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

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

(1) lstat() which follows symbolic links in all path elements but the
    rightmost one
(2) remove() which follows symbolic links in all path elements but the
    rightmost one

This patch converts local_remove() to rely on opendir_nofollow(),
fstatat(AT_SYMLINK_NOFOLLOW) to fix (1) and unlinkat() to fix (2).

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 a0e640a8)
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

@@ -1022,54 +1022,32 @@ err_out:
 
 static int local_remove(FsContext *ctx, const char *path)
 {
-    int err;
     struct stat stbuf;
-    char *buffer;
+    char *dirpath = g_path_get_dirname(path);
+    char *name = g_path_get_basename(path);
+    int flags = 0;
+    int dirfd;
+    int err = -1;
 
-    if (ctx->export_flags & V9FS_SM_MAPPED_FILE) {
-        buffer = rpath(ctx, path);
-        err =  lstat(buffer, &stbuf);
-        g_free(buffer);
-        if (err) {
-            goto err_out;
-        }
-        /*
-         * If directory remove .virtfs_metadata contained in the
-         * directory
-         */
-        if (S_ISDIR(stbuf.st_mode)) {
-            buffer = g_strdup_printf("%s/%s/%s", ctx->fs_root,
-                                     path, VIRTFS_META_DIR);
-            err = remove(buffer);
-            g_free(buffer);
-            if (err < 0 && errno != ENOENT) {
-                /*
-                 * We didn't had the .virtfs_metadata file. May be file created
-                 * in non-mapped mode ?. Ignore ENOENT.
-                 */
-                goto err_out;
-            }
-        }
-        /*
-         * Now remove the name from parent directory
-         * .virtfs_metadata directory
-         */
-        buffer = local_mapped_attr_path(ctx, path);
-        err = remove(buffer);
-        g_free(buffer);
-        if (err < 0 && errno != ENOENT) {
-            /*
-             * We didn't had the .virtfs_metadata file. May be file created
-             * in non-mapped mode ?. Ignore ENOENT.
-             */
-            goto err_out;
-        }
+    dirfd = local_opendir_nofollow(ctx, dirpath);
+    if (dirfd) {
+        goto out;
     }
 
-    buffer = rpath(ctx, path);
-    err = remove(buffer);
-    g_free(buffer);
+    if (fstatat(dirfd, path, &stbuf, AT_SYMLINK_NOFOLLOW) < 0) {
+        goto err_out;
+    }
+
+    if (S_ISDIR(stbuf.st_mode)) {
+        flags |= AT_REMOVEDIR;
+    }
+
+    err = local_unlinkat_common(ctx, dirfd, name, flags);
 err_out:
+    close_preserve_errno(dirfd);
+out:
+    g_free(name);
+    g_free(dirpath);
     return err;
 }