Unverified Commit 602acfb5 authored by Mickaël Salaün's avatar Mickaël Salaün
Browse files

landlock: Optimize stack usage when !CONFIG_AUDIT 



Until now, each landlock_request struct were allocated on the stack, even
if not really used, because is_access_to_paths_allowed() unconditionally
modified the passed references.  Even if the changed landlock_request
variables are not used, the compiler is not smart enough to detect this
case.

To avoid this issue, explicitly disable the related code when
CONFIG_AUDIT is not set, which enables elision of log_request_parent*
and associated caller's stack variables thanks to dead code elimination.
This makes it possible to reduce the stack frame by 32 bytes for the
path_link and path_rename hooks, and by 20 bytes for most other
filesystem hooks.

Here is a summary of scripts/stackdelta before and after this change
when CONFIG_AUDIT is disabled:

  current_check_refer_path    560  320  -240
  current_check_access_path   328  184  -144
  hook_file_open              328  184  -144
  is_access_to_paths_allowed  376  360  -16

Also, add extra pointer checks to be more future-proof.

Cc: Günther Noack <gnoack@google.com>
Reported-by: default avatarTingmao Wang <m@maowtm.org>
Closes: https://lore.kernel.org/r/eb86863b-53b0-460b-b223-84dd31d765b9@maowtm.org
Fixes: 2fc80c69 ("landlock: Log file-related denials")
Link: https://lore.kernel.org/r/20251219142302.744917-2-mic@digikod.net


Reviewed-by: default avatarGünther Noack <gnoack3000@gmail.com>
[mic: Improve stack usage measurement accuracy with scripts/stackdelta]
Signed-off-by: default avatarMickaël Salaün <mic@digikod.net>
parent 6548fb52

security/landlock/fs.c

+9 −2
Original line number Diff line number Diff line
@@ -939,7 +939,12 @@ static bool is_access_to_paths_allowed( 
	}

	path_put(&walker_path);



	if (!allowed_parent1) {

	/*

	 * Check CONFIG_AUDIT to enable elision of log_request_parent* and

	 * associated caller's stack variables thanks to dead code elimination.

	 */

#ifdef CONFIG_AUDIT

	if (!allowed_parent1 && log_request_parent1) {

		log_request_parent1->type = LANDLOCK_REQUEST_FS_ACCESS;

		log_request_parent1->audit.type = LSM_AUDIT_DATA_PATH;

		log_request_parent1->audit.u.path = *path;
@@ -949,7 +954,7 @@ static bool is_access_to_paths_allowed( 
			ARRAY_SIZE(*layer_masks_parent1);

	}



	if (!allowed_parent2) {

	if (!allowed_parent2 && log_request_parent2) {

		log_request_parent2->type = LANDLOCK_REQUEST_FS_ACCESS;

		log_request_parent2->audit.type = LSM_AUDIT_DATA_PATH;

		log_request_parent2->audit.u.path = *path;
@@ -958,6 +963,8 @@ static bool is_access_to_paths_allowed( 
		log_request_parent2->layer_masks_size =

			ARRAY_SIZE(*layer_masks_parent2);

	}

#endif /* CONFIG_AUDIT */



	return allowed_parent1 && allowed_parent2;

}