All locking and unlocking of mutexes should be performed in the same module and at the same level of abstraction. Failure to follow this recommendation can lead to some lock or unlock operations not being executed by the multithreaded program as designed, eventually resulting in deadlock, race conditions, or other security vulnerabilities, depending on the mutex type.
A common consequence of improper locking is for a mutex to be unlocked twice, via two calls to mtx_unlock(). This can cause the unlock operation to return errors. In the case of recursive mutexes, an error is returned only if the lock count is 0 (making the mutex available to other threads) and a call to mtx_unlock() is made.
Noncompliant Code Example
In this noncompliant code example for a simplified multithreaded banking system, imagine an account with a required minimum balance. The code would need to verify that all debit transactions are allowable. Suppose a call is made to debit() asking to withdraw funds that would bring account_balance below MIN_BALANCE, which would result in two calls to mtx_unlock(). In this example, because the mutex is defined statically, the mutex type is implementation-defined.
#include <threads.h>
enum { MIN_BALANCE = 50 };
int account_balance;
mtx_t mp;
/* Initialize mp */
int verify_balance(int amount) {
if (account_balance - amount < MIN_BALANCE) {
/* Handle error condition */
if (mtx_unlock(&mp) == thrd_error) {
/* Handle error */
}
return -1;
}
return 0;
}
void debit(int amount) {
if (mtx_lock(&mp) == thrd_error) {
/* Handle error */
}
if (verify_balance(amount) == -1) {
if (mtx_unlock(&mp) == thrd_error) {
/* Handle error */
}
return;
}
account_balance -= amount;
if (mtx_unlock(&mp) == thrd_error) {
/* Handle error */
}
}
Compliant Solution
This compliant solution unlocks the mutex only in the same module and at the same level of abstraction at which it is locked. This technique ensures that the code will not attempt to unlock the mutex twice.
#include <threads.h>
enum { MIN_BALANCE = 50 };
static int account_balance;
static mtx_t mp;
/* Initialize mp */
static int verify_balance(int amount) {
if (account_balance - amount < MIN_BALANCE) {
return -1; /* Indicate error to caller */
}
return 0; /* Indicate success to caller */
}
int debit(int amount) {
if (mtx_lock(&mp) == thrd_error) {
return -1; /* Indicate error to caller */
}
if (verify_balance(amount)) {
mtx_unlock(&mp);
return -1; /* Indicate error to caller */
}
account_balance -= amount;
if (mtx_unlock(&mp) == thrd_error) {
return -1; /* Indicate error to caller */
}
return 0; /* Indicate success */
}
Risk Assessment
Improper use of mutexes can result in denial-of-service attacks or the unexpected termination of a multithreaded program.
Recommendation | Severity | Likelihood | Remediation Cost | Priority | Level |
|---|---|---|---|---|---|
CON01-C | Low | Probable | Medium | P4 | L3 |
Automated Detection
| Tool | Version | Checker | Description |
|---|---|---|---|
| Astrée | 24.04 | Supported, but no explicit checker | |
| CodeSonar | 8.1p0 | CONCURRENCY.LOCK.NOLOCK CONCURRENCY.LOCK.NOUNLOCK | Missing Lock Acquisition Missing Lock Release |
| Coverity | 6.5 | LOCK | Fully implemented |
| Parasoft C/C++test | 2023.1 | CERT_C-CON01-a | Do not abandon unreleased locks |
| PC-lint Plus | 1.4 | 454, 455, 456 | Partially supported: acquire and release synchronization primitives within the same scope |
| Polyspace Bug Finder | R2024a | Checks for:
Rec. fully covered. |
Bibliography
| [IEEE Std 1003.1:2013] | XSH, System Interfaces, pthread_mutex_initXSH, System Interfaces, pthread_mutex_lock, pthread_mutex_unlockXSH, System Interfaces, pthread_mutexattr_init |
12 Comments
David Svoboda
Good rule so far. Comments:
Unknown User (krishant)
Summary of the changes made:
Is POS47-C really an exception? The CCE for that rule follows this recommendation, even though the NCCE's do not.
David Svoboda
I think it is, so I added the exception. Everything else looks good.
Martin Sebor
A few comments:
volatilequalifier in the examples is unnecessary. I suggest to remove it for the sake of clarity (see also POS03-C. Do not use volatile as a synchronization primitive and DCL34-C. Use volatile for data that cannot be cached).David Svoboda
Martin, I made all the changes you suggested, except for the following:
The two main hazards with locking bugs are deadlocks and race conditions AKA data races, which the locks are designed to prevent. I don't have any particular details of worse things that can happen. (I expect a double unlock might cause an out-of-bounds read or a null pointer dereference on some platform.)
I think we've been using an external table to map CWE references and CERT rules. So this association belongs on the wiki, but not necessarily here.
Martin Sebor
Great! I suspect you might be right about double unlock potentially having the same effects as double free even though I don't see anything to support that hypothesis in the Solaris
mutex_unlock()code or in glibcpthread_mutex_unlock().Are you fine with renaming the practice?
David Svoboda
Yes, I adjusted the title as you suggested.
BTW double free() is considerably worse than I was suggesting...in the right circumstances, a double free() can permit an attacker to run shellcode. I don't see how that is possible with a double unlock. AFAICT a double unlock is undefined behavior, so what happens next is up to the implementation. I just suspect it might cause an out-of-bounds read or null pointer dereference. Which might lead to a program crash. Which is much less harmful than shellcode.
EDIT: I suppose a double-unlock could lead to executable shellcode if it caused a double-free. I still think its very unlikely.
Martin Sebor
Looks good, thanks!
Re: double unlock and double free, I was thinking that if
pthread_mutex_unlock()resulted in unlinking the mutex from a linked list as it seems to in the Solaris implementation (see the call toqueue_lock()inmutex_wakeup()) then it could have the same effect as double free (i.e., writing arbitrary values to arbitrary memory). It does seem like it would be pretty hard to control though.It might be fun to try to produce an exploit – if only I had a few weeks of free time on my hands...
Santiago Urueña
I think the
verify_balancefunction of the compliant solution is not OK because it is reading theaccount_balanceglobal variable without first acquiring the mutex. POSIX avoids race conditions preventing memory conflicts (i.e. two threads cannot access the same memory location at the same time, and at least one is a write), so just locking the mutex from the writer thread is not enough: The reader must also lock the mutex before reading the shared variable to ensure this thread does not read it while being modified by other thread (is not guaranteed to be an atomic operation).Martin Sebor
I think the intent is that
verify_balance()is a implementation function that can only be called fromdebit()when the mutex is locked. I tried to make it clearer by declaring the function static.However, there was a race condition in the compliant solution in assigning to the global variable
ret. Since the variable isn't necessary to demonstrate the problem or the solution I removed it.Santiago Urueña
Thanks for the clarification, I though
verify_balance()was meant to be called concurrently. No data race is possible then.Good idea using
staticfor private objects and functions!Dean Sutherland
This is clearly good advice. Perhaps we should add the true requirement, which permits more possible placements than this. That true requirement is this:
where the domination and post-domination properties are defined over the executing thread's global control-flow graph. Further, intervening acquisition or release of the mutex is permitted only for recursive mutexes and only when the acquisitions and releases are correctly balanced.
Thinking a bit more, even that requirement isn't (quite) the true minimal requirement. We actually need a (set of) mutex acquisitions and a matching set of mutex releases where the acquisition(s) collectively dominate the release(s) and the releases collectively post-dominate the acquisitions (and the rule about intervening operations on the same mutex still holds). But that's getting rather baroque.