[PATCH v19 08/12] landlock: Add syscall implementation

[Arnd Bergmann]

On Tue, Jul 7, 2020 at 8:10 PM Mickaël Salaün <mic at digikod.net> wrote:
>
> This system call, inspired from seccomp(2) and bpf(2), is designed to be
> used by unprivileged processes to sandbox themselves.  It has the same
> usage restrictions as seccomp(2): the caller must have the no_new_privs
> attribute set or have CAP_SYS_ADMIN in the current user namespace.
>
> Here are the motivations for this new syscall:
> * A sandboxed process may not have access to file systems, including
>   /dev, /sys or /proc, but it should still be able to add more
>   restrictions to itself.
> * Neither prctl(2) nor seccomp(2) (which was used in a previous version)
>   fit well with the current definition of a Landlock security policy.
> * It is quite easy to whitelist this syscall with seccomp-bpf to enable
>   all processes to use it.  It is also easy to filter specific commands
>   or options to restrict a process to a subset of Landlock features.
>
> There is currently four commands:
> * LANDLOCK_CMD_GET_FEATURES: Gets the supported features (required for
>   backward and forward compatibility, and best-effort security).
> * LANDLOCK_CMD_CREATE_RULESET: Creates a ruleset and returns its file
>   descriptor.
> * LANDLOCK_CMD_ADD_RULE: Adds a rule (e.g. file hierarchy access) to a
>   ruleset, identified by the dedicated file descriptor.
> * LANDLOCK_CMD_ENFORCE_RULESET: Enforces a ruleset on the current thread
>   and its future children (similar to seccomp).

I never paid attention to the patch series so far, so I'm sorry if this
has all been discussed before, but I think the syscall prototype needs
to be different, with *much* less extensibility built in.

> Each command has at least one option, which enables to define the
> attribute types passed to the syscall.  All attribute types (structures)
> are checked at build time to ensure that they don't contain holes and
> that they are aligned the same way for each architecture.  The struct
> landlock_attr_features contains __u32 options_* fields which is enough
> to store 32-bits syscall arguments, and __u16 size_attr_* fields which
> is enough for the maximal struct size (i.e. page size) passed through
> the landlock syscall.  The other fields can have __u64 type for flags
> and bitfields, and __s32 type for file descriptors.
>
> See the user and kernel documentation for more details (provided by a
> following commit): Documentation/security/landlock/

System calls with their own sub-commands have turned out to be a
bad idea many times in the past and cause more problems than they
solve. See sys_ipc, sys_socketcall and sys_futex for common examples.

The first step I would recommend is to split out the individual commands
into separate syscalls. For each one of those, pick a simple prototype
that can do what it needs, with one 'flags' argument for extensibility.

> +/**
> + * DOC: options_intro
> + *
> + * These options may be used as second argument of sys_landlock().  Each
> + * command have a dedicated set of options, represented as bitmasks.  For two
> + * different commands, their options may overlap.  Each command have at least
> + * one option defining the used attribute type.  This also enables to always
> + * have a usable &struct landlock_attr_features (i.e. filled with bits).
> + */
> +
> +/**
> + * DOC: options_get_features
> + *
> + * Options for ``LANDLOCK_CMD_GET_FEATURES``
> + * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> + *
> + * - %LANDLOCK_OPT_GET_FEATURES: the attr type is `struct
> + *   landlock_attr_features`.
> + */
> +#define LANDLOCK_OPT_GET_FEATURES                      (1 << 0)

For each command, you currently have one attribute that is defined
to have a name directly corresponding to the command, making it
entirely redundant. I'd suggest just check the 'flags' argument for
being zero at syscall entry for future extension if you think there may
be a need to extend it, or completely leave out attributes and flags.

> +static int syscall_create_ruleset(const void __user *const attr_ptr,
> +               const size_t attr_size)
> +{
> +       struct landlock_attr_ruleset attr_ruleset;
> +       struct landlock_ruleset *ruleset;
> +       int err, ruleset_fd;
> +
> +       /* Copies raw user space buffer. */
> +       err = copy_struct_if_any_from_user(&attr_ruleset, sizeof(attr_ruleset),
> +                       offsetofend(typeof(attr_ruleset), handled_access_fs),
> +                       attr_ptr, attr_size);
> +       if (err)
> +               return err;
> +
> +       /* Checks content (and 32-bits cast). */
> +       if ((attr_ruleset.handled_access_fs | _LANDLOCK_ACCESS_FS_MASK) !=
> +                       _LANDLOCK_ACCESS_FS_MASK)
> +               return -EINVAL;
> +
> +       /* Checks arguments and transforms to kernel struct. */
> +       ruleset = landlock_create_ruleset(attr_ruleset.handled_access_fs);
> +       if (IS_ERR(ruleset))
> +               return PTR_ERR(ruleset);
> +
> +       /* Creates anonymous FD referring to the ruleset. */
> +       ruleset_fd = anon_inode_getfd("landlock-ruleset", &ruleset_fops,
> +                       ruleset, O_RDWR | O_CLOEXEC);
> +       if (ruleset_fd < 0)
> +               landlock_put_ruleset(ruleset);
> +       return ruleset_fd;
> +}

It looks like all you need here today is a single argument bit, plus
possibly some room for extensibility. I would suggest removing all
the extra bits and using a syscall like

SYSCALL_DEFINE1(landlock_create_ruleset, u32, flags);

I don't really see how this needs any variable-length arguments,
it really doesn't do much.

To be on the safe side, you might split up the flags into either the
upper/lower 16 bits or two u32 arguments, to allow both compatible
(ignored by older kernels if flag is set) and incompatible (return error
when an unknown flag is set) bits.

> +struct landlock_attr_path_beneath {
> +       /**
> +        * @ruleset_fd: File descriptor tied to the ruleset which should be
> +        * extended with this new access.
> +        */
> +       __s32 ruleset_fd;
> +       /**
> +        * @parent_fd: File descriptor, open with ``O_PATH``, which identify
> +        * the parent directory of a file hierarchy, or just a file.
> +        */
> +       __s32 parent_fd;
> +       /**
> +        * @allowed_access: Bitmask of allowed actions for this file hierarchy
> +        * (cf. `Filesystem flags`_).
> +        */
> +       __u64 allowed_access;
> +};

> +static int syscall_add_rule_path_beneath(const void __user *const attr_ptr,
> +               const size_t attr_size)
> +{
> +       struct landlock_attr_path_beneath attr_path_beneath;
> +       struct path path;
> +       struct landlock_ruleset *ruleset;
> +       int err;

Similarly, it looks like this wants to be

SYSCALL_DEFINE3(landlock_add_rule_path_beneath, int, ruleset, int,
path, __u32, flags)

I don't see any need to extend this in a way that wouldn't already
be served better by adding another system call. You might argue
that 'flags' and 'allowed_access' could be separate, with the latter
being an indirect in/out argument here, like

SYSCALL_DEFINE4(landlock_add_rule_path_beneath, int, ruleset, int, path,
                           __u64 *, allowed_acces, __u32, flags)

> +static int syscall_enforce_ruleset(const void __user *const attr_ptr,
> +               const size_t attr_size)

Here it seems like you just need to pass the file descriptor, or maybe

SYSCALL_DEFINE2(landlock_enforce, int, ruleset, __u32 flags);

if you need flags for extensibility.

      Arnd