[PATCH v6 2/6] landlock: Implement LANDLOCK_ADD_RULE_NO_INHERIT userspace api

[Justin Suess]

Implements the syscall side flag handling and kernel api headers for the
LANDLOCK_ADD_RULE_NO_INHERIT flag.

Signed-off-by: Justin Suess <utilityemal77 at gmail.com>
---

Notes:
    v5..v6 changes:
    
      * None
    
    v4..v5 changes:
    
      * Moved syscall handling to this patch and moved out flag definition
        to allow independent build.
    
    v3..v4 changes:
    
      * Changed documentation to reflect protections now apply to VFS root
        instead of the mountpoint.
    
    v2..v3 changes:
    
      * Extended documentation for flag inheritance suppression on
        LANDLOCK_ADD_RULE_NO_INHERIT.
      * Extended the flag validation rules in the syscall.
      * Added mention of no inherit in empty rules in add_rule_path_beneath
        as per Tingmao Wang's suggestion.
      * Added check for useless no-inherit flag in networking rules.

 security/landlock/ruleset.c  | 19 ++++++++++++++++++-
 security/landlock/syscalls.c | 16 ++++++++++++----
 2 files changed, 30 insertions(+), 5 deletions(-)

diff --git a/security/landlock/ruleset.c b/security/landlock/ruleset.c
index a58af26db201..adc965de8e4e 100644
--- a/security/landlock/ruleset.c
+++ b/security/landlock/ruleset.c
@@ -254,8 +254,13 @@ static int insert_rule(struct landlock_ruleset *const ruleset,
 				return -EINVAL;
 			if (WARN_ON_ONCE(this->layers[0].level != 0))
 				return -EINVAL;
+			/* Merge the flags into the rules */
 			this->layers[0].access |= (*layers)[0].access;
 			this->layers[0].flags.quiet |= (*layers)[0].flags.quiet;
+			this->layers[0].flags.no_inherit |=
+				(*layers)[0].flags.no_inherit;
+			this->layers[0].flags.has_no_inherit_descendant |=
+				(*layers)[0].flags.has_no_inherit_descendant;
 			return 0;
 		}
 
@@ -314,7 +319,10 @@ int landlock_insert_rule(struct landlock_ruleset *const ruleset,
 		.level = 0,
 		.flags = {
 			.quiet = !!(flags & LANDLOCK_ADD_RULE_QUIET),
-		},
+			.no_inherit = !!(flags & LANDLOCK_ADD_RULE_NO_INHERIT),
+			.has_no_inherit_descendant =
+				!!(flags & LANDLOCK_ADD_RULE_NO_INHERIT),
+		}
 	} };
 
 	build_check_layer();
@@ -661,9 +669,18 @@ bool landlock_unmask_layers(const struct landlock_rule *const rule,
 		unsigned long access_bit;
 		bool is_empty;
 
+		/* Skip layers that already have no inherit flags. */
+		if (rule_flags &&
+		    (rule_flags->no_inherit_masks & layer_bit))
+			continue;
+
 		/* Collect rule flags for each layer. */
 		if (rule_flags && layer->flags.quiet)
 			rule_flags->quiet_masks |= layer_bit;
+		if (rule_flags && layer->flags.no_inherit)
+			rule_flags->no_inherit_masks |= layer_bit;
+		if (rule_flags && layer->flags.has_no_inherit_descendant)
+			rule_flags->no_inherit_desc_masks |= layer_bit;
 
 		/*
 		 * Records in @layer_masks which layer grants access to each requested
diff --git a/security/landlock/syscalls.c b/security/landlock/syscalls.c
index 5cf1183bb596..cffe7d944ae5 100644
--- a/security/landlock/syscalls.c
+++ b/security/landlock/syscalls.c
@@ -352,7 +352,7 @@ static int add_rule_path_beneath(struct landlock_ruleset *const ruleset,
 	/*
 	 * Informs about useless rule: empty allowed_access (i.e. deny rules)
 	 * are ignored in path walks.  However, the rule is not useless if it
-	 * is there to hold a quiet flag
+	 * is there to hold a quiet or no inherit flag.
 	 */
 	if (!flags && !path_beneath_attr.allowed_access)
 		return -ENOMSG;
@@ -407,6 +407,10 @@ static int add_rule_net_port(struct landlock_ruleset *ruleset,
 	if (flags & LANDLOCK_ADD_RULE_QUIET && !ruleset->quiet_masks.net)
 		return -EINVAL;
 
+	/* No inherit is always useless for this scope */
+	if (flags & LANDLOCK_ADD_RULE_NO_INHERIT)
+		return -EINVAL;
+
 	/* Denies inserting a rule with port greater than 65535. */
 	if (net_port_attr.port > U16_MAX)
 		return -EINVAL;
@@ -424,7 +428,7 @@ static int add_rule_net_port(struct landlock_ruleset *ruleset,
  * @rule_type: Identify the structure type pointed to by @rule_attr:
  *             %LANDLOCK_RULE_PATH_BENEATH or %LANDLOCK_RULE_NET_PORT.
  * @rule_attr: Pointer to a rule (matching the @rule_type).
- * @flags: Must be 0 or %LANDLOCK_ADD_RULE_QUIET.
+ * @flags: Must be 0 or %LANDLOCK_ADD_RULE_QUIET and/or %LANDLOCK_ADD_RULE_NO_INHERIT.
  *
  * This system call enables to define a new rule and add it to an existing
  * ruleset.
@@ -462,8 +466,12 @@ SYSCALL_DEFINE4(landlock_add_rule, const int, ruleset_fd,
 
 	if (!is_initialized())
 		return -EOPNOTSUPP;
-
-	if (flags && flags != LANDLOCK_ADD_RULE_QUIET)
+	/* Checks flag existence */
+	if (flags & ~(LANDLOCK_ADD_RULE_QUIET | LANDLOCK_ADD_RULE_NO_INHERIT))
+		return -EINVAL;
+	/* No inherit may only apply on path_beneath rules. */
+	if ((flags & LANDLOCK_ADD_RULE_NO_INHERIT) &&
+	    rule_type != LANDLOCK_RULE_PATH_BENEATH)
 		return -EINVAL;
 
 	/* Gets and checks the ruleset. */
-- 
2.51.0