V642. Function result is saved inside the 'byte' type variable. Significant bits may be lost. This may break the program's logic.

The analyzer has detected a potential error: a function result is saved into a variable whose size is only 8 or 16 bits. It may be inadmissible for some functions that return a status of the 'int' type: significant bits may get lost.

Consider the following example of incorrect code:

char c = memcmp(buf1, buf2, n);
if (c != 0)
{
  ...
}

The 'memcmp' function returns the following values of the 'int' type:

• < 0 - buf1 less than buf2;
• 0 - buf1 identical to buf2;
• > 0 - buf1 greater than buf2;

Note that "> 0" means any numbers, not 1. It can be 2, 3, 100, 256, 1024, 5555 and so on. It means that this result cannot be stored in a 'char'-variable, as significant bits may be thrown off, which will violate the program execution logic.

What is dangerous about such errors is that the returned value may depend on the architecture and an implementation of a particular function on this architecture. For instance, the program may work correctly in the 32-bit mode and incorrectly in the 64-bit mode.

This is the fixed code:

int c = memcmp(buf1, buf2, n);
if (c != 0)
{
  ...
}

Some of you might think that this danger is farfetched. But this error caused a severe vulnerability in MySQL/MariaDB up to versions 5.1.61, 5.2.11, 5.3.5, 5.5.22. The point is that when a MySQL /MariaDB user logins, the token (SHA of the password and hash) is calculated and compared to the expected value returned by the 'memcmp' function. On some platforms the returned value might fall out of the range [-128..127]. As a result, in 1 case in 256 the procedure of comparing the hash with the expected value always returns 'true' regardless of the hash. It means that an intruder can use a simple bash-command to get root access to the vulnerable MySQL server even if he/she doesn't know the password. This breach is caused by the following code contained in the file 'sql/password.c':

typedef char my_bool;
...
my_bool check(...) {
  return memcmp(...);
}

This issue is described in more detail here: Security vulnerability in MySQL/MariaDB.

This diagnostic is classified as: