Fix issue of listen before chmod on Unix sockets (CVE-2023-45145) (redis#12671)

Before this commit, Unix socket setup performed chmod(2) on the socket
file after calling listen(2). Depending on what umask is used, this
could leave the file with the wrong permissions for a short period of
time. As a result, another process could exploit this race condition and
establish a connection that would otherwise not be possible.

We now make sure the socket permissions are set up prior to calling
listen(2).

(cherry picked from commit 1119eca)

Co-authored-by: Yossi Gottlieb <yossigo@gmail.com>

Author: oranagra

src/anet.c

@@ -417,13 +417,16 @@ int anetUnixGenericConnect(char *err, const char *path, int flags)
     return s;
 }
 
-static int anetListen(char *err, int s, struct sockaddr *sa, socklen_t len, int backlog) {
+static int anetListen(char *err, int s, struct sockaddr *sa, socklen_t len, int backlog, mode_t perm) {
     if (bind(s,sa,len) == -1) {
         anetSetError(err, "bind: %s", strerror(errno));
         close(s);
         return ANET_ERR;
     }
 
+    if (sa->sa_family == AF_LOCAL && perm)
+        chmod(((struct sockaddr_un *) sa)->sun_path, perm);
+
     if (listen(s, backlog) == -1) {
         anetSetError(err, "listen: %s", strerror(errno));
         close(s);
@@ -467,7 +470,7 @@ static int _anetTcpServer(char *err, int port, char *bindaddr, int af, int backl
 
         if (af == AF_INET6 && anetV6Only(err,s) == ANET_ERR) goto error;
         if (anetSetReuseAddr(err,s) == ANET_ERR) goto error;
-        if (anetListen(err,s,p->ai_addr,p->ai_addrlen,backlog) == ANET_ERR) s = ANET_ERR;
+        if (anetListen(err,s,p->ai_addr,p->ai_addrlen,backlog,0) == ANET_ERR) s = ANET_ERR;
         goto end;
     }
     if (p == NULL) {
@@ -508,10 +511,8 @@ int anetUnixServer(char *err, char *path, mode_t perm, int backlog)
     memset(&sa,0,sizeof(sa));
     sa.sun_family = AF_LOCAL;
     redis_strlcpy(sa.sun_path,path,sizeof(sa.sun_path));
-    if (anetListen(err,s,(struct sockaddr*)&sa,sizeof(sa),backlog) == ANET_ERR)
+    if (anetListen(err,s,(struct sockaddr*)&sa,sizeof(sa),backlog,perm) == ANET_ERR)
         return ANET_ERR;
-    if (perm)
-        chmod(sa.sun_path, perm);
     return s;
 }