Re: [PATCH 1/2] landlock: Support truncate(2).

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

On Fri, Jul 08, 2022 at 01:17:32PM +0200, Mickaël Salaün wrote:
> No final dot for a subject please.

Done. Will be fixed in the next version.

>
> On 07/07/2022 22:06, Günther Noack wrote:
> > Add support for restricting the use of the truncate(2) and
> > ftruncate(2) family of syscalls with Landlock.
> >
> > This change also updates the Landlock ABI version and updates the
> > existing Landlock tests to match the new ABI version.
> >
> > Technically, unprivileged processes can already restrict the use of
> > truncate(2) with seccomp-bpf.
> >
> > Using Landlock instead of seccomp-bpf has the folowwing advantages:
>
> typo: following

Done. Will be fixed in the next version.

>
> >
> > - it doesn't require the use of BPF (conceptually simpler)
> >
> > - callers don't need to keep track of lists of syscall numbers for
> >    different architectures and kernel versions
> >
> > - the restriction policy can be configured per file hierarchy.
> >
> > Signed-off-by: Günther Noack <gnoack3000@gmail.com>
> > ---
> >   include/uapi/linux/landlock.h                | 2 ++
> >   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 ++++---
> >   6 files changed, 17 insertions(+), 7 deletions(-)
> >
> > diff --git a/include/uapi/linux/landlock.h b/include/uapi/linux/landlock.h
> > index 23df4e0e8ace..2351050d4773 100644
> > --- a/include/uapi/linux/landlock.h
> > +++ b/include/uapi/linux/landlock.h
> > @@ -134,6 +134,7 @@ struct landlock_path_beneath_attr {
> >    *   directory) parent.  Otherwise, such actions are denied with errno set to
> >    *   EACCES.  The EACCES errno prevails over EXDEV to let user space
> >    *   efficiently deal with an unrecoverable error.
> > + * - %LANDLOCK_ACCESS_FS_TRUNCATE%: Truncate a file.
>
> We need to specify the ABI version starting to support this right.

Done. Will be fixed in the next version.

>
> >    *
> >    * .. warning::
>
> You need to remove truncate(2) from this warning block.

Done. Will be fixed in the next version.

>
> >    *
> > @@ -160,6 +161,7 @@ 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)
>
> I created ACCESS_LAST to store the last access right while avoiding to copy
> it in ACCESS_FILE or ACCESS_ALL, and then avoid forgetting about new access
> right, but I now think it is not worth it and I prefer your approach which
> will be easier to maintain.

OK, thanks.

>
> >   /* clang-format on */

--