Re: [PATCH v2 1/4] landlock: Support file truncation

["Günther Noack" <gnoack3000@gmail.com>]

On Thu, Jul 28, 2022 at 06:25:52PM +0200, Mickaël Salaün wrote:
>
> On 12/07/2022 23:14, Günther Noack wrote:
> > Introduce the LANDLOCK_ACCESS_FS_TRUNCATE flag for file truncation.
> >
> > This flag hooks into the path_truncate LSM hook 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 includes minor documentation changes to
> > document the flag.
> >
> > Signed-off-by: Günther Noack <gnoack3000@gmail.com>
> > Link: https://lore.kernel.org/all/20220707200612.132705-2-gnoack3000@gmail.com/
>
> Please don't include Link tags pointing to the previous version of the
> patch. You could add it after a "---" but you already added a link to the v1
> in your cover letter, which is great. I'll add fresh links pointing to your
> emails when I'll merge your changes.
>
> Also, don't add anything after your Signed-off-by, it should be the last
> line of your commit message.

Thanks, noted and fixed for the next version.

>
>
> > ---
> >   Documentation/userspace-api/landlock.rst     |  5 +++++
> >   include/uapi/linux/landlock.h                | 13 +++++++++----
> >   security/landlock/fs.c                       |  9 ++++++++-
> >   security/landlock/limits.h                   |  2 +-
> >   security/landlock/syscalls.c                 |  2 +-
> >   tools/testing/selftests/landlock/base_test.c |  2 +-
> >   tools/testing/selftests/landlock/fs_test.c   |  7 ++++---
> >   7 files changed, 29 insertions(+), 11 deletions(-)
> >
> > diff --git a/Documentation/userspace-api/landlock.rst b/Documentation/userspace-api/landlock.rst
> > index b8ea59493964..b86fd94ae797 100644
> > --- a/Documentation/userspace-api/landlock.rst
> > +++ b/Documentation/userspace-api/landlock.rst
> > @@ -380,6 +380,11 @@ by the Documentation/admin-guide/cgroup-v1/memory.rst.
> >   Previous limitations
> >   ====================
> > +File truncation (ABI 1-2)
>
> I think "ABI < 3" should be easier to understand.

Agreed, done.

>
>
> > +-------------------------
> > +
> > +File truncation was not restrictable in ABI 1-2, so it was always permitted.
>
> What about:
> File truncation couldn't be denied before the third Landlock ABI, so it is
> always allowed for kernels only supporting the first to the second ABI.

Sounds good, done.

Will include these fixes in the next version. Thank you for the review!

--Günther

>
>
> > +
> >   File renaming and linking (ABI 1)
> >   ---------------------------------
> > diff --git a/include/uapi/linux/landlock.h b/include/uapi/linux/landlock.h
> > index 23df4e0e8ace..9ca7f9d0d862 100644
> > --- a/include/uapi/linux/landlock.h
> > +++ b/include/uapi/linux/landlock.h
> > @@ -96,7 +96,12 @@ struct landlock_path_beneath_attr {
> >    *
> >    * - %LANDLOCK_ACCESS_FS_EXECUTE: Execute a file.
> >    * - %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 open(2) using O_TRUNC or creat(2).
> >    * - %LANDLOCK_ACCESS_FS_READ_FILE: Open a file with read access.
> > + * - %LANDLOCK_ACCESS_FS_TRUNCATE: Truncate a file through file truncation
> > + *   APIs like truncate(2), ftruncate(2), open(2) with O_TRUNC or creat(2).
> > + *   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 +144,9 @@ 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 +164,7 @@ 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 */
> > diff --git a/security/landlock/fs.c b/security/landlock/fs.c
> > index ec5a6247cd3e..c57f581a9cd5 100644
> > --- a/security/landlock/fs.c
> > +++ b/security/landlock/fs.c
> > @@ -146,7 +146,8 @@ static struct landlock_object *get_inode_object(struct inode *const inode)
> >   #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 */
> >   /*
> > @@ -1140,6 +1141,11 @@ 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)
> > @@ -1192,6 +1198,7 @@ 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_open, hook_file_open),
> >   };
> > 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/syscalls.c b/security/landlock/syscalls.c
> > index 735a0865ea11..f4d6fc7ed17f 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/tools/testing/selftests/landlock/base_test.c b/tools/testing/selftests/landlock/base_test.c
> > index da9290817866..72cdae277b02 100644
> > --- a/tools/testing/selftests/landlock/base_test.c
> > +++ b/tools/testing/selftests/landlock/base_test.c
> > @@ -75,7 +75,7 @@ TEST(abi_version)
> >   	const struct landlock_ruleset_attr ruleset_attr = {
> >   		.handled_access_fs = LANDLOCK_ACCESS_FS_READ_FILE,
> >   	};
> > -	ASSERT_EQ(2, landlock_create_ruleset(NULL, 0,
> > +	ASSERT_EQ(3, landlock_create_ruleset(NULL, 0,
> >   					     LANDLOCK_CREATE_RULESET_VERSION));
> >   	ASSERT_EQ(-1, landlock_create_ruleset(&ruleset_attr, 0,
> > diff --git a/tools/testing/selftests/landlock/fs_test.c b/tools/testing/selftests/landlock/fs_test.c
> > index 21a2ce8fa739..cb77eaa01c91 100644
> > --- a/tools/testing/selftests/landlock/fs_test.c
> > +++ b/tools/testing/selftests/landlock/fs_test.c
> > @@ -399,9 +399,10 @@ TEST_F_FORK(layout1, inval)
> >   #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)
> > -#define ACCESS_LAST LANDLOCK_ACCESS_FS_REFER
> > +#define ACCESS_LAST LANDLOCK_ACCESS_FS_TRUNCATE
> >   #define ACCESS_ALL ( \
> >   	ACCESS_FILE | \
> > @@ -415,7 +416,7 @@ TEST_F_FORK(layout1, inval)
> >   	LANDLOCK_ACCESS_FS_MAKE_FIFO | \
> >   	LANDLOCK_ACCESS_FS_MAKE_BLOCK | \
> >   	LANDLOCK_ACCESS_FS_MAKE_SYM | \
> > -	ACCESS_LAST)
> > +	LANDLOCK_ACCESS_FS_REFER)
> >   /* clang-format on */

--