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Mutexes are often used for critical resources to prevent multiple threads accessing them from causing a data race by accessing shared resources at the same time. Sometimes, when locking mutexes, deadlock will happen when multiple threads hold each other's lock, and the program come to a halt. There are four requirements for deadlockconsequently deadlocks. Four conditions are required for deadlock to occur:

Mutual Exclusionexclusion
Hold and Waitwait
No Preemptionpreemption
Circular Waitwait

Deadlock needs Each deadlock requires all four . Therefore, to prevent deadlock one just need to avoid conditions, so preventing deadlock requires preventing any one of the four . The advice of this guideline conditions. One simple solution is to require locking lock the mutexes in a predefined order to prevent , which prevents circular wait.

Noncompliant Code Example

Based on The behavior of this noncompliant code example depends on the runtime environment and the scheduler on the operating system, the following code will have different behaviors. However, with proper timing, the code will deadlock in which thr1 tries platform's scheduler. The program is susceptible to deadlock if thread thr1 attempts to lock ba2's mutex while thr2 tries at the same time thread thr2 attempts to lock on ba1's mutex and the program will not progressin the deposit() function.

Code Block

bgColor	#ffcccc
lang	c


#include <stdio<stdlib.h>
#include <pthread<threads.h>
#include <stdlib.h>
 
typedef struct {
  int balance;
  pthreadmtx_mutex_t balance_mutex; 
} bank_account;

typedef struct {
  bank_account *from;
  bank_account *to;
  int amount;
} deposit_thr_argstransaction;

/* return negative on error */
int void create_bank_account(bank_account **ba,
                         int initial_amount) {

  bank_account *nba = (bank_account *)malloc(
    sizeof(bank_account)
  );
  if (nba == NULL) {
    /* Handle return -1;error */
  }

  nba->balance = initial_amount;
  pthread_mutexif (thrd_success
      != mtx_init(&nba->balance_mutex, NULL);

  *ba = nba;

  return 0mtx_plain)) {
    /* Handle error */
  }

  *ba = nba;
}


voidint *deposit(void *ptr) {
	
  deposit_thr_argstransaction *args = (deposit_thr_argstransaction *)ptr;

  pthread_mutexif (thrd_success != mtx_lock(&(args->from->balance_mutex)); {
    /* Handle error */
  }

  /* notNot enough balance to transfer */
  if (args->from->balance < args->amount) {
    pthread_mutexif (thrd_success
        != mtx_unlock(&(args->from->balance_mutex)); {
      /* Handle error */
    }
    return NULL;-1; /* Indicate error */
  }

  pthread_mutexif (thrd_success != mtx_lock(&(args->to->balance_mutex)); {
    /* Handle error */
  }

  args->from->balance -= args->amount;
  args->to->balance += args->amount;

  pthread_mutexif (thrd_success
      != mtx_unlock(&(args->from->balance_mutex));
  pthread_mutex {
    /* Handle error */
  }

  if (thrd_success
      != mtx_unlock(&(args->to->balance_mutex)) {
    /* Handle error */
  }

  free(ptr);

  return NULL0;
}

int main(void) {

  pthreadthrd_t thr1, thr2;
  inttransaction err*arg1;
  transaction *arg2;
  bank_account *ba1,;
  bank_account *ba2;

  err = create_bank_account(&ba1, 1000);
  if (err < 0) 
    exit(err)create_bank_account(&ba2, 1000);

  errarg1 = create_bank_account(&ba2, 1000(transaction *)malloc(sizeof(transaction));
  if (errarg1 <== 0NULL) {
    exit(err);
/* Handle error */
  }
  deposit_thr_args *arg1 = arg2 = (transaction *)malloc(sizeof(deposit_thr_argstransaction));
  deposit_thr_args *if (arg2 == malloc(sizeof(deposit_thr_args));
NULL) {
    /* Handle error */
  }
  arg1->from = ba1;
  arg1->to = ba2;
  arg1->amount = 100;

  arg2->from = ba2;
  arg2->to = ba1;
  arg2->amount = 100;

  /* performPerform the depositdeposits */
  pthreadif (thrd_success
     != thrd_create(&thr1, NULL, deposit, (void *)arg1);
  pthread) {
    /* Handle error */
  }
  if (thrd_success
      != thrd_create(&thr2, NULL, deposit, (void *)arg2));
 {
    /* Handle error */
  pthread_exit(NULL);
}
  return 0;
}

Compliant Solution

The This compliant solution to the deadlock problem is to lock in predefined order. In the following example, each thread will lock based on bank_account's id in increasing order. This way circular wait problem is avoided and when one thread requires a lock will guarantee it will require the next lockeliminates the circular wait condition by establishing a predefined order for locking in the deposit() function. Each thread will lock on the basis of the bank_account ID, which is set when the bank_account struct is initialized.

Code Block

bgColor	#ccccff
lang	c


#include <stdio<stdlib.h>
#include <pthread<threads.h>
#include <stdlib.h> 

typedef struct {
  int balance;
  pthread_mutexmtx_t balance_mutex; 
 
  unsigned/* intShould id;not /*change readafter onlyinitialization and*/
 should neverunsigned be changed */int id;
} bank_account;

typedef struct {
  bank_account *from;
  bank_account *to;
  int amount;
} deposit_thr_argstransaction;

unsigned int global_id = 1;

/* return negative on error */
intvoid create_bank_account(bank_account **ba,
                         int initial_amount) {

  bank_account *nba = (bank_account *)malloc(
    sizeof(bank_account)
  );

  if (nba == NULL) {
    /* return -1;Handle error */
  }

  nba->balance = initial_amount;
  if pthread_mutex(thrd_success
      != mtx_init(&nba->balance_mutex, NULL);mtx_plain)) {
    /* Handle error */
  }

  nba->id = global_id++;

  *ba = nba;

  return 0;
}


voidint *deposit(void *ptr) {
	
  deposit_thr_argstransaction *args = (deposit_thr_args *)ptrtransaction *)ptr;
  int result = -1;
  mtx_t *first;
  mtx_t *second;

  if (args->from->id == args->to->id) 
		{
    return NULL;
-1; /* Indicate error */
  }

  /* ensureEnsure proper ordering for locking */
  if (args->from->id < args->to->id) {
     pthread_mutex_lock(&(first = &args->from->balance_mutex));
    second = pthread_mutex_lock(&(args->to->balance_mutex));
  } else {
    pthread_mutex_lock(&(first = &args->to->balance_mutex));
    pthread_mutex_lock(&(second = &args->from->balance_mutex));
  }
  if (thrd_success != mtx_lock(first)) {
    /* notHandle enougherror balance*/
 to transfer */}
  if (args->from->balance < args->amountthrd_success != mtx_lock(second)) {
    pthread_mutex_unlock(&(args->from->balance_mutex));
    pthread_mutex_unlock(&(args->to->balance_mutex));
    return NULL;
  }

  /* Handle error */
  }

  /* Not enough balance to transfer */
  if (args->from->balance ->= args->amount;) {
    args->to>from->balance +-= args->amount;

  pthread_mutex_unlock(&(args->from->balance_mutex));
  pthread_mutex_unlock(&(args->to->balance_mutex));

  return NULL;
}

int main() {

  pthread_t thr1, thr2 += args->amount;
    result = 0;
  int err;}

  bank_account *ba1, *ba2;

  err = create_bank_account(&ba1, 1000);
  if (err < 0) if (thrd_success != mtx_unlock(second)) {
    exit(err);

  err = create_bank_account(&ba2, 1000);/* Handle error */
  }
  if (err < 0) 
    exit(err);

  deposit_thr_args *arg1 = malloc(sizeof(deposit_thr_args));
  deposit_thr_args *arg2 = malloc(sizeof(deposit_thr_args));

  arg1->from = ba1;
  arg1->to = ba2;
  arg1->amount = 100;

  arg2->from = ba2;
  arg2->to = ba1;
  arg2->amount = 100;

  /* perform the deposit */
  pthread_create(&thr1, NULL, deposit, (void *)arg1);
  pthread_create(&thr2, NULL, deposit, (void *)arg2);

  pthread_exit(NULL);

  return 0;
}

Risk Assessment

Deadlock causes multiple threads to not be able to progress and thus halt the executing program. This is a potential denial-of-service attack when the attacker can force deadlock situations. It's probable that deadlock will occur in multi-thread programs that manage multiple resources. Some automation for detecting deadlock can be implemented in which the detector can try different inputs and wait for a timeout. The fixes can be done manually.

thrd_success != mtx_unlock(first)) {
    /* Handle error */
  }
  free(ptr);
  return result;
}

Risk Assessment

Deadlock prevents multiple threads from progressing, halting program execution. A denial-of-service attack is possible if the attacker can create the conditions for deadlock.

Rule

Recommendation

	Severity	Likelihood	Remediation Cost	Priority	Level

Priority

POS43

CON35-C

Low

low

Probable

probable

Medium

medium

P4

L3

P3

References

Wiki Markup

\[[pthread_mutex | https://computing.llnl.gov/tutorials/pthreads/#Mutexes]\]  pthread_mutex tutorial
\[[MITRE CWE:764 | http://cwe.mitre.org/data/definitions/764.html]\] Multiple Locks of Critical Resources
\[[Bryant 03|AA. References#Bryant 03]\] Chapter 13, Concurrent Programming