Merge tag 'landlock-6.2-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/mic/linux

Pull landlock updates from Mickaël Salaün:
 "This adds file truncation support to Landlock, contributed by Günther
  Noack. As described by Günther [1], the goal of these patches is to
  work towards a more complete coverage of file system operations that
  are restrictable with Landlock.

  The known set of currently unsupported file system operations in
  Landlock is described at [2]. Out of the operations listed there,
  truncate is the only one that modifies file contents, so these patches
  should make it possible to prevent the direct modification of file
  contents with Landlock.

  The new LANDLOCK_ACCESS_FS_TRUNCATE access right covers both the
  truncate(2) and ftruncate(2) families of syscalls, as well as open(2)
  with the O_TRUNC flag. This includes usages of creat() in the case
  where existing regular files are overwritten.

  Additionally, this introduces a new Landlock security blob associated
  with opened files, to track the available Landlock access rights at
  the time of opening the file. This is in line with Unix's general
  approach of checking the read and write permissions during open(), and
  associating this previously checked authorization with the opened
  file. An ongoing patch documents this use case [3].

  In order to treat truncate(2) and ftruncate(2) calls differently in an
  LSM hook, we split apart the existing security_path_truncate hook into
  security_path_truncate (for truncation by path) and
  security_file_truncate (for truncation of previously opened files)"

Link: https://lore.kernel.org/r/20221018182216.301684-1-gnoack3000@gmail.com [1]
Link: https://www.kernel.org/doc/html/v6.1/userspace-api/landlock.html#filesystem-flags [2]
Link: https://lore.kernel.org/r/20221209193813.972012-1-mic@digikod.net [3]

* tag 'landlock-6.2-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/mic/linux:
  samples/landlock: Document best-effort approach for LANDLOCK_ACCESS_FS_REFER
  landlock: Document Landlock's file truncation support
  samples/landlock: Extend sample tool to support LANDLOCK_ACCESS_FS_TRUNCATE
  selftests/landlock: Test ftruncate on FDs created by memfd_create(2)
  selftests/landlock: Test FD passing from restricted to unrestricted processes
  selftests/landlock: Locally define __maybe_unused
  selftests/landlock: Test open() and ftruncate() in multiple scenarios
  selftests/landlock: Test file truncation support
  landlock: Support file truncation
  landlock: Document init_layer_masks() helper
  landlock: Refactor check_access_path_dual() into is_access_to_paths_allowed()
  security: Create file_truncate hook from path_truncate hook

8 files changed, 213 insertions, 57 deletions

diff --git a/security/apparmor/lsm.c b/security/apparmor/lsm.c
index f34675f7c3df..b751d6253977 100644
--- a/security/apparmor/lsm.c
+++ b/security/apparmor/lsm.c
@@ -333,6 +333,11 @@ static int apparmor_path_truncate(const struct path *path)
 	return common_perm_cond(OP_TRUNC, path, MAY_WRITE | AA_MAY_SETATTR);
 }
 
+static int apparmor_file_truncate(struct file *file)
+{
+	return apparmor_path_truncate(&file->f_path);
+}
+
 static int apparmor_path_symlink(const struct path *dir, struct dentry *dentry,
 				 const char *old_name)
 {
@@ -1241,6 +1246,7 @@ static struct security_hook_list apparmor_hooks[] __lsm_ro_after_init = {
 	LSM_HOOK_INIT(mmap_file, apparmor_mmap_file),
 	LSM_HOOK_INIT(file_mprotect, apparmor_file_mprotect),
 	LSM_HOOK_INIT(file_lock, apparmor_file_lock),
+	LSM_HOOK_INIT(file_truncate, apparmor_file_truncate),
 
 	LSM_HOOK_INIT(getprocattr, apparmor_getprocattr),
 	LSM_HOOK_INIT(setprocattr, apparmor_setprocattr),
diff --git a/security/landlock/fs.c b/security/landlock/fs.c
index 64ed7665455f..adcea0fe7e68 100644
--- a/security/landlock/fs.c
+++ b/security/landlock/fs.c
@@ -146,7 +146,8 @@ retry:
 #define ACCESS_FILE ( \
 	LANDLOCK_ACCESS_FS_EXECUTE | \
 	LANDLOCK_ACCESS_FS_WRITE_FILE | \
-	LANDLOCK_ACCESS_FS_READ_FILE)
+	LANDLOCK_ACCESS_FS_READ_FILE | \
+	LANDLOCK_ACCESS_FS_TRUNCATE)
 /* clang-format on */
 
 /*
@@ -297,6 +298,19 @@ get_handled_accesses(const struct landlock_ruleset *const domain)
 	return access_dom & LANDLOCK_MASK_ACCESS_FS;
 }
 
+/**
+ * init_layer_masks - Initialize layer masks from an access request
+ *
+ * Populates @layer_masks such that for each access right in @access_request,
+ * the bits for all the layers are set where this access right is handled.
+ *
+ * @domain: The domain that defines the current restrictions.
+ * @access_request: The requested access rights to check.
+ * @layer_masks: The layer masks to populate.
+ *
+ * Returns: An access mask where each access right bit is set which is handled
+ * in any of the active layers in @domain.
+ */
 static inline access_mask_t
 init_layer_masks(const struct landlock_ruleset *const domain,
 		 const access_mask_t access_request,
@@ -430,7 +444,7 @@ is_eacces(const layer_mask_t (*const layer_masks)[LANDLOCK_NUM_ACCESS_FS],
 }
 
 /**
- * check_access_path_dual - Check accesses for requests with a common path
+ * is_access_to_paths_allowed - Check accesses for requests with a common path
  *
  * @domain: Domain to check against.
  * @path: File hierarchy to walk through.
@@ -465,14 +479,10 @@ is_eacces(const layer_mask_t (*const layer_masks)[LANDLOCK_NUM_ACCESS_FS],
  * allow the request.
  *
  * Returns:
- * - 0 if the access request is granted;
- * - -EACCES if it is denied because of access right other than
- *   LANDLOCK_ACCESS_FS_REFER;
- * - -EXDEV if the renaming or linking would be a privileged escalation
- *   (according to each layered policies), or if LANDLOCK_ACCESS_FS_REFER is
- *   not allowed by the source or the destination.
+ * - true if the access request is granted;
+ * - false otherwise.
  */
-static int check_access_path_dual(
+static bool is_access_to_paths_allowed(
 	const struct landlock_ruleset *const domain,
 	const struct path *const path,
 	const access_mask_t access_request_parent1,
@@ -492,17 +502,17 @@ static int check_access_path_dual(
 	(*layer_masks_child2)[LANDLOCK_NUM_ACCESS_FS] = NULL;
 
 	if (!access_request_parent1 && !access_request_parent2)
-		return 0;
+		return true;
 	if (WARN_ON_ONCE(!domain || !path))
-		return 0;
+		return true;
 	if (is_nouser_or_private(path->dentry))
-		return 0;
+		return true;
 	if (WARN_ON_ONCE(domain->num_layers < 1 || !layer_masks_parent1))
-		return -EACCES;
+		return false;
 
 	if (unlikely(layer_masks_parent2)) {
 		if (WARN_ON_ONCE(!dentry_child1))
-			return -EACCES;
+			return false;
 		/*
 		 * For a double request, first check for potential privilege
 		 * escalation by looking at domain handled accesses (which are
@@ -513,7 +523,7 @@ static int check_access_path_dual(
 		is_dom_check = true;
 	} else {
 		if (WARN_ON_ONCE(dentry_child1 || dentry_child2))
-			return -EACCES;
+			return false;
 		/* For a simple request, only check for requested accesses. */
 		access_masked_parent1 = access_request_parent1;
 		access_masked_parent2 = access_request_parent2;
@@ -622,24 +632,7 @@ jump_up:
 	}
 	path_put(&walker_path);
 
-	if (allowed_parent1 && allowed_parent2)
-		return 0;
-
-	/*
-	 * This prioritizes EACCES over EXDEV for all actions, including
-	 * renames with RENAME_EXCHANGE.
-	 */
-	if (likely(is_eacces(layer_masks_parent1, access_request_parent1) ||
-		   is_eacces(layer_masks_parent2, access_request_parent2)))
-		return -EACCES;
-
-	/*
-	 * Gracefully forbids reparenting if the destination directory
-	 * hierarchy is not a superset of restrictions of the source directory
-	 * hierarchy, or if LANDLOCK_ACCESS_FS_REFER is not allowed by the
-	 * source or the destination.
-	 */
-	return -EXDEV;
+	return allowed_parent1 && allowed_parent2;
 }
 
 static inline int check_access_path(const struct landlock_ruleset *const domain,
@@ -649,8 +642,10 @@ static inline int check_access_path(const struct landlock_ruleset *const domain,
 	layer_mask_t layer_masks[LANDLOCK_NUM_ACCESS_FS] = {};
 
 	access_request = init_layer_masks(domain, access_request, &layer_masks);
-	return check_access_path_dual(domain, path, access_request,
-				      &layer_masks, NULL, 0, NULL, NULL);
+	if (is_access_to_paths_allowed(domain, path, access_request,
+				       &layer_masks, NULL, 0, NULL, NULL))
+		return 0;
+	return -EACCES;
 }
 
 static inline int current_check_access_path(const struct path *const path,
@@ -711,8 +706,9 @@ static inline access_mask_t maybe_remove(const struct dentry *const dentry)
  * file.  While walking from @dir to @mnt_root, we record all the domain's
  * allowed accesses in @layer_masks_dom.
  *
- * This is similar to check_access_path_dual() but much simpler because it only
- * handles walking on the same mount point and only checks one set of accesses.
+ * This is similar to is_access_to_paths_allowed() but much simpler because it
+ * only handles walking on the same mount point and only checks one set of
+ * accesses.
  *
  * Returns:
  * - true if all the domain access rights are allowed for @dir;
@@ -857,10 +853,11 @@ static int current_check_refer_path(struct dentry *const old_dentry,
 		access_request_parent1 = init_layer_masks(
 			dom, access_request_parent1 | access_request_parent2,
 			&layer_masks_parent1);
-		return check_access_path_dual(dom, new_dir,
-					      access_request_parent1,
-					      &layer_masks_parent1, NULL, 0,
-					      NULL, NULL);
+		if (is_access_to_paths_allowed(
+			    dom, new_dir, access_request_parent1,
+			    &layer_masks_parent1, NULL, 0, NULL, NULL))
+			return 0;
+		return -EACCES;
 	}
 
 	access_request_parent1 |= LANDLOCK_ACCESS_FS_REFER;
@@ -886,11 +883,27 @@ static int current_check_refer_path(struct dentry *const old_dentry,
 	 * parent access rights.  This will be useful to compare with the
 	 * destination parent access rights.
 	 */
-	return check_access_path_dual(dom, &mnt_dir, access_request_parent1,
-				      &layer_masks_parent1, old_dentry,
-				      access_request_parent2,
-				      &layer_masks_parent2,
-				      exchange ? new_dentry : NULL);
+	if (is_access_to_paths_allowed(
+		    dom, &mnt_dir, access_request_parent1, &layer_masks_parent1,
+		    old_dentry, access_request_parent2, &layer_masks_parent2,
+		    exchange ? new_dentry : NULL))
+		return 0;
+
+	/*
+	 * This prioritizes EACCES over EXDEV for all actions, including
+	 * renames with RENAME_EXCHANGE.
+	 */
+	if (likely(is_eacces(&layer_masks_parent1, access_request_parent1) ||
+		   is_eacces(&layer_masks_parent2, access_request_parent2)))
+		return -EACCES;
+
+	/*
+	 * Gracefully forbids reparenting if the destination directory
+	 * hierarchy is not a superset of restrictions of the source directory
+	 * hierarchy, or if LANDLOCK_ACCESS_FS_REFER is not allowed by the
+	 * source or the destination.
+	 */
+	return -EXDEV;
 }
 
 /* Inode hooks */
@@ -1142,9 +1155,23 @@ static int hook_path_rmdir(const struct path *const dir,
 	return current_check_access_path(dir, LANDLOCK_ACCESS_FS_REMOVE_DIR);
 }
 
+static int hook_path_truncate(const struct path *const path)
+{
+	return current_check_access_path(path, LANDLOCK_ACCESS_FS_TRUNCATE);
+}
+
 /* File hooks */
 
-static inline access_mask_t get_file_access(const struct file *const file)
+/**
+ * get_required_file_open_access - Get access needed to open a file
+ *
+ * @file: File being opened.
+ *
+ * Returns the access rights that are required for opening the given file,
+ * depending on the file type and open mode.
+ */
+static inline access_mask_t
+get_required_file_open_access(const struct file *const file)
 {
 	access_mask_t access = 0;
 
@@ -1162,19 +1189,95 @@ static inline access_mask_t get_file_access(const struct file *const file)
 	return access;
 }
 
+static int hook_file_alloc_security(struct file *const file)
+{
+	/*
+	 * Grants all access rights, even if most of them are not checked later
+	 * on. It is more consistent.
+	 *
+	 * Notably, file descriptors for regular files can also be acquired
+	 * without going through the file_open hook, for example when using
+	 * memfd_create(2).
+	 */
+	landlock_file(file)->allowed_access = LANDLOCK_MASK_ACCESS_FS;
+	return 0;
+}
+
 static int hook_file_open(struct file *const file)
 {
+	layer_mask_t layer_masks[LANDLOCK_NUM_ACCESS_FS] = {};
+	access_mask_t open_access_request, full_access_request, allowed_access;
+	const access_mask_t optional_access = LANDLOCK_ACCESS_FS_TRUNCATE;
 	const struct landlock_ruleset *const dom =
 		landlock_get_current_domain();
 
 	if (!dom)
 		return 0;
+
 	/*
-	 * Because a file may be opened with O_PATH, get_file_access() may
-	 * return 0.  This case will be handled with a future Landlock
+	 * Because a file may be opened with O_PATH, get_required_file_open_access()
+	 * may return 0.  This case will be handled with a future Landlock
 	 * evolution.
 	 */
-	return check_access_path(dom, &file->f_path, get_file_access(file));
+	open_access_request = get_required_file_open_access(file);
+
+	/*
+	 * We look up more access than what we immediately need for open(), so
+	 * that we can later authorize operations on opened files.
+	 */
+	full_access_request = open_access_request | optional_access;
+
+	if (is_access_to_paths_allowed(
+		    dom, &file->f_path,
+		    init_layer_masks(dom, full_access_request, &layer_masks),
+		    &layer_masks, NULL, 0, NULL, NULL)) {
+		allowed_access = full_access_request;
+	} else {
+		unsigned long access_bit;
+		const unsigned long access_req = full_access_request;
+
+		/*
+		 * Calculate the actual allowed access rights from layer_masks.
+		 * Add each access right to allowed_access which has not been
+		 * vetoed by any layer.
+		 */
+		allowed_access = 0;
+		for_each_set_bit(access_bit, &access_req,
+				 ARRAY_SIZE(layer_masks)) {
+			if (!layer_masks[access_bit])
+				allowed_access |= BIT_ULL(access_bit);
+		}
+	}
+
+	/*
+	 * For operations on already opened files (i.e. ftruncate()), it is the
+	 * access rights at the time of open() which decide whether the
+	 * operation is permitted. Therefore, we record the relevant subset of
+	 * file access rights in the opened struct file.
+	 */
+	landlock_file(file)->allowed_access = allowed_access;
+
+	if ((open_access_request & allowed_access) == open_access_request)
+		return 0;
+
+	return -EACCES;
+}
+
+static int hook_file_truncate(struct file *const file)
+{
+	/*
+	 * Allows truncation if the truncate right was available at the time of
+	 * opening the file, to get a consistent access check as for read, write
+	 * and execute operations.
+	 *
+	 * Note: For checks done based on the file's Landlock allowed access, we
+	 * enforce them independently of whether the current thread is in a
+	 * Landlock domain, so that open files passed between independent
+	 * processes retain their behaviour.
+	 */
+	if (landlock_file(file)->allowed_access & LANDLOCK_ACCESS_FS_TRUNCATE)
+		return 0;
+	return -EACCES;
 }
 
 static struct security_hook_list landlock_hooks[] __lsm_ro_after_init = {
@@ -1194,8 +1297,11 @@ static struct security_hook_list landlock_hooks[] __lsm_ro_after_init = {
 	LSM_HOOK_INIT(path_symlink, hook_path_symlink),
 	LSM_HOOK_INIT(path_unlink, hook_path_unlink),
 	LSM_HOOK_INIT(path_rmdir, hook_path_rmdir),
+	LSM_HOOK_INIT(path_truncate, hook_path_truncate),
 
+	LSM_HOOK_INIT(file_alloc_security, hook_file_alloc_security),
 	LSM_HOOK_INIT(file_open, hook_file_open),
+	LSM_HOOK_INIT(file_truncate, hook_file_truncate),
 };
 
 __init void landlock_add_fs_hooks(void)
diff --git a/security/landlock/fs.h b/security/landlock/fs.h
index 8db7acf9109b..488e4813680a 100644
--- a/security/landlock/fs.h
+++ b/security/landlock/fs.h
@@ -37,6 +37,24 @@ struct landlock_inode_security {
 };
 
 /**
+ * struct landlock_file_security - File security blob
+ *
+ * This information is populated when opening a file in hook_file_open, and
+ * tracks the relevant Landlock access rights that were available at the time
+ * of opening the file. Other LSM hooks use these rights in order to authorize
+ * operations on already opened files.
+ */
+struct landlock_file_security {
+	/**
+	 * @allowed_access: Access rights that were available at the time of
+	 * opening the file. This is not necessarily the full set of access
+	 * rights available at that time, but it's the necessary subset as
+	 * needed to authorize later operations on the open file.
+	 */
+	access_mask_t allowed_access;
+};
+
+/**
  * struct landlock_superblock_security - Superblock security blob
  *
  * Enable hook_sb_delete() to wait for concurrent calls to release_inode().
@@ -50,6 +68,12 @@ struct landlock_superblock_security {
 	atomic_long_t inode_refs;
 };
 
+static inline struct landlock_file_security *
+landlock_file(const struct file *const file)
+{
+	return file->f_security + landlock_blob_sizes.lbs_file;
+}
+
 static inline struct landlock_inode_security *
 landlock_inode(const struct inode *const inode)
 {
diff --git a/security/landlock/limits.h b/security/landlock/limits.h
index b54184ab9439..82288f0e9e5e 100644
--- a/security/landlock/limits.h
+++ b/security/landlock/limits.h
@@ -18,7 +18,7 @@
 #define LANDLOCK_MAX_NUM_LAYERS		16
 #define LANDLOCK_MAX_NUM_RULES		U32_MAX
 
-#define LANDLOCK_LAST_ACCESS_FS		LANDLOCK_ACCESS_FS_REFER
+#define LANDLOCK_LAST_ACCESS_FS		LANDLOCK_ACCESS_FS_TRUNCATE
 #define LANDLOCK_MASK_ACCESS_FS		((LANDLOCK_LAST_ACCESS_FS << 1) - 1)
 #define LANDLOCK_NUM_ACCESS_FS		__const_hweight64(LANDLOCK_MASK_ACCESS_FS)
 
diff --git a/security/landlock/setup.c b/security/landlock/setup.c
index f8e8e980454c..3f196d2ce4f9 100644
--- a/security/landlock/setup.c
+++ b/security/landlock/setup.c
@@ -19,6 +19,7 @@ bool landlock_initialized __lsm_ro_after_init = false;
 
 struct lsm_blob_sizes landlock_blob_sizes __lsm_ro_after_init = {
 	.lbs_cred = sizeof(struct landlock_cred_security),
+	.lbs_file = sizeof(struct landlock_file_security),
 	.lbs_inode = sizeof(struct landlock_inode_security),
 	.lbs_superblock = sizeof(struct landlock_superblock_security),
 };
diff --git a/security/landlock/syscalls.c b/security/landlock/syscalls.c
index 2ca0ccbd905a..245cc650a4dc 100644
--- a/security/landlock/syscalls.c
+++ b/security/landlock/syscalls.c
@@ -129,7 +129,7 @@ static const struct file_operations ruleset_fops = {
 	.write = fop_dummy_write,
 };
 
-#define LANDLOCK_ABI_VERSION 2
+#define LANDLOCK_ABI_VERSION 3
 
 /**
  * sys_landlock_create_ruleset - Create a new ruleset
diff --git a/security/security.c b/security/security.c
index bdc295ad5fba..b967e035b456 100644
--- a/security/security.c
+++ b/security/security.c
@@ -185,11 +185,12 @@ static void __init lsm_set_blob_size(int *need, int *lbs)
 {
 	int offset;
 
-	if (*need > 0) {
-		offset = *lbs;
-		*lbs += *need;
-		*need = offset;
-	}
+	if (*need <= 0)
+		return;
+
+	offset = ALIGN(*lbs, sizeof(void *));
+	*lbs = offset + *need;
+	*need = offset;
 }
 
 static void __init lsm_set_blob_sizes(struct lsm_blob_sizes *needed)
@@ -1694,6 +1695,11 @@ int security_file_open(struct file *file)
 	return fsnotify_perm(file, MAY_OPEN);
 }
 
+int security_file_truncate(struct file *file)
+{
+	return call_int_hook(file_truncate, 0, file);
+}
+
 int security_task_alloc(struct task_struct *task, unsigned long clone_flags)
 {
 	int rc = lsm_task_alloc(task);
diff --git a/security/tomoyo/tomoyo.c b/security/tomoyo/tomoyo.c
index 71e82d855ebf..af04a7b7eb28 100644
--- a/security/tomoyo/tomoyo.c
+++ b/security/tomoyo/tomoyo.c
@@ -135,6 +135,18 @@ static int tomoyo_path_truncate(const struct path *path)
 }
 
 /**
+ * tomoyo_file_truncate - Target for security_file_truncate().
+ *
+ * @file: Pointer to "struct file".
+ *
+ * Returns 0 on success, negative value otherwise.
+ */
+static int tomoyo_file_truncate(struct file *file)
+{
+	return tomoyo_path_truncate(&file->f_path);
+}
+
+/**
  * tomoyo_path_unlink - Target for security_path_unlink().
  *
  * @parent: Pointer to "struct path".
@@ -545,6 +557,7 @@ static struct security_hook_list tomoyo_hooks[] __lsm_ro_after_init = {
 	LSM_HOOK_INIT(bprm_check_security, tomoyo_bprm_check_security),
 	LSM_HOOK_INIT(file_fcntl, tomoyo_file_fcntl),
 	LSM_HOOK_INIT(file_open, tomoyo_file_open),
+	LSM_HOOK_INIT(file_truncate, tomoyo_file_truncate),
 	LSM_HOOK_INIT(path_truncate, tomoyo_path_truncate),
 	LSM_HOOK_INIT(path_unlink, tomoyo_path_unlink),
 	LSM_HOOK_INIT(path_mkdir, tomoyo_path_mkdir),