floats) {
  base::span<const uint8_t> byte_floats = base::as_byte_span(floats);
  // Or skip the first() if you want to CHECK at runtime that all of `out` has
  // been written to.
  out.first(byte_floats.size()).copy_from(byte_floats);
}

Reading from a byte span

Instead of turning a span<const uint8_t> into a pointer of a larger type, which can cause Undefined Behaviour, read values out of the byte span and convert each one as a value (not as a pointer).

Using subspan() and endian-aware conversion FloatFromLittleEndian:

void read_floats(base::span<const uint8_t> in, float& f1, float& f2) {
  f1 = base::FloatFromLittleEndian(in.subspan<0, 4>());
  f2 = base::FloatFromLittleEndian(in.subspan<4, 4>());
}

Using SpanReader and endian-aware U32FromBigEndian():

void read_values(base::span<const uint8_t> in, int& i1, int& i2, int& i3) {
  auto reader = base::SpanReader(in);
  i1 = base::U32FromBigEndian(*reader.Read<4>());
  i2 = base::U32FromBigEndian(*reader.Read<4>());
  // SpanReader has a built-in shortcut to do the same thing.
  CHECK(reader.ReadU32BigEndian(i3));
  // Verify we read the whole input. We can put a size parameter in the `in`
  // span to push this check to compile-time when it's a constant.
  CHECK_EQ(reader.remaining(), 0u);
}

Patterns for spanification

Most pointer issues ought to be resolved by converting to containers. In particular, one common conversion is to replace T* pointers with base::span<T> in a process known as spanification, since most pointers are unowned references into an array (or vector). The appropriate replacement for the pointer is base::span.

Copying arrays (memcpy)

You have:

uint8_t array1[12];
uint8_t array2[16];
uint64_t array3[2];
memcpy(array1, array2 + 8, 4);
memcpy(array1 + 4, array3, 8);

Spanified:

uint8_t array1[12];
uint8_t array2[16];
uint64_t array3[2];
base::span(array1).first(4u).copy_from(base::span(array2).subspan(8u, 4u));
base::span(array1).subspan(4u).copy_from(base::as_byte_span(array3).first(8u));

// Use `split_at()` to ensure `array1` is fully written.
auto [from2, from3] = base::span(array1).split_at(4u);
from2.copy_from(base::span(array2).subspan(8u, 4u));
from3.copy_from(base::as_byte_span(array3).first(8u));

// This can even be ensured at compile time (if sizes and offsets are all
// constants).
auto [from2, from3] = base::span(array1).split_at<4u>();
from2.copy_from(base::span(array2).subspan<8u, 4u>());
from3.copy_from(base::as_byte_span(array3).first<8u>());

Zeroing arrays (memset)

std::ranges::fill works on any range/container and won't write out of bounds. Converting arbitrary types into a byte array (through base::as_writable_byte_span) is only valid when the type holds trivial types such as primitives. Unlike memset, a constructed object can be given as the value to use as in std::ranges_fill(container, Object()).

You have:

uint8_t array1[12];
uint64_t array2[2];
Object array3[4];
memset(array1, 0, 12);
memset(array2, 0, 2 * sizeof(uint64_t));
memset(array3, 0, 4 * sizeof(Object));

Spanified:

uint8_t array1[12];
uint64_t array2[2];
Object array3[4];
std::ranges::fill(array1, 0u);
std::ranges::fill(array2, 0u);
std::ranges::fill(base::as_writable_byte_span(array3), 0u);

Comparing arrays (memcmp)

You have:

uint8_t array1[12] = {};
uint8_t array2[12] = {};
bool eq = memcmp(array1, array2, sizeof(array1)) == 0;
bool less = memcmp(array1, array2, sizeof(array1)) < 0;

// In tests.
for (size_t i = 0; i < sizeof(array1); ++i) {
  SCOPED_TRACE(i);
  EXPECT_EQ(array1[i], array2[i]);
}

Spanified:

uint8_t array1[12] = {};
uint8_t array2[12] = {};
// If one side is a span, the other will convert to span too.
bool eq = base::span(array1) == array2;
bool less = base::span(array1) < array2;

// In tests.
EXPECT_EQ(base::span(array1), array2);

Copying array into an integer

You have:

uint8_t array[44] = {};
uint32_t v1;
memcpy(&v1, array, sizeof(v1));  // Front.
uint64_t v2;
memcpy(&v2, array + 6, sizeof(v2));  // Middle.

Spanified:

#include "base/numerics/byte_conversions.h"
...
uint8_t array[44] = {};
uint32_t v1 = base::U32FromLittleEndian(base::span(array).first<4u>());  // Front.
uint64_t v2 = base::U64FromLittleEndian(base::span(array).subspan<6u, 8u>());  // Middle.

Copy an array into an integer via cast

Note: This pattern is prone to more UB than out-of-bounds access. It is UB to cast pointers if the result is not aligned, so these cases are often buggy or were only correct due to subtle assumptions on buffer alignment. The spanified version avoids this pitfalls. It has no alignment requirement.

You have:

uint8_t array[44] = {};
uint32_t v1 = *reinterpret_cast<const uint32_t*>(array);  // Front.
uint64_t v2 = *reinterpret_cast<const uint64_t*>(array + 16);  // Middle.

Spanified:

#include "base/numerics/byte_conversions.h"
...
uint8_t array[44] = {};
uint32_t v1 = base::U32FromLittleEndian(base::span(array).first<4u>());  // Front.
uint64_t v2 = base::U64FromLittleEndian(base::span(array).subspan<16u, 8u>());  // Middle.

Making a byte array (span<uint8_t>) from a string (or any array/range)

You have:

std::string str = "hello world";
func_with_const_ptr_size(reinterpret_cast<const uint8_t*>(str.data()), str.size());
func_with_mut_ptr_size(reinterpret_cast<uint8_t*>(str.data()), str.size());

Spanified:

std::string str = "hello world";
base::span<const uint8_t> bytes = base::as_byte_span(str);
func_with_const_ptr_size(bytes.data(), bytes.size());
base::span<uint8_t> mut_bytes = base::as_writable_byte_span(str);
func_with_mut_ptr_size(mut_bytes.data(), mut_bytes.size());

// Replace pointer and size with a span, though.
func_with_const_span(base::as_byte_span(str));
func_with_mut_span(base::as_writable_byte_span(str));

Making a byte array (span<uint8_t>) from an object

You have:

uint8_t array[8];
uint64_t val;
two_byte_arrays(array, reinterpret_cast<const uint8_t*>(&val));

Spanified:

uint8_t array[8];
uint64_t val;
base::span<uint8_t> val_span = base::byte_span_from_ref(val);
two_byte_arrays(array, val_span.data());

// Replace an unbounded pointer a span, though.
two_byte_spans(base::span(array), base::byte_span_from_ref(val));

Functions with array pointer parameters

Functions that receive a pointer argument into an array may read or write out of bounds of that array if subsequent manual size calculations are incorrect. Such functions should be avoided if possible, or marked with the UNSAFE_BUFFER_USAGE attribute macro otherwise. This macro propagates to their callers that they must be called from inside an UNSAFE_BUFFERS() region (along with a corresponding safety comment explaining how the caller knows the call will be safe).

The same is true for functions that accept an iterator instead of a range type. Some examples of each are memcpy() and std::copy().

Again, calling such functions is unsafe and should be avoided. Replace such functions with an API built on base::span or other range types which prevents any chance of OOB memory access. For instance, replace memcpy(), std::copy() and std::ranges::copy() with base::span::copy_from(). And replace memset() with std::ranges::fill().