The C Standard, 6.7.2.1, discusses the layout of structure fields. It specifies that non-bit-field members are aligned in an implementation-defined manner and that there may be padding within or at the end of a structure. Furthermore, initializing the members of the structure does not guarantee initialization of the padding bytes. The C Standard, 6.2.6.1, paragraph 6 [ISO/IEC 9899:2011], states:

When a value is stored in an object of structure or union type, including in a member object, the bytes of the object representation that correspond to any padding bytes take unspecified values.

When passing a pointer to a structure across a trust boundary to a different trusted domain, programmers must ensure that the padding bytes of these structures do not contain sensitive information.

Noncompliant Code Example

This noncompliant code example runs in kernel space and copies data from struct test to user space. However, padding bytes may be used within the structure, for example, to ensure the proper alignment of the structure members. These padding bytes may contain sensitive information, which may then be leaked when the data is copied to user space.

#include <stddef.h>

struct test {
  int a;
  char b;
  int c;
};

/* Safely copy bytes to user space */
extern int copy_to_user(void *dest, void *src, size_t size);

void do_stuff(void *usr_buf) {
  struct test arg = {.a = 1, .b = 2, .c = 3};
  copy_to_user(usr_buf, &arg, sizeof(arg));
}

Noncompliant Code Example (memset())

The padding bytes can be explicitly initialized by calling memset():

#include <string.h>

struct test {
  int a;
  char b;
  int c;
};

/* Safely copy bytes to user space */
extern int copy_to_user(void *dest, void *src, size_t size);

void do_stuff(void *usr_buf) {
  struct test arg;

  /* Set all bytes (including padding bytes) to zero */
  memset(&arg, 0, sizeof(arg));

  arg.a = 1;
  arg.b = 2;
  arg.c = 3;

  copy_to_user(usr_buf, &arg, sizeof(arg));
}

However, compilers are free to implement arg.b = 2 by setting the low byte of a 32-bit register to 2, leaving the high bytes unchanged, and storing all 32 bits of the register into memory. This could leak the high-order bytes resident in the register to a user.

Compliant Solution

This compliant solution serializes the structure data before copying it to an untrusted context:

#include <stddef.h>
 
struct test {
  int a;
  char b;
  int c;
};
 
/* Safely copy bytes to user space */
extern int copy_to_user(void *dest, void *src, size_t size);
 
void do_stuff(void *usr_buf) {
  struct test arg = {.a = 1, .b = 2, .c = 3};
  /* May be larger than strictly needed */
  unsigned char buf[sizeof(arg)];
  size_t offset = 0;
  
  memcpy(buf + offset, &arg.a, sizeof(arg.a));
  offset += sizeof(arg.a);
  memcpy(buf + offset, &arg.b, sizeof(arg.b));
  offset += sizeof(arg.b);
  memcpy(buf + offset, &arg.c, sizeof(arg.c));
  offset += sizeof(arg.c);

  copy_to_user(usr_buf, buf, offset /* size of info copied */);
} 

This code ensures that no uninitialized padding bytes are copied to unprivileged users. Note that the structure copied to user space is now a packed structure and that the copy_to_user() function would need to unpack it to re-create the original padded structure.

Compliant Solution (Padding Bytes)

Padding bytes can be explicitly declared as fields within the structure. This solution is not portable, however, because it depends on the implementation and target memory architecture. The following solution is specific to the x86-32 architecture:

#include <assert.h>
#include <stddef.h>

struct test {
  int a;
  char b;
  char padding_1, padding_2, padding_3;
  int c;
};

/* Safely copy bytes to user space */
extern int copy_to_user(void *dest, void *src, size_t size);

void do_stuff(void *usr_buf) {
  /* Ensure c is the next byte after the last padding byte */
  static_assert(offsetof(struct test, c) ==
                offsetof(struct test, padding_3) + 1,
                "Structure contains intermediate padding");
  /* Ensure there is no trailing padding */
  static_assert(sizeof(struct test) ==
                offsetof(struct test, c) + sizeof(int),
                "Structure contains trailing padding");
  struct test arg = {.a = 1, .b = 2, .c = 3};
  arg.padding_1 = 0;
  arg.padding_2 = 0;
  arg.padding_3 = 0;
  copy_to_user(usr_buf, &arg, sizeof(arg));
}

The C Standard static_assert() macro accepts a constant expression and an error message. The expression is evaluated at compile time and, if false, the compilation is terminated and the error message is output (see DCL03-C. Use a static assertion to test the value of a constant expression for more details). The explicit insertion of the padding bytes into the struct should ensure that no additional padding bytes are added by the compiler, and consequently both static assertions should be true. However, it is necessary to validate these assumptions to ensure that the solution is correct for a particular implementation.

Compliant Solution (Structure Packing—GCC)

GCC allows specifying declaration attributes using the keyword __attribute__((__packed__)). When this attribute is present, the compiler will not add padding bytes for memory alignment unless otherwise required to by the _Alignas alignment specifier, and it will attempt to place fields at adjacent memory offsets when possible.

#include <stddef.h>

struct test {
  int a;
  char b;
  int c;
} __attribute__((__packed__));

/* Safely copy bytes to user space */
extern int copy_to_user(void *dest, void *src, size_t size);

void do_stuff(void *usr_buf) {
  struct test arg = {.a = 1, .b = 2, .c = 3};
  copy_to_user(usr_buf, &arg, sizeof(arg));
}

Compliant Solution (Structure Packing—Microsoft Visual Studio)

Microsoft Visual Studio  supports #pragma pack() to suppress padding bytes [MSDN]. The compiler adds padding bytes for memory alignment depending on the current packing mode but still honors alignment specified by __declspec(align()). In this compliant solution, the packing mode is set to 1 in an attempt to ensure all fields are given adjacent offsets:

#include <stddef.h>

#pragma pack(push, 1) /* 1 byte */
struct test {
  int a;
  char b;
  int c;
};
#pragma pack(pop)
 
/* Safely copy bytes to user space */
extern int copy_to_user(void *dest, void *src, size_t size);

void do_stuff(void *usr_buf) {
  struct test arg = {1, 2, 3};
  copy_to_user(usr_buf, &arg, sizeof(arg));
}

The pack pragma takes effect at the first struct declaration after the pragma is seen.

Risk Assessment

Padding bytes might contain sensitive data because the C Standard allows any padding bytes to take unspecified values. A pointer to such a structure could be passed to other functions, causing information leakage.

Related Vulnerabilities

Numerous vulnerabilities in the Linux Kernel have resulted from violations of this rule. CVE-2010-4083 describes a vulnerability in which the semctl() system call allows unprivileged users to read uninitialized kernel stack memory, because various fields of a semid_ds struct declared on the stack are not altered or zeroed before being copied back to the user. CVE-2010-3881 describes a vulnerability in which structure padding and reserved fields in certain data structures in QEMU-KVM were not initialized properly before being copied to user space. A privileged host user with access to /dev/kvm could use this flaw to leak kernel stack memory to user space. CVE-2010-3477 describes a kernel information leak in act_police where incorrectly initialized structures in the traffic-control dump code may allow the disclosure of kernel memory to user space applications.

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

Bibliography