Landlock updates for v6.11-rc1

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Merge tag 'landlock-6.11-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/mic/linux

Pull landlock updates from Mickaël Salaün:
 "This simplifies code and improves documentation"

* tag 'landlock-6.11-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/mic/linux:
  landlock: Various documentation improvements
  landlock: Clarify documentation for struct landlock_ruleset_attr
  landlock: Use bit-fields for storing handled layer access masks

Documentation/userspace-api/landlock.rst

@@ -8,7 +8,7 @@ Landlock: unprivileged access control
 =====================================
 
 :Author: Mickaël Salaün
-:Date: April 2024
+:Date: July 2024
 
 The goal of Landlock is to enable to restrict ambient rights (e.g. global
 filesystem or network access) for a set of processes.  Because Landlock

include/uapi/linux/landlock.h

@@ -12,29 +12,36 @@
 #include <linux/types.h>
 
 /**
- * struct landlock_ruleset_attr - Ruleset definition
+ * struct landlock_ruleset_attr - Ruleset definition.
  *
- * Argument of sys_landlock_create_ruleset().  This structure can grow in
- * future versions.
+ * Argument of sys_landlock_create_ruleset().
+ *
+ * This structure defines a set of *handled access rights*, a set of actions on
+ * different object types, which should be denied by default when the ruleset is
+ * enacted.  Vice versa, access rights that are not specifically listed here are
+ * not going to be denied by this ruleset when it is enacted.
+ *
+ * For historical reasons, the %LANDLOCK_ACCESS_FS_REFER right is always denied
+ * by default, even when its bit is not set in @handled_access_fs.  In order to
+ * add new rules with this access right, the bit must still be set explicitly
+ * (cf. `Filesystem flags`_).
+ *
+ * The explicit listing of *handled access rights* is required for backwards
+ * compatibility reasons.  In most use cases, processes that use Landlock will
+ * *handle* a wide range or all access rights that they know about at build time
+ * (and that they have tested with a kernel that supported them all).
+ *
+ * This structure can grow in future Landlock versions.
  */
 struct landlock_ruleset_attr {
 	/**
-	 * @handled_access_fs: Bitmask of actions (cf. `Filesystem flags`_)
-	 * that is handled by this ruleset and should then be forbidden if no
-	 * rule explicitly allow them: it is a deny-by-default list that should
-	 * contain as much Landlock access rights as possible. Indeed, all
-	 * Landlock filesystem access rights that are not part of
-	 * handled_access_fs are allowed.  This is needed for backward
-	 * compatibility reasons.  One exception is the
-	 * %LANDLOCK_ACCESS_FS_REFER access right, which is always implicitly
-	 * handled, but must still be explicitly handled to add new rules with
-	 * this access right.
+	 * @handled_access_fs: Bitmask of handled filesystem actions
+	 * (cf. `Filesystem flags`_).
 	 */
 	__u64 handled_access_fs;
 	/**
-	 * @handled_access_net: Bitmask of actions (cf. `Network flags`_)
-	 * that is handled by this ruleset and should then be forbidden if no
-	 * rule explicitly allow them.
+	 * @handled_access_net: Bitmask of handled network actions (cf. `Network
+	 * flags`_).
 	 */
 	__u64 handled_access_net;
 };
@@ -97,20 +104,21 @@ struct landlock_path_beneath_attr {
  */
 struct landlock_net_port_attr {
 	/**
-	 * @allowed_access: Bitmask of allowed access network for a port
+	 * @allowed_access: Bitmask of allowed network actions for a port
 	 * (cf. `Network flags`_).
 	 */
 	__u64 allowed_access;
 	/**
 	 * @port: Network port in host endianness.
 	 *
-	 * It should be noted that port 0 passed to :manpage:`bind(2)` will
-	 * bind to an available port from a specific port range. This can be
-	 * configured thanks to the ``/proc/sys/net/ipv4/ip_local_port_range``
-	 * sysctl (also used for IPv6). A Landlock rule with port 0 and the
-	 * ``LANDLOCK_ACCESS_NET_BIND_TCP`` right means that requesting to bind
-	 * on port 0 is allowed and it will automatically translate to binding
-	 * on the related port range.
+	 * It should be noted that port 0 passed to :manpage:`bind(2)` will bind
+	 * to an available port from the ephemeral port range.  This can be
+	 * configured with the ``/proc/sys/net/ipv4/ip_local_port_range`` sysctl
+	 * (also used for IPv6).
+	 *
+	 * A Landlock rule with port 0 and the ``LANDLOCK_ACCESS_NET_BIND_TCP``
+	 * right means that requesting to bind on port 0 is allowed and it will
+	 * automatically translate to binding on the related port range.
 	 */
 	__u64 port;
 };
@@ -131,10 +139,10 @@ struct landlock_net_port_attr {
  * The following access rights apply only to files:
  *
  * - %LANDLOCK_ACCESS_FS_EXECUTE: Execute a file.
- * - %LANDLOCK_ACCESS_FS_WRITE_FILE: Open a file with write access. Note that
- *   you might additionally need the %LANDLOCK_ACCESS_FS_TRUNCATE right in order
- *   to overwrite files with :manpage:`open(2)` using ``O_TRUNC`` or
- *   :manpage:`creat(2)`.
+ * - %LANDLOCK_ACCESS_FS_WRITE_FILE: Open a file with write access.  When
+ *   opening files for writing, you will often additionally need the
+ *   %LANDLOCK_ACCESS_FS_TRUNCATE right.  In many cases, these system calls
+ *   truncate existing files when overwriting them (e.g., :manpage:`creat(2)`).
  * - %LANDLOCK_ACCESS_FS_READ_FILE: Open a file with read access.
  * - %LANDLOCK_ACCESS_FS_TRUNCATE: Truncate a file with :manpage:`truncate(2)`,
  *   :manpage:`ftruncate(2)`, :manpage:`creat(2)`, or :manpage:`open(2)` with
@@ -256,7 +264,7 @@ struct landlock_net_port_attr {
  * These flags enable to restrict a sandboxed process to a set of network
  * actions. This is supported since the Landlock ABI version 4.
  *
- * TCP sockets with allowed actions:
+ * The following access rights apply to TCP port numbers:
  *
  * - %LANDLOCK_ACCESS_NET_BIND_TCP: Bind a TCP socket to a local port.
  * - %LANDLOCK_ACCESS_NET_CONNECT_TCP: Connect an active TCP socket to

security/landlock/limits.h

@@ -21,12 +21,10 @@
 #define LANDLOCK_LAST_ACCESS_FS		LANDLOCK_ACCESS_FS_IOCTL_DEV
 #define LANDLOCK_MASK_ACCESS_FS		((LANDLOCK_LAST_ACCESS_FS << 1) - 1)
 #define LANDLOCK_NUM_ACCESS_FS		__const_hweight64(LANDLOCK_MASK_ACCESS_FS)
-#define LANDLOCK_SHIFT_ACCESS_FS	0
 
 #define LANDLOCK_LAST_ACCESS_NET	LANDLOCK_ACCESS_NET_CONNECT_TCP
 #define LANDLOCK_MASK_ACCESS_NET	((LANDLOCK_LAST_ACCESS_NET << 1) - 1)
 #define LANDLOCK_NUM_ACCESS_NET		__const_hweight64(LANDLOCK_MASK_ACCESS_NET)
-#define LANDLOCK_SHIFT_ACCESS_NET	LANDLOCK_NUM_ACCESS_FS
 
 /* clang-format on */
 

security/landlock/ruleset.c

@@ -169,13 +169,9 @@ static void build_check_ruleset(void)
 		.num_rules = ~0,
 		.num_layers = ~0,
 	};
-	typeof(ruleset.access_masks[0]) access_masks = ~0;
 
 	BUILD_BUG_ON(ruleset.num_rules < LANDLOCK_MAX_NUM_RULES);
 	BUILD_BUG_ON(ruleset.num_layers < LANDLOCK_MAX_NUM_LAYERS);
-	BUILD_BUG_ON(access_masks <
-		     ((LANDLOCK_MASK_ACCESS_FS << LANDLOCK_SHIFT_ACCESS_FS) |
-		      (LANDLOCK_MASK_ACCESS_NET << LANDLOCK_SHIFT_ACCESS_NET)));
 }
 
 /**

security/landlock/ruleset.h

@@ -39,10 +39,10 @@ static_assert(BITS_PER_TYPE(access_mask_t) >= LANDLOCK_NUM_ACCESS_NET);
 static_assert(sizeof(unsigned long) >= sizeof(access_mask_t));
 
 /* Ruleset access masks. */
-typedef u32 access_masks_t;
-/* Makes sure all ruleset access rights can be stored. */
-static_assert(BITS_PER_TYPE(access_masks_t) >=
-	      LANDLOCK_NUM_ACCESS_FS + LANDLOCK_NUM_ACCESS_NET);
+struct access_masks {
+	access_mask_t fs : LANDLOCK_NUM_ACCESS_FS;
+	access_mask_t net : LANDLOCK_NUM_ACCESS_NET;
+};
 
 typedef u16 layer_mask_t;
 /* Makes sure all layers can be checked. */
@@ -226,7 +226,7 @@ struct landlock_ruleset {
 			 * layers are set once and never changed for the
 			 * lifetime of the ruleset.
 			 */
-			access_masks_t access_masks[];
+			struct access_masks access_masks[];
 		};
 	};
 };
@@ -265,8 +265,7 @@ landlock_add_fs_access_mask(struct landlock_ruleset *const ruleset,
 
 	/* Should already be checked in sys_landlock_create_ruleset(). */
 	WARN_ON_ONCE(fs_access_mask != fs_mask);
-	ruleset->access_masks[layer_level] |=
-		(fs_mask << LANDLOCK_SHIFT_ACCESS_FS);
+	ruleset->access_masks[layer_level].fs |= fs_mask;
 }
 
 static inline void
@@ -278,17 +277,14 @@ landlock_add_net_access_mask(struct landlock_ruleset *const ruleset,
 
 	/* Should already be checked in sys_landlock_create_ruleset(). */
 	WARN_ON_ONCE(net_access_mask != net_mask);
-	ruleset->access_masks[layer_level] |=
-		(net_mask << LANDLOCK_SHIFT_ACCESS_NET);
+	ruleset->access_masks[layer_level].net |= net_mask;
 }
 
 static inline access_mask_t
 landlock_get_raw_fs_access_mask(const struct landlock_ruleset *const ruleset,
 				const u16 layer_level)
 {
-	return (ruleset->access_masks[layer_level] >>
-		LANDLOCK_SHIFT_ACCESS_FS) &
-	       LANDLOCK_MASK_ACCESS_FS;
+	return ruleset->access_masks[layer_level].fs;
 }
 
 static inline access_mask_t
@@ -304,9 +300,7 @@ static inline access_mask_t
 landlock_get_net_access_mask(const struct landlock_ruleset *const ruleset,
 			     const u16 layer_level)
 {
-	return (ruleset->access_masks[layer_level] >>
-		LANDLOCK_SHIFT_ACCESS_NET) &
-	       LANDLOCK_MASK_ACCESS_NET;
+	return ruleset->access_masks[layer_level].net;
 }
 
 bool landlock_unmask_layers(const struct landlock_rule *const rule,

security/landlock/syscalls.c

@@ -378,8 +378,7 @@ static int add_rule_net_port(struct landlock_ruleset *ruleset,
  *		with the new rule.
  * @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 (only of type &struct
- *             landlock_path_beneath_attr for now).
+ * @rule_attr: Pointer to a rule (matching the @rule_type).
  * @flags: Must be 0.
  *
  * This system call enables to define a new rule and add it to an existing
@@ -390,18 +389,20 @@ static int add_rule_net_port(struct landlock_ruleset *ruleset,
  * - %EOPNOTSUPP: Landlock is supported by the kernel but disabled at boot time;
  * - %EAFNOSUPPORT: @rule_type is %LANDLOCK_RULE_NET_PORT but TCP/IP is not
  *   supported by the running kernel;
- * - %EINVAL: @flags is not 0, or inconsistent access in the rule (i.e.
+ * - %EINVAL: @flags is not 0;
+ * - %EINVAL: The rule accesses are inconsistent (i.e.
  *   &landlock_path_beneath_attr.allowed_access or
- *   &landlock_net_port_attr.allowed_access is not a subset of the
- *   ruleset handled accesses), or &landlock_net_port_attr.port is
- *   greater than 65535;
- * - %ENOMSG: Empty accesses (e.g. &landlock_path_beneath_attr.allowed_access);
+ *   &landlock_net_port_attr.allowed_access is not a subset of the ruleset
+ *   handled accesses)
+ * - %EINVAL: &landlock_net_port_attr.port is greater than 65535;
+ * - %ENOMSG: Empty accesses (e.g. &landlock_path_beneath_attr.allowed_access is
+ *   0);
  * - %EBADF: @ruleset_fd is not a file descriptor for the current thread, or a
  *   member of @rule_attr is not a file descriptor as expected;
  * - %EBADFD: @ruleset_fd is not a ruleset file descriptor, or a member of
  *   @rule_attr is not the expected file descriptor type;
  * - %EPERM: @ruleset_fd has no write access to the underlying ruleset;
- * - %EFAULT: @rule_attr inconsistency.
+ * - %EFAULT: @rule_attr was not a valid address.
  */
 SYSCALL_DEFINE4(landlock_add_rule, const int, ruleset_fd,
 		const enum landlock_rule_type, rule_type,