Prevent unauthenticated client from easily consuming lots of memory (CVE-2021-32675)

This change sets a low limit for multibulk and bulk length in the
protocol for unauthenticated connections, so that they can't easily
cause redis to allocate massive amounts of memory by sending just a few
characters on the network.
The new limits are 10 arguments of 16kb each (instead of 1m of 512mb)

src/networking.c

@@ -107,6 +107,15 @@ static void clientSetDefaultAuth(client *c) {
                        !(c->user->flags & USER_FLAG_DISABLED);
 }
 
+int authRequired(client *c) {
+    /* Check if the user is authenticated. This check is skipped in case
+     * the default user is flagged as "nopass" and is active. */
+    int auth_required = (!(DefaultUser->flags & USER_FLAG_NOPASS) ||
+                          (DefaultUser->flags & USER_FLAG_DISABLED)) &&
+                        !c->authenticated;
+    return auth_required;
+}
+
 client *createClient(connection *conn) {
     client *c = zmalloc(sizeof(client));
 
@@ -1860,6 +1869,10 @@ int processMultibulkBuffer(client *c) {
             addReplyError(c,"Protocol error: invalid multibulk length");
             setProtocolError("invalid mbulk count",c);
             return C_ERR;
+        } else if (ll > 10 && authRequired(c)) {
+            addReplyError(c, "Protocol error: unauthenticated multibulk length");
+            setProtocolError("unauth mbulk count", c);
+            return C_ERR;
         }
 
         c->qb_pos = (newline-c->querybuf)+2;
@@ -1907,6 +1920,10 @@ int processMultibulkBuffer(client *c) {
                 addReplyError(c,"Protocol error: invalid bulk length");
                 setProtocolError("invalid bulk length",c);
                 return C_ERR;
+            } else if (ll > 16384 && authRequired(c)) {
+                addReplyError(c, "Protocol error: unauthenticated bulk length");
+                setProtocolError("unauth bulk length", c);
+                return C_ERR;
             }
 
             c->qb_pos = newline-c->querybuf+2;

src/server.c

@@ -4001,13 +4001,8 @@ int processCommand(client *c) {
     int is_may_replicate_command = (c->cmd->flags & (CMD_WRITE | CMD_MAY_REPLICATE)) ||
                                    (c->cmd->proc == execCommand && (c->mstate.cmd_flags & (CMD_WRITE | CMD_MAY_REPLICATE)));
 
-    /* Check if the user is authenticated. This check is skipped in case
-     * the default user is flagged as "nopass" and is active. */
-    int auth_required = (!(DefaultUser->flags & USER_FLAG_NOPASS) ||
-                          (DefaultUser->flags & USER_FLAG_DISABLED)) &&
-                        !c->authenticated;
-    if (auth_required) {
-        /* AUTH and HELLO and no auth modules are valid even in
+    if (authRequired(c)) {
+        /* AUTH and HELLO and no auth commands are valid even in
          * non-authenticated state. */
         if (!(c->cmd->flags & CMD_NO_AUTH)) {
             rejectCommand(c,shared.noautherr);

src/server.h

@@ -1897,6 +1897,7 @@ void protectClient(client *c);
 void unprotectClient(client *c);
 void initThreadedIO(void);
 client *lookupClientByID(uint64_t id);
+int authRequired(client *c);
 
 #ifdef __GNUC__
 void addReplyErrorFormat(client *c, const char *fmt, ...)

tests/unit/auth.tcl

@@ -24,6 +24,22 @@ start_server {tags {"auth"} overrides {requirepass foobar}} {
         r set foo 100
         r incr foo
     } {101}
+
+    test {For unauthenticated clients multibulk and bulk length are limited} {
+        set rr [redis [srv "host"] [srv "port"] 0 $::tls]
+        $rr write "*100\r\n"
+        $rr flush
+        catch {[$rr read]} e
+        assert_match {*unauthenticated multibulk length*} $e
+        $rr close
+
+        set rr [redis [srv "host"] [srv "port"] 0 $::tls]
+        $rr write "*1\r\n\$100000000\r\n"
+        $rr flush
+        catch {[$rr read]} e
+        assert_match {*unauthenticated bulk length*} $e
+        $rr close
+    }
 }
 
 start_server {tags {"auth_binary_password"}} {