CWE Rule 560

This issue occurs when umask commands have arguments specified in the style of arguments to chmod and provide possibly unintended permissions. For instance:

For new files, the umask argument or the mask value specifies which permissions not to set, in other words, which permissions to remove. The mask is bitwise-negated and then applied to new file permissions. In contrast, chmod sets the permissions as you specify them.

If you use chmod-style arguments, you specify opposite permissions of what you want. This mistake can give external users unintended read/write access to new files and folders.

To fix this defect, perform both of these tasks:

You can see the umask value by calling,

umask

umask -S

#include <stdio.h>
#include <assert.h>
#include <sys/types.h>
#include <sys/stat.h>

typedef mode_t (*umask_func)(mode_t);

const mode_t default_mode = (
    S_IRUSR    /* 00400 */ 
    | S_IWUSR  /* 00200 */ 
    | S_IRGRP  /* 00040 */ 
    | S_IWGRP  /* 00020 */
    | S_IROTH  /* 00004 */
    | S_IWOTH  /* 00002 */
    );         /* 00666 (i.e. -rw-rw-rw-) */

static void my_umask(mode_t mode)
{
    umask(mode);  //Noncompliant
}

int umask_use(mode_t m)
{
    my_umask(default_mode);
    return 0;
}

This example uses a function called my_umask to set the default mask mode. However, the default_mode variable gives the permissions 666 or -rw-rw-rw. umask negates this value. However, this negation means the default mask mode turns off read/write permissions for the user, group users, and other outside users.

One possible correction is to negate the default_mode argument to my_umask. This correction nullifies the negation umask for new files.

#include <stdio.h>
#include <assert.h>
#include <sys/types.h>
#include <sys/stat.h>

typedef mode_t (*umask_func)(mode_t);

const mode_t default_mode = (
    S_IRUSR    /* 00400 */ 
    | S_IWUSR  /* 00200 */ 
    | S_IRGRP  /* 00040 */ 
    | S_IWGRP  /* 00020 */
    | S_IROTH  /* 00004 */
    | S_IWOTH  /* 00002 */
    );         /* 00666 (i.e. -rw-rw-rw-) */

static void my_umask(mode_t mode)
{
    umask(mode);
}

int umask_use(mode_t m)
{
    my_umask(~default_mode);
    return 0;
}