sockets: improve error reporting if UNIX socket path is too long

The 'struct sockaddr_un' only allows 108 bytes for the socket
path.

If the user supplies a path, QEMU uses snprintf() to silently
truncate it when too long. This is undesirable because the user
will then be unable to connect to the path they asked for.

If the user doesn't supply a path, QEMU builds one based on
TMPDIR, but if that leads to an overlong path, it mistakenly
uses error_setg_errno() with a stale errno value, because
snprintf() does not set errno on truncation.

In solving this the code needed some refactoring to ensure we
don't pass 'un.sun_path' directly to any APIs which expect
NUL-terminated strings, because the path is not required to
be terminated.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>
Message-Id: <20170525155300.22743-1-berrange@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

util/qemu-sockets.c

@@ -845,6 +845,8 @@ static int unix_listen_saddr(UnixSocketAddress *saddr,
 {
     struct sockaddr_un un;
     int sock, fd;
+    char *pathbuf = NULL;
+    const char *path;
 
     sock = qemu_socket(PF_UNIX, SOCK_STREAM, 0);
     if (sock < 0) {
@@ -852,20 +854,22 @@ static int unix_listen_saddr(UnixSocketAddress *saddr,
         return -1;
     }
 
-    memset(&un, 0, sizeof(un));
-    un.sun_family = AF_UNIX;
-    if (saddr->path && strlen(saddr->path)) {
-        snprintf(un.sun_path, sizeof(un.sun_path), "%s", saddr->path);
+    if (saddr->path && saddr->path[0]) {
+        path = saddr->path;
     } else {
         const char *tmpdir = getenv("TMPDIR");
         tmpdir = tmpdir ? tmpdir : "/tmp";
-        if (snprintf(un.sun_path, sizeof(un.sun_path), "%s/qemu-socket-XXXXXX",
-                     tmpdir) >= sizeof(un.sun_path)) {
-            error_setg_errno(errp, errno,
-                             "TMPDIR environment variable (%s) too large", tmpdir);
-            goto err;
-        }
+        path = pathbuf = g_strdup_printf("%s/qemu-socket-XXXXXX", tmpdir);
+    }
 
+    if (strlen(path) > sizeof(un.sun_path)) {
+        error_setg(errp, "UNIX socket path '%s' is too long", path);
+        error_append_hint(errp, "Path must be less than %zu bytes\n",
+                          sizeof(un.sun_path));
+        goto err;
+    }
+
+    if (pathbuf != NULL) {
         /*
          * This dummy fd usage silences the mktemp() unsecure warning.
          * Using mkstemp() doesn't make things more secure here
@@ -873,24 +877,25 @@ static int unix_listen_saddr(UnixSocketAddress *saddr,
          * to unlink first and thus re-open the race window.  The
          * worst case possible is bind() failing, i.e. a DoS attack.
          */
-        fd = mkstemp(un.sun_path);
+        fd = mkstemp(pathbuf);
         if (fd < 0) {
             error_setg_errno(errp, errno,
-                             "Failed to make a temporary socket name in %s", tmpdir);
+                             "Failed to make a temporary socket %s", pathbuf);
             goto err;
         }
         close(fd);
-        if (update_addr) {
-            g_free(saddr->path);
-            saddr->path = g_strdup(un.sun_path);
-        }
     }
 
-    if (unlink(un.sun_path) < 0 && errno != ENOENT) {
+    if (unlink(path) < 0 && errno != ENOENT) {
         error_setg_errno(errp, errno,
-                         "Failed to unlink socket %s", un.sun_path);
+                         "Failed to unlink socket %s", path);
         goto err;
     }
+
+    memset(&un, 0, sizeof(un));
+    un.sun_family = AF_UNIX;
+    strncpy(un.sun_path, path, sizeof(un.sun_path));
+
     if (bind(sock, (struct sockaddr*) &un, sizeof(un)) < 0) {
         error_setg_errno(errp, errno, "Failed to bind socket to %s", un.sun_path);
         goto err;
@@ -900,9 +905,16 @@ static int unix_listen_saddr(UnixSocketAddress *saddr,
         goto err;
     }
 
+    if (update_addr && pathbuf) {
+        g_free(saddr->path);
+        saddr->path = pathbuf;
+    } else {
+        g_free(pathbuf);
+    }
     return sock;
 
 err:
+    g_free(pathbuf);
     closesocket(sock);
     return -1;
 }
@@ -932,9 +944,16 @@ static int unix_connect_saddr(UnixSocketAddress *saddr,
         qemu_set_nonblock(sock);
     }
 
+    if (strlen(saddr->path) > sizeof(un.sun_path)) {
+        error_setg(errp, "UNIX socket path '%s' is too long", saddr->path);
+        error_append_hint(errp, "Path must be less than %zu bytes\n",
+                          sizeof(un.sun_path));
+        goto err;
+    }
+
     memset(&un, 0, sizeof(un));
     un.sun_family = AF_UNIX;
-    snprintf(un.sun_path, sizeof(un.sun_path), "%s", saddr->path);
+    strncpy(un.sun_path, saddr->path, sizeof(un.sun_path));
 
     /* connect to peer */
     do {
@@ -956,13 +975,18 @@ static int unix_connect_saddr(UnixSocketAddress *saddr,
     }
 
     if (rc < 0) {
-        error_setg_errno(errp, -rc, "Failed to connect socket");
-        close(sock);
-        sock = -1;
+        error_setg_errno(errp, -rc, "Failed to connect socket %s",
+                         saddr->path);
+        goto err;
     }
 
     g_free(connect_state);
     return sock;
+
+ err:
+    close(sock);
+    g_free(connect_state);
+    return -1;
 }
 
 #else