landlock: Add the errata interface

Some fixes may require user space to check if they are applied on the
running kernel before using a specific feature.  For instance, this
applies when a restriction was previously too restrictive and is now
getting relaxed (e.g. for compatibility reasons).  However, non-visible
changes for legitimate use (e.g. security fixes) do not require an
erratum.

Because fixes are backported down to a specific Landlock ABI, we need a
way to avoid cherry-pick conflicts.  The solution is to only update a
file related to the lower ABI impacted by this issue.  All the ABI files
are then used to create a bitmask of fixes.

The new errata interface is similar to the one used to get the supported
Landlock ABI version, but it returns a bitmask instead because the order
of fixes may not match the order of versions, and not all fixes may
apply to all versions.

The actual errata will come with dedicated commits.  The description is
not actually used in the code but serves as documentation.

Create the landlock_abi_version symbol and use its value to check errata
consistency.

Update test_base's create_ruleset_checks_ordering tests and add errata
tests.

This commit is backportable down to the first version of Landlock.

Fixes: 3532b0b4352c ("landlock: Enable user space to infer supported features")
Cc: Günther Noack <gnoack@google.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20250318161443.279194-3-mic@digikod.net
Signed-off-by: Mickaël Salaün <mic@digikod.net>

1 files changed, 30 insertions, 0 deletions

diff --git a/security/landlock/setup.c b/security/landlock/setup.c
index c71832a8e369..0c85ea27e409 100644
--- a/security/landlock/setup.c
+++ b/security/landlock/setup.c
@@ -6,12 +6,14 @@
  * Copyright © 2018-2020 ANSSI
  */
 
+#include <linux/bits.h>
 #include <linux/init.h>
 #include <linux/lsm_hooks.h>
 #include <uapi/linux/lsm.h>
 
 #include "common.h"
 #include "cred.h"
+#include "errata.h"
 #include "fs.h"
 #include "net.h"
 #include "setup.h"
@@ -31,8 +33,36 @@ struct lsm_blob_sizes landlock_blob_sizes __ro_after_init = {
 	.lbs_superblock = sizeof(struct landlock_superblock_security),
 };
 
+int landlock_errata __ro_after_init;
+
+static void __init compute_errata(void)
+{
+	size_t i;
+
+#ifndef __has_include
+	/*
+	 * This is a safeguard to make sure the compiler implements
+	 * __has_include (see errata.h).
+	 */
+	WARN_ON_ONCE(1);
+	return;
+#endif
+
+	for (i = 0; landlock_errata_init[i].number; i++) {
+		const int prev_errata = landlock_errata;
+
+		if (WARN_ON_ONCE(landlock_errata_init[i].abi >
+				 landlock_abi_version))
+			continue;
+
+		landlock_errata |= BIT(landlock_errata_init[i].number - 1);
+		WARN_ON_ONCE(prev_errata == landlock_errata);
+	}
+}
+
 static int __init landlock_init(void)
 {
+	compute_errata();
 	landlock_add_cred_hooks();
 	landlock_add_task_hooks();
 	landlock_add_fs_hooks();