MSC22-C. Use the setjmp(), longjmp() facility securely

setjmp should only be invoked from one of the contexts listed in §7.13.1.1 of [[ISO/IEC 9899:1999]]. Invoking setjmp outside of one of these contexts results in undefined behavior (see Undefined Behavior 119).

After invoking longjmp, non-volatile-qualified local objects should not be accessed if their values could have changed since the invocation of setjmp. Their value in this case is considered indeterminate and accessing them is undefined behavior (see Undefined Behavior 121, 10).

longjmp should never be used to return control to a function that has terminated execution (see Undefined Behavior 120).

This recommendation is related to SIG32-C. Do not call longjmp() from inside a signal handler and ENV32-C. All atexit handlers must return normally.

Noncompliant Code Example

The following noncompliant code example calls setjmp in an assignment statement, resulting in undefined behavior.

jmp_buf buf;

void f() {
  int i = setjmp(buf);
  if (i == 0) {
    g();
  } else {
    /* longjmp was invoked */
  }
}

void g() {
  /* ... */
  longjmp(buf, 1);
}

Compliant Solution

Placing the call to setjmp in the if statement and (optionally) comparing it with a constant integer removes the undefined behavior.

jmp_buf buf;

void f() {
  if (setjmp(buf) == 0) {
    g();
  } else {
    /* longjmp was invoked */
  }
}

void g() {
  /* ... */
  longjmp(buf, 1);
}

Noncompliant Code Example

Any attempt to longjmp to a function that has terminated execution results in possibly exploitable undefined behavior.

jmp_buf buf;

void f() {
  g();
  h();
  return;
}

void g() {
  if (setjmp(buf) != 0) {
    /* longjmp was invoked*/
  }
  return;
}

void h() {
  /* ... */
  longjmp(buf, 1);
}

Compliant Solution

longjmp should only be used when the function containing the corresponding setjmp is guaranteed not to have terminated execution, as in the following example.

jmp_buf buf;

void f() {
  if (setjmp(buf) != 0) {
    /* longjmp was invoked */
  }
  h();
  return;
}

void h() {
  /* ... */
  longjmp(buf, 1);
}

Noncompliant Code Example

Non-volatile-qualified objects local to the function that invoked the corresponding setjmp have indeterminate values after longjmp has been executed if their value has been changed since the invocation of setjmp.

jmp_buf buf;

void f() {
  int i = 0;
  if (setjmp(buf) != 0) {
    printf("%i\n", i);
    /* ... */
  }
  i = 2;
  g();
}

void g() {
  /* ... */
  longjmp(buf, 1);
}

Compliant Solution

If an object local to the function that invoked setjmp needs to be accessed after longjmp returns control to the function, the object should be volatile-qualified.

jmp_buf buf;

void f() {
  volatile int i = 0;
  if (setjmp(buf) != 0) {
    printf("%i\n", i);
    /* ... */
  }
  i = 2;
  g();
}

void g() {
  /* ... */
  longjmp(buf, 1);
}

Risk Assessment

References

[[ISO/IEC 9899:1999]] Section 7.13, "Nonlocal jumps <setjmp.h>"
[[ISO/IEC 9899:1999]] Section J.2, "Portability issues"