Security: CONFIG SET client-output-buffer-limit overflow fixed.

This commit fixes a vunlerability reported by Cory Duplantis
of Cisco Talos, see TALOS-2016-0206 for reference.

CONFIG SET client-output-buffer-limit accepts as client class "master"
which is actually only used to implement CLIENT KILL. The "master" class
has ID 3. What happens is that the global structure:

    server.client_obuf_limits[class]

Is accessed with class = 3. However it is a 3 elements array, so writing
the 4th element means to write up to 24 bytes of memory *after* the end
of the array, since the structure is defined as:

    typedef struct clientBufferLimitsConfig {
        unsigned long long hard_limit_bytes;
        unsigned long long soft_limit_bytes;
        time_t soft_limit_seconds;
    } clientBufferLimitsConfig;

EVALUATION OF IMPACT:

Checking what's past the boundaries of the array in the global
'server' structure, we find AOF state fields:

    clientBufferLimitsConfig client_obuf_limits[CLIENT_TYPE_OBUF_COUNT];
    /* AOF persistence */
    int aof_state;                  /* AOF_(ON|OFF|WAIT_REWRITE) */
    int aof_fsync;                  /* Kind of fsync() policy */
    char *aof_filename;             /* Name of the AOF file */
    int aof_no_fsync_on_rewrite;    /* Don't fsync if a rewrite is in prog. */
    int aof_rewrite_perc;           /* Rewrite AOF if % growth is > M and... */
    off_t aof_rewrite_min_size;     /* the AOF file is at least N bytes. */
    off_t aof_rewrite_base_size;    /* AOF size on latest startup or rewrite. */
    off_t aof_current_size;         /* AOF current size. */

Writing to most of these fields should be harmless and only cause problems in
Redis persistence that should not escalate to security problems.
However unfortunately writing to "aof_filename" could be potentially a
security issue depending on the access pattern.

Searching for "aof.filename" accesses in the source code returns many different
usages of the field, including using it as input for open(), logging to the
Redis log file or syslog, and calling the rename() syscall.

It looks possible that attacks could lead at least to informations
disclosure of the state and data inside Redis. However note that the
attacker must already have access to the server. But, worse than that,
it looks possible that being able to change the AOF filename can be used
to mount more powerful attacks: like overwriting random files with AOF
data (easily a potential security issue as demostrated here:
http://antirez.com/news/96), or even more subtle attacks where the
AOF filename is changed to a path were a malicious AOF file is loaded
in order to exploit other potential issues when the AOF parser is fed
with untrusted input (no known issue known currently).

The fix checks the places where the 'master' class is specifiedf in
order to access configuration data structures, and return an error in
this cases.

WHO IS AT RISK?

The "master" client class was introduced in Redis in Jul 28 2015.
Every Redis instance released past this date is not vulnerable
while all the releases after this date are. Notably:

    Redis 3.0.x is NOT vunlerable.
    Redis 3.2.x IS vulnerable.
    Redis unstable is vulnerable.

In order for the instance to be at risk, at least one of the following
conditions must be true:

    1. The attacker can access Redis remotely and is able to send
       the CONFIG SET command (often banned in managed Redis instances).

    2. The attacker is able to control the "redis.conf" file and
       can wait or trigger a server restart.

The problem was fixed 26th September 2016 in all the releases affected.

src/config.c

@@ -616,8 +616,9 @@ void loadServerConfigFromString(char *config) {
             unsigned long long hard, soft;
             int soft_seconds;
 
-            if (class == -1) {
+            if (class == -1 || class == CLIENT_TYPE_MASTER) {
-                err = "Unrecognized client limit class";
+                err = "Unrecognized client limit class: the user specified "
+                "an invalid one, or 'master' which has no buffer limits.";
                 goto loaderr;
             }
             hard = memtoll(argv[2],NULL);
@@ -906,7 +907,8 @@ void configSetCommand(client *c) {
             long val;
 
             if ((j % 4) == 0) {
-                if (getClientTypeByName(v[j]) == -1) {
+                int class = getClientTypeByName(v[j]);
+                if (class == -1 || class == CLIENT_TYPE_MASTER) {
                     sdsfreesplitres(v,vlen);
                     goto badfmt;
                 }