landlock: Support file truncation

Introduce the LANDLOCK_ACCESS_FS_TRUNCATE flag for file truncation.

This flag hooks into the path_truncate, file_truncate and
file_alloc_security LSM hooks and covers file truncation using
truncate(2), ftruncate(2), open(2) with O_TRUNC, as well as creat().

This change also increments the Landlock ABI version, updates
corresponding selftests, and updates code documentation to document
the flag.

In security/security.c, allocate security blobs at pointer-aligned
offsets. This fixes the problem where one LSM's security blob can
shift another LSM's security blob to an unaligned address (reported
by Nathan Chancellor).

The following operations are restricted:

open(2): requires the LANDLOCK_ACCESS_FS_TRUNCATE right if a file gets
implicitly truncated as part of the open() (e.g. using O_TRUNC).

Notable special cases:
* open(..., O_RDONLY|O_TRUNC) can truncate files as well in Linux
* open() with O_TRUNC does *not* need the TRUNCATE right when it
  creates a new file.

truncate(2) (on a path): requires the LANDLOCK_ACCESS_FS_TRUNCATE
right.

ftruncate(2) (on a file): requires that the file had the TRUNCATE
right when it was previously opened. File descriptors acquired by
other means than open(2) (e.g. memfd_create(2)) continue to support
truncation with ftruncate(2).

Cc: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Günther Noack <gnoack3000@gmail.com>
Acked-by: Paul Moore <paul@paul-moore.com> (LSM)
Link: https://lore.kernel.org/r/20221018182216.301684-5-gnoack3000@gmail.com
Signed-off-by: Mickaël Salaün <mic@digikod.net>

1 files changed, 99 insertions, 5 deletions

diff --git a/security/landlock/fs.c b/security/landlock/fs.c
index 87fde50eb550..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 */
 
 /*
@@ -1154,9 +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;
 
@@ -1174,19 +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 = {
@@ -1206,8 +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)