}
struct stat s;
- int result = stat(path, &s);
+ int result = lstat(path, &s);
if (result == 0) {
return s.st_mode;
}
else {
- /* errno will be set already by stat() */
+ /* errno will be set already by lstat() */
return result;
}
}
+/**
+ * @brief Determine if the given path might refer to an (unsafe) hard link.
+ *
+ * @param path
+ * The path to test.
+ *
+ * @return true if we are certain that @c path does not refer to a hard
+ * link, and false otherwise. In case of error, false is returned,
+ * because we are not sure that @c path is not a hard link.
+ */
+bool is_hardlink_safe(const char* path) {
+ if (path == NULL) {
+ return false;
+ }
+ struct stat s;
+ int result = lstat(path, &s);
+ if (result == 0) {
+ return (s.st_nlink == 1 || S_ISDIR(s.st_mode));
+ }
+ else {
+ return false;
+ }
+}
+
+
/**
* @brief Determine whether or not the given path is a regular file.
*
}
struct stat s;
- int result = stat(path, &s);
+ int result = lstat(path, &s);
if (result == 0) {
return S_ISREG(s.st_mode);
}
}
struct stat s;
- int result = stat(path, &s);
+ int result = lstat(path, &s);
if (result == 0) {
return S_ISDIR(s.st_mode);
}
/**
- * @brief Inherit the default ACL from @c parent to @c path.
- *
- * The @c parent parameter does not necessarily need to be the parent
- * of @c path, although that will usually be the case. This overwrites
- * any existing default ACL on @c path.
+ * @brief Set @c acl as the default ACL on @c path if it's a directory.
*
- * @param parent
- * The parent directory whose ACL we want to inherit.
+ * This overwrites any existing default ACL on @c path. If no default
+ * ACL exists, then one is created. If @c path is not a directory, we
+ * return ACL_FAILURE but no error is raised.
*
* @param path
- * The target directory whose ACL we wish to overwrite (or create).
+ * The target directory whose ACL we wish to replace or create.
+ *
+ * @param acl
+ * The ACL to set as default on @c path.
*
* @return
- * - @c ACL_SUCCESS - The default ACL was inherited successfully.
- * - @c ACL_FAILURE - Either @c parent or @c path is not a directory.
+ * - @c ACL_SUCCESS - The default ACL was assigned successfully.
+ * - @c ACL_FAILURE - If @c path is not a directory.
* - @c ACL_ERROR - Unexpected library error.
*/
-int inherit_default_acl(const char* path, const char* parent) {
-
- /* Our return value. */
- int result = ACL_SUCCESS;
+int assign_default_acl(const char* path, acl_t acl) {
if (path == NULL) {
errno = ENOENT;
return ACL_ERROR;
}
- if (!is_directory(path) || !is_directory(parent)) {
+ if (!is_directory(path)) {
return ACL_FAILURE;
}
- acl_t parent_acl = acl_get_file(parent, ACL_TYPE_DEFAULT);
- if (parent_acl == (acl_t)NULL) {
- perror("inherit_default_acl (acl_get_file)");
- return ACL_ERROR;
- }
-
- acl_t path_acl = acl_dup(parent_acl);
+ /* Our return value; success unless something bad happens. */
+ int result = ACL_SUCCESS;
+ acl_t path_acl = acl_dup(acl);
if (path_acl == (acl_t)NULL) {
perror("inherit_default_acl (acl_dup)");
- acl_free(parent_acl);
- return ACL_ERROR;
+ return ACL_ERROR; /* Nothing to clean up in this case. */
}
int sf_result = acl_set_file(path, ACL_TYPE_DEFAULT, path_acl);
if (sf_result == -1) {
perror("inherit_default_acl (acl_set_file)");
result = ACL_ERROR;
- goto cleanup;
}
- cleanup:
acl_free(path_acl);
return result;
}
return ACL_ERROR;
}
+ /* Refuse to operate on hard links, which can be abused by an
+ * attacker to trick us into changing the ACL on a file we didn't
+ * intend to; namely the "target" of the hard link. To truly prevent
+ * that sort of mischief, we should be using file descriptors for
+ * the target and its parent directory. Then modulo a tiny race
+ * condition, we would be sure that "path" and "parent" don't change
+ * their nature between the time that we test them and when we
+ * utilize them. For contrast, the same attacker is free to replace
+ * "path" with a hard link after is_hardlink_safe() has returned
+ * "true" below.
+ *
+ * Unfortunately, our API is lacking in this area. For example,
+ * acl_set_fd() is only capable of setting the ACL_TYPE_ACCESS list,
+ * and not the ACL_TYPE_DEFAULT. Apparently the only way to operate
+ * on default ACLs is through the path name, which is inherently
+ * unreliable since the acl_*_file() calls themselves might follow
+ * links (both hard and symbolic).
+ *
+ * Some improvement could still be made by using descriptors where
+ * possible -- this would shrink the exploit window -- but for now
+ * we use a naive implementation that only keeps honest men honest.
+ */
+ if (!is_hardlink_safe(path)) {
+ return ACL_FAILURE;
+ }
+
if (!is_regular_file(path) && !is_directory(path)) {
return ACL_FAILURE;
}
}
/* If it's a directory, inherit the parent's default. */
- int inherit_result = inherit_default_acl(path, parent);
+ int inherit_result = assign_default_acl(path, defacl);
if (inherit_result == ACL_ERROR) {
perror("apply_default_acl (inherit_acls)");
result = ACL_ERROR;
* typos, too.
*/
if (!path_accessible(target)) {
- fprintf(stderr, "%s: %s: no such file or directory\n", argv[0], target);
+ fprintf(stderr, "%s: %s: No such file or directory\n", argv[0], target);
result = EXIT_FAILURE;
continue;
}