LSM: Add "contents" flag to kernel_read_file hook

As with the kernel_load_data LSM hook, add a "contents" flag to the
kernel_read_file LSM hook that indicates whether the LSM can expect
a matching call to the kernel_post_read_file LSM hook with the full
contents of the file. With the coming addition of partial file read
support for kernel_read_file*() API, the LSM will no longer be able
to always see the entire contents of a file during the read calls.

For cases where the LSM must read examine the complete file contents,
it will need to do so on its own every time the kernel_read_file
hook is called with contents=false (or reject such cases). Adjust all
existing LSMs to retain existing behavior.

Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Mimi Zohar <zohar@linux.ibm.com>
Link: https://lore.kernel.org/r/20201002173828.2099543-12-keescook@chromium.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

security/integrity/ima/ima_main.c

@@ -602,6 +602,7 @@ void ima_post_path_mknod(struct dentry *dentry)
  * ima_read_file - pre-measure/appraise hook decision based on policy
  * @file: pointer to the file to be measured/appraised/audit
  * @read_id: caller identifier
+ * @contents: whether a subsequent call will be made to ima_post_read_file()
  *
  * Permit reading a file based on policy. The policy rules are written
  * in terms of the policy identifier.  Appraising the integrity of
@@ -609,8 +610,15 @@ void ima_post_path_mknod(struct dentry *dentry)
  *
  * For permission return 0, otherwise return -EACCES.
  */
-int ima_read_file(struct file *file, enum kernel_read_file_id read_id)
+int ima_read_file(struct file *file, enum kernel_read_file_id read_id,
+		  bool contents)
 {
+	/* Reject all partial reads during appraisal. */
+	if (!contents) {
+		if (ima_appraise & IMA_APPRAISE_ENFORCE)
+			return -EACCES;
+	}
+
 	/*
 	 * Do devices using pre-allocated memory run the risk of the
 	 * firmware being accessible to the device prior to the completion

security/loadpin/loadpin.c

@@ -118,11 +118,21 @@ static void loadpin_sb_free_security(struct super_block *mnt_sb)
 	}
 }
 
-static int loadpin_read_file(struct file *file, enum kernel_read_file_id id)
+static int loadpin_read_file(struct file *file, enum kernel_read_file_id id,
+			     bool contents)
 {
 	struct super_block *load_root;
 	const char *origin = kernel_read_file_id_str(id);
 
+	/*
+	 * If we will not know that we'll be seeing the full contents
+	 * then we cannot trust a load will be complete and unchanged
+	 * off disk. Treat all contents=false hooks as if there were
+	 * no associated file struct.
+	 */
+	if (!contents)
+		file = NULL;
+
 	/* If the file id is excluded, ignore the pinning. */
 	if ((unsigned int)id < ARRAY_SIZE(ignore_read_file_id) &&
 	    ignore_read_file_id[id]) {
@@ -179,7 +189,7 @@ static int loadpin_read_file(struct file *file, enum kernel_read_file_id id)
 
 static int loadpin_load_data(enum kernel_load_data_id id, bool contents)
 {
-	return loadpin_read_file(NULL, (enum kernel_read_file_id) id);
+	return loadpin_read_file(NULL, (enum kernel_read_file_id) id, contents);
 }
 
 static struct security_hook_list loadpin_hooks[] __lsm_ro_after_init = {

security/security.c

@@ -1672,14 +1672,15 @@ int security_kernel_module_request(char *kmod_name)
 	return integrity_kernel_module_request(kmod_name);
 }
 
-int security_kernel_read_file(struct file *file, enum kernel_read_file_id id)
+int security_kernel_read_file(struct file *file, enum kernel_read_file_id id,
+			      bool contents)
 {
 	int ret;
 
-	ret = call_int_hook(kernel_read_file, 0, file, id);
+	ret = call_int_hook(kernel_read_file, 0, file, id, contents);
 	if (ret)
 		return ret;
-	return ima_read_file(file, id);
+	return ima_read_file(file, id, contents);
 }
 EXPORT_SYMBOL_GPL(security_kernel_read_file);
 

security/selinux/hooks.c

@@ -4003,13 +4003,14 @@ static int selinux_kernel_module_from_file(struct file *file)
 }
 
 static int selinux_kernel_read_file(struct file *file,
-				    enum kernel_read_file_id id)
+				    enum kernel_read_file_id id,
+				    bool contents)
 {
 	int rc = 0;
 
 	switch (id) {
 	case READING_MODULE:
-		rc = selinux_kernel_module_from_file(file);
+		rc = selinux_kernel_module_from_file(contents ? file : NULL);
 		break;
 	default:
 		break;