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landlock: Support file truncation

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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>
This commit is contained in:
Günther Noack
2022-10-18 20:22:09 +02:00
committed by Mickaël Salaün
parent 52a13488d1
commit b9f5ce27c8
9 changed files with 153 additions and 21 deletions
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21
include/uapi/linux/landlock.h
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@@ -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 */