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hash_equals: Leak less lengths differences. (Bug 67939) #792

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hash_equals: Leak less lengths differences. (Bug 67939) #792

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2369d7b
hash_equals: Don't leak length difference.
dunglas Aug 31, 2014
4516782
hash_equals: Add optimizations inspired from CPython
dunglas Aug 31, 2014
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29 changes: 19 additions & 10 deletions ext/hash/hash.c
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Original file line number Diff line number Diff line change
Expand Up @@ -730,12 +730,13 @@ PHP_FUNCTION(hash_pbkdf2)

/* {{{ proto bool hash_equals(string known_string, string user_string)
Compares two strings using the same time whether they're equal or not.
A difference in length will leak */
Even with the implemented mitigation, a difference in length will leak.
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Why is this removed? It's still true.

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Fixed.

Inspired by CPython's algorithm: https://github.com/python/cpython/blob/c7688b44387d116522ff53c0927169db45969f0e/Modules/_operator.c#L175 */
PHP_FUNCTION(hash_equals)
{
zval *known_zval, *user_zval;
char *known_str, *user_str;
int result = 0, j;
volatile const char *known_str, *user_str;
volatile int known_len, user_len, result, j;

if (zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "zz", &known_zval, &user_zval) == FAILURE) {
return;
Expand All @@ -752,16 +753,24 @@ PHP_FUNCTION(hash_equals)
RETURN_FALSE;
}

if (Z_STRLEN_P(known_zval) != Z_STRLEN_P(user_zval)) {
RETURN_FALSE;
}
known_len = Z_STRLEN_P(known_zval);
user_len = Z_STRLEN_P(user_zval);

known_str = Z_STRVAL_P(known_zval);
user_str = Z_STRVAL_P(user_zval);

/* This is security sensitive code. Do not optimize this for speed. */
for (j = 0; j < Z_STRLEN_P(known_zval); j++) {
result |= known_str[j] ^ user_str[j];
/* This is security sensitive code. Do not optimize this for speed nor use else. */
if (known_len == user_len) {
known_str = *((volatile const char **) &Z_STRVAL_P(known_zval));
result = 0;
}

if (known_len != user_len) {
known_str = Z_STRVAL_P(user_zval);
result = 1;
}

for (j = 0; j < user_len; j++) {
result |= *known_str++ ^ *user_str++;
}

RETURN_BOOL(0 == result);
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