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, 16 insertions, 5 deletions

diff --git a/include/uapi/linux/landlock.h b/include/uapi/linux/landlock.h
index 9c4bcc37a455..f3223f964691 100644
--- a/include/uapi/linux/landlock.h
+++ b/include/uapi/linux/landlock.h
@@ -95,8 +95,19 @@ struct landlock_path_beneath_attr {
  * A file can only receive these access rights:
  *
  * - %LANDLOCK_ACCESS_FS_EXECUTE: Execute a file.
- * - %LANDLOCK_ACCESS_FS_WRITE_FILE: Open a file with write access.
+ * - %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_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
+ *   ``O_TRUNC``. Whether an opened file can be truncated with
+ *   :manpage:`ftruncate(2)` is determined during :manpage:`open(2)`, in the
+ *   same way as read and write permissions are checked during
+ *   :manpage:`open(2)` using %LANDLOCK_ACCESS_FS_READ_FILE and
+ *   %LANDLOCK_ACCESS_FS_WRITE_FILE. This access right is available since the
+ *   third version of the Landlock ABI.
  *
  * A directory can receive access rights related to files or directories.  The
  * following access right is applied to the directory itself, and the
@@ -139,10 +150,9 @@ struct landlock_path_beneath_attr {
  *
  *   It is currently not possible to restrict some file-related actions
  *   accessible through these syscall families: :manpage:`chdir(2)`,
- *   :manpage:`truncate(2)`, :manpage:`stat(2)`, :manpage:`flock(2)`,
- *   :manpage:`chmod(2)`, :manpage:`chown(2)`, :manpage:`setxattr(2)`,
- *   :manpage:`utime(2)`, :manpage:`ioctl(2)`, :manpage:`fcntl(2)`,
- *   :manpage:`access(2)`.
+ *   :manpage:`stat(2)`, :manpage:`flock(2)`, :manpage:`chmod(2)`,
+ *   :manpage:`chown(2)`, :manpage:`setxattr(2)`, :manpage:`utime(2)`,
+ *   :manpage:`ioctl(2)`, :manpage:`fcntl(2)`, :manpage:`access(2)`.
  *   Future Landlock evolutions will enable to restrict them.
  */
 /* clang-format off */
@@ -160,6 +170,7 @@ struct landlock_path_beneath_attr {
 #define LANDLOCK_ACCESS_FS_MAKE_BLOCK			(1ULL << 11)
 #define LANDLOCK_ACCESS_FS_MAKE_SYM			(1ULL << 12)
 #define LANDLOCK_ACCESS_FS_REFER			(1ULL << 13)
+#define LANDLOCK_ACCESS_FS_TRUNCATE			(1ULL << 14)
 /* clang-format on */
 
 #endif /* _UAPI_LINUX_LANDLOCK_H */