A compiler might insert padding bytes to a structure to ensure that structure members appear in the correct location. Initializing the members of the structure does not always initialize the padding bytes.

The C Standard, subclause 6.2.6.1 [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.

It is annotated with the following (footnote 51):

Thus, for example, structure assignment need not copy any padding bits.

As a result of being uninitialized, padding bytes could have random data (sensitive data). This structure could be passed to functions that do not have privilege. For example, there have been instances in the Linux kernel when uninitialized stack bytes were leaked to unprivileged users as a result of copying structures to user space.

Some compilers do not initialize the padding bytes if all the members of the structure are initialized.

Noncompliant Code Example

In this noncompliant code example, the padding bytes after char b may not be initialized:

#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};

  /* ... Perform operations on arg ... */

  /* Copy arg to user space */
  copy_to_user(usr_buf, &arg, sizeof(arg));

  /* ... */
}

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

#include <stddef.h>
#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;

  /* Initializes all bytes (including padding bytes) of the struct to zero */
  memset(&arg, 0, sizeof(arg));

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

  /* ... Perform operations on arg ... */

  /* Copy arg to user space */
  copy_to_user(usr_buf, &arg, sizeof(arg));

  /* ... */
}

Here, the compiler could implement arg.b = 2 in the following way:

• 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,

thereby leaking stack bytes to an unprivileged user. This may not be the case with all compilers, but compilers are free to implement it in their own way, so this example could leak data under some specific compiler.

Compliant Solution (Structure Packing—GCC)

GCC allows specifying attributes of variables and structures using the keyword __attribute__((__packed__)). This means that the GCC will not add any padding bytes (for memory alignment) and will make variables or fields immediately next to each other.

#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};

  /* ... Perform operations on arg ... */

  /* Copy arg to user space */
  copy_to_user(usr_buf, &arg, sizeof(arg));

  /* ... */
}

Compliant Solution (Structure Packing—MSVC)

Microsoft Visual Studio supports the #pragma pack() instead of the __packed__ attribute to ensure no padding bytes are added [MSDN]:

#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};

  /* ... Perform operations on arg ... */

  /* Copy arg to user space */
  copy_to_user(usr_buf, &arg, sizeof(arg));

  /* ... */
}

The pack pragma takes effect at the first struct declaration after the pragma is seen. The alignment of a member will be on a boundary that is a multiple of 1 byte.

Compliant Solution (Adding Padding bytes)

The padding bytes could be explicitly declared. It should be done carefully on the basis of the memory architecture.

The following solution assumes it will be run on an IA-32 architecture:

#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) {

  /* Make sure c is the next byte after the last padding byte */
  static_assert(offsetof(struct test, c) == \
    offsetof(struct test, padding_3) + 1, \
    "Error: not compiling for IA-32");

  struct test arg = {.a=1,.b=2,.c=3};
  arg.padding_1 = 0;
  arg.padding_2 = 0;
  arg.padding_3 = 0;

  /* ... Perform operations on arg ... */

  /* Copy arg to user space */
  copy_to_user(usr_buf, &arg, sizeof(arg));

  /* ... */
}

The static_assert() macro, a feature of the C Standard, 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 example, in an IA-32 machine, the explicit insertion of the padding bytes into the struct should ensure that no "invisible" padding bytes are added by the compiler, and consequently the expression in static_assert should be true.

Explicitly,

offsetof(struct test, c ) = 8

and

offsetof(struct test, padding_3 ) = 7,

and consequently the expression should return 1.

However, if you were compiling for a different architecture and the compiler added padding bytes, then the memory would not be contiguous and the expression would return 0.

This approach ensures that no padding bytes are inserted.

Compliant Solution

If setting memset of the original structure to 0 (as mentioned in the noncompliant example) does not work under some compilers, then copy the original structure to an unsigned char memory and pass that memory to the user as follows:

#include <stddef.h>
#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 = {.a=1,.b=2,.c=3};
  unsigned char r[sizeof(arg)];

  /* ... Perform operations on arg ... */

  /* Just before passing arg to the function */
  memset(r, 0, sizeof(r));
  memset(r+offsetof(struct test,a), arg.a, sizeof(arg.a));
  memset(r+offsetof(struct test,b), arg.b, sizeof(arg.b));
  memset(r+offsetof(struct test,c), arg.c, sizeof(arg.c));
  /* Now pass r to the function */
  copy_to_user(usr_buf, r, sizeof(r));

  /* ... */
}

This code ensures that no uninitialized padding bytes are copied to unprivileged users.

Risk Assessment

Bibliography