...
| Code Block | ||||
|---|---|---|---|---|
| ||||
jmp_buf buf; unsigned char b[] = {0xe5, 0x06, 0x40, 0x00, 0x00, 0x00, 0x00, 0x00}; int main(void) { setup(); do_stuff(); return 0; } void setup(void) { f(); } void f(void) { g(); } #include <setjmp.h> #include <stdio.h> #include <stdlib.h> static jmp_buf buf; static void bad(void); static void g(void) { if (setjmp(buf) == 0) { printf("setjmp() invoked\n"); } else { printf("longjmp() invoked\n"); } } static void do_stufff(void) { g(); } static void char a[8]; memcpy(a, b, 8)setup(void) { f(); } void do_stuff(void) { void (*b)(void) = bad; /* ... */ longjmp(buf, 1); } static void bad(void) { printf("Should not be called!\n"); exit(1); } int main(void) { setup(); do_stuff(); } |
Implementation Details
Compiled for x86at -64 O0 using GCC 4.1.2 on Linux, the preceding 7.5 or Clang 8.0 on Ubuntu 18.04 (Linux for x86-64), the preceding example outputs the following when run:
...
Because g() has finished executing at the time longjmp() is called, it is no longer on the stack. When do_stuff() is invoked, its stack frame occupies the same memory as the old stack frame of g(). In this case, a was located in the same location as the return address of function g(). The call to memcpy() assignment of b overwrites the return address, so when longjmp() sends control back to function g(), the function returns to the wrong address (in this case, to function bad()).
...
| Code Block | ||||
|---|---|---|---|---|
| ||||
#include <setjmp.h> #include <stdio.h> #include <stdlib.h> static jmp_buf buf; unsignedstatic char b[] = {0xe5, 0x06, 0x40, 0x00, 0x00, 0x00, 0x00, 0x00}void bad(void); intvoid maindo_stuff(void) { ifvoid (setjmp*b)(bufvoid) == 0) { printf("setjmp() invoked\n"); } else { bad; /* ... */ longjmp(buf, 1); } static void bad(void) { printf("longjmp() invokedShould not be called!\n"); } do_stuff(exit(1); return 0; } voidint do_stuffmain(void) { char a[8]; memcpy(a, b, 8); /* ... */ longjmp(buf, 1); } void bad(void)if (setjmp(buf) == 0) { printf("setjmp() invoked\n"); } else { printf("Should not be called!longjmp() invoked\n"); exit(1} do_stuff(); } |
There is no risk of overwriting a return address because the stack frame of main() (the function that invoked setjmp()) is still on the stack; so when do_stuff() is invoked, the two stack frames will not overlap.
...
| Code Block | ||||
|---|---|---|---|---|
| ||||
jmp_buf buf;
void f(void) {
int i = 0;
if (setjmp(buf) != 0) {
printf("%i\n", i);
/* ... */
}
i = 2;
g();
}
void g(void) {
/* ... */
longjmp(buf, 1);
}
|
Implementation Details
Calling f() will print 2 if you compile with -O0, but will print 0 if you compile with -O2. This involves using GCC 7.5 or Clang 8.0 on Ubuntu 18.04 (Linux x86-64).
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:
| Code Block | ||||
|---|---|---|---|---|
| ||||
jmp_buf buf;
void f(void) {
volatile int i = 0;
if (setjmp(buf) != 0) {
printf("%i\n", i);
/* ... */
}
i = 2;
g();
}
void g(void) {
/* ... */
longjmp(buf, 1);
}
|
This will now correctly print 2 regardless of optimization level.
Risk Assessment
Recommendation | Severity | Likelihood | Remediation Cost | Priority | Level |
|---|---|---|---|---|---|
MSC22-C | Low | Probable | Medium | P4 | L3 |
Automated Detection
setjmp and longjmp cause deviation from normal control flow
| Tool | Version | Checker | Description | ||||||
|---|---|---|---|---|---|---|---|---|---|
| CodeSonar |
| BADFUNC.LONGJMP BADFUNC.SETJMP | Use of longjmp Use of setjmp | ||||||
| LDRA tool suite |
| 43 S | Enhanced enforcement | ||||||
| Parasoft C/C++test |
| CERT_C-MSC22-a | The facilities provided by <setjmp.h> should not be used | |||||||
| Polyspace Bug Finder |
| CERT C: Rec. MSC22-C | Checks for use of setjmp/longjmp (rec. fully covered) |
| SonarQube C/C++ Plugin |
| S982 |
...
...