9pfs: local: chown: don't follow symlinks

The local_chown() callback is vulnerable to symlink attacks because it
calls:

(1) lchown() which follows symbolic links for all path elements but the
    rightmost one
(2) local_set_xattr()->setxattr() which follows symbolic links for all
    path elements
(3) local_set_mapped_file_attr() which calls in turn local_fopen() and
    mkdir(), both functions following symbolic links for all path
    elements but the rightmost one

This patch converts local_chown() to rely on open_nofollow() and
fchownat() to fix (1), as well as local_set_xattrat() and
local_set_mapped_file_attrat() to fix (2) and (3) respectively.

This partly fixes CVE-2016-9602.
Signed-off-by: NGreg Kurz <groug@kaod.org>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
(cherry picked from commit d369f207)
Signed-off-by: NGreg Kurz <gkurz@linux.vnet.ibm.com>
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>

hw/9pfs/9p-local.c

@@ -1161,23 +1161,31 @@ static int local_truncate(FsContext *ctx, V9fsPath *fs_path, off_t size)
 
 static int local_chown(FsContext *fs_ctx, V9fsPath *fs_path, FsCred *credp)
 {
-    char *buffer;
+    char *dirpath = g_path_get_dirname(fs_path->data);
+    char *name = g_path_get_basename(fs_path->data);
     int ret = -1;
-    char *path = fs_path->data;
+    int dirfd;
+
+    dirfd = local_opendir_nofollow(fs_ctx, dirpath);
+    if (dirfd == -1) {
+        goto out;
+    }
 
     if ((credp->fc_uid == -1 && credp->fc_gid == -1) ||
         (fs_ctx->export_flags & V9FS_SM_PASSTHROUGH) ||
         (fs_ctx->export_flags & V9FS_SM_NONE)) {
-        buffer = rpath(fs_ctx, path);
-        ret = lchown(buffer, credp->fc_uid, credp->fc_gid);
-        g_free(buffer);
+        ret = fchownat(dirfd, name, credp->fc_uid, credp->fc_gid,
+                       AT_SYMLINK_NOFOLLOW);
     } else if (fs_ctx->export_flags & V9FS_SM_MAPPED) {
-        buffer = rpath(fs_ctx, path);
-        ret = local_set_xattr(buffer, credp);
-        g_free(buffer);
+        ret = local_set_xattrat(dirfd, name, credp);
     } else if (fs_ctx->export_flags & V9FS_SM_MAPPED_FILE) {
-        return local_set_mapped_file_attr(fs_ctx, path, credp);
+        ret = local_set_mapped_file_attrat(dirfd, name, credp);
     }
+
+    close_preserve_errno(dirfd);
+out:
+    g_free(name);
+    g_free(dirpath);
     return ret;
 }