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Update time independent string compare to use hash length (#9759)

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* Update time independent string compare to use hash length
This commit is contained in:
Madelyn Olson 2022-06-03 09:30:28 -07:00 committed by GitHub
parent a18c91d642
commit 4ad166235e
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GPG Key ID: 4AEE18F83AFDEB23
2 changed files with 6 additions and 36 deletions
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41
src/acl.c
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@ -144,7 +144,7 @@ void ACLFreeLogEntry(void *le);
int ACLSetSelector(aclSelector *selector, const char *op, size_t oplen);
/* The length of the string representation of a hashed password. */
#define HASH_PASSWORD_LEN SHA256_BLOCK_SIZE*2
#define HASH_PASSWORD_LEN (SHA256_BLOCK_SIZE*2)
/* =============================================================================
* Helper functions for the rest of the ACL implementation
@ -153,42 +153,13 @@ int ACLSetSelector(aclSelector *selector, const char *op, size_t oplen);
/* Return zero if strings are the same, non-zero if they are not.
* The comparison is performed in a way that prevents an attacker to obtain
* information about the nature of the strings just monitoring the execution
* time of the function.
*
* Note that limiting the comparison length to strings up to 512 bytes we
* can avoid leaking any information about the password length and any
* possible branch misprediction related leak.
* time of the function. Note: The two strings must be the same length.
*/
int time_independent_strcmp(char *a, char *b) {
char bufa[CONFIG_AUTHPASS_MAX_LEN], bufb[CONFIG_AUTHPASS_MAX_LEN];
/* The above two strlen perform len(a) + len(b) operations where either
* a or b are fixed (our password) length, and the difference is only
* relative to the length of the user provided string, so no information
* leak is possible in the following two lines of code. */
unsigned int alen = strlen(a);
unsigned int blen = strlen(b);
unsigned int j;
int time_independent_strcmp(char *a, char *b, int len) {
int diff = 0;
/* We can't compare strings longer than our static buffers.
* Note that this will never pass the first test in practical circumstances
* so there is no info leak. */
if (alen > sizeof(bufa) || blen > sizeof(bufb)) return 1;
memset(bufa,0,sizeof(bufa)); /* Constant time. */
memset(bufb,0,sizeof(bufb)); /* Constant time. */
/* Again the time of the following two copies is proportional to
* len(a) + len(b) so no info is leaked. */
memcpy(bufa,a,alen);
memcpy(bufb,b,blen);
/* Always compare all the chars in the two buffers without
* conditional expressions. */
for (j = 0; j < sizeof(bufa); j++) {
diff |= (bufa[j] ^ bufb[j]);
for (int j = 0; j < len; j++) {
diff |= (a[j] ^ b[j]);
}
/* Length must be equal as well. */
diff |= alen ^ blen;
return diff; /* If zero strings are the same. */
}
@ -1414,7 +1385,7 @@ int ACLCheckUserCredentials(robj *username, robj *password) {
sds hashed = ACLHashPassword(password->ptr,sdslen(password->ptr));
while((ln = listNext(&li))) {
sds thispass = listNodeValue(ln);
if (!time_independent_strcmp(hashed, thispass)) {
if (!time_independent_strcmp(hashed, thispass, HASH_PASSWORD_LEN)) {
sdsfree(hashed);
return C_OK;
}

1
src/server.h
View File

@ -114,7 +114,6 @@ typedef long long ustime_t; /* microsecond time type. */
#define LOG_MAX_LEN 1024 /* Default maximum length of syslog messages.*/
#define AOF_REWRITE_ITEMS_PER_CMD 64
#define AOF_ANNOTATION_LINE_MAX_LEN 1024
#define CONFIG_AUTHPASS_MAX_LEN 512
#define CONFIG_RUN_ID_SIZE 40
#define RDB_EOF_MARK_SIZE 40
#define CONFIG_REPL_BACKLOG_MIN_SIZE (1024*16) /* 16k */