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, 24 insertions, 0 deletions

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)
 {