Re: [PATCH v22 02/12] landlock: Add ruleset and domain management

Mickaël Salaün
2020-10-29T09:35:33Z

On 29/10/2020 02:05, Jann Horn wrote:
> On Tue, Oct 27, 2020 at 9:04 PM Mickaël Salaün <mic@digikod.net> wrote:
>> A Landlock ruleset is mainly a red-black tree with Landlock rules as
>> nodes. This enables quick update and lookup to match a requested access
>> e.g., to a file. A ruleset is usable through a dedicated file
>> descriptor (cf. following commit implementing syscalls) which enables a
>> process to create and populate a ruleset with new rules.
>>
>> A domain is a ruleset tied to a set of processes. This group of rules
>> defines the security policy enforced on these processes and their future
>> children. A domain can transition to a new domain which is the
>> intersection of all its constraints and those of a ruleset provided by
>> the current process. This modification only impact the current process.
>> This means that a process can only gain more constraints (i.e. lose
>> accesses) over time.
>>
>> Cc: James Morris <jmorris@namei.org>
>> Cc: Jann Horn <jannh@google.com>
>> Cc: Kees Cook <keescook@chromium.org>
>> Cc: Serge E. Hallyn <serge@hallyn.com>
>> Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com>
>
> Reviewed-by: Jann Horn <jannh@google.com>

Thanks.

>
> with some nits:
>
> [...]
>> +static struct landlock_ruleset *create_ruleset(void)
>> +{
>> + struct landlock_ruleset *new_ruleset;
>> +
>> + new_ruleset = kzalloc(sizeof(*new_ruleset), GFP_KERNEL_ACCOUNT);
>> + if (!new_ruleset)
>> + return ERR_PTR(-ENOMEM);
>> + refcount_set(&new_ruleset->usage, 1);
>> + mutex_init(&new_ruleset->lock);
>> + /*
>> + * root = RB_ROOT
>
> This should probably be done explicitly, even though it's currently a
> no-op, in case the implementation of RB_ROOT changes in the future.

OK, I'll do it for RB_ROOT.

>
>> + * hierarchy = NULL
>> + * nb_rules = 0
>> + * nb_layers = 0
>> + * fs_access_mask = 0
>> + */
>> + return new_ruleset;
>> +}
> [...]
>> +/**
>> + * landlock_insert_rule - Insert a rule in a ruleset
>> + *
>> + * @ruleset: The ruleset to be updated.
>> + * @rule: Read-only payload to be inserted (not own by this function).
>
> s/own/owned/

OK

>
>> + * @is_merge: If true, intersects access rights and updates the rule's layers
>> + * (e.g. merge two rulesets), else do a union of access rights and
>> + * keep the rule's layers (e.g. extend a ruleset)
>> + *
>> + * Assumptions:
>> + *
>> + * - An inserted rule cannot be removed.
>> + * - The underlying kernel object must be held by the caller.
>> + */
>> +int landlock_insert_rule(struct landlock_ruleset *const ruleset,
>> + struct landlock_rule *const rule, const bool is_merge)
> [...]
>> +static int merge_ruleset(struct landlock_ruleset *const dst,
>> + struct landlock_ruleset *const src)
>> +{
>> + struct landlock_rule *walker_rule, *next_rule;
>> + int err = 0;
>> +
>> + might_sleep();
>> + if (!src)
>> + return 0;
>> + /* Only merge into a domain. */
>> + if (WARN_ON_ONCE(!dst || !dst->hierarchy))
>> + return -EFAULT;
>> +
>> + mutex_lock(&dst->lock);
>> + mutex_lock_nested(&src->lock, 1);
>
> Maybe add a comment like this above these two lines? "Ruleset locks
> are ordered by time of ruleset creation; dst is newer than src."

OK

>
> Also, maybe s/1/SINGLE_DEPTH_NESTING/.

OK

>
>
>> + /*
>> + * Makes a new layer, but only increments the number of layers after
>> + * the rules are inserted.
>> + */
>> + if (dst->nb_layers == sizeof(walker_rule->layers) * BITS_PER_BYTE) {
>> + err = -E2BIG;
>> + goto out_unlock;
>> + }
>> + dst->fs_access_mask |= src->fs_access_mask;
>> +
>> + /* Merges the @src tree. */
>> + rbtree_postorder_for_each_entry_safe(walker_rule, next_rule,
>> + &src->root, node) {
>> + err = landlock_insert_rule(dst, walker_rule, true);
>> + if (err)
>> + goto out_unlock;
>> + }
>> + dst->nb_layers++;
>> +
>> +out_unlock:
>> + mutex_unlock(&src->lock);
>> + mutex_unlock(&dst->lock);
>> + return err;
>> +}
> [...]
>> diff --git a/security/landlock/ruleset.h b/security/landlock/ruleset.h
> [...]
>> +struct landlock_rule {
>> + /**
>> + * @node: Node in the red-black tree.
>
> s/the red-black tree/the ruleset's red-black tree/

OK

>
>> + */
>> + struct rb_node node;
>> + /**
>> + * @object: Pointer to identify a kernel object (e.g. an inode). This
>> + * is used as a key for this ruleset element. This pointer is set once
>> + * and never modified. It always point to an allocated object because
>
> s/point/points/

OK

>
>> + * each rule increment the refcount of there object.
>
> s/increment/increments/
> s/there/its/

OK

>
>> + */
>> + struct landlock_object *object;
>> + /**
>> + * @layers: Bitfield to identify the layers which resulted to @access
>> + * from different consecutive intersections.
>> + */
>> + u64 layers;
>> + /**
>> + * @access: Bitfield of allowed actions on the kernel object. They are
>> + * relative to the object type (e.g. %LANDLOCK_ACTION_FS_READ). This
>> + * may be the result of the merged access rights (boolean AND) from
>> + * multiple layers referring to the same object.
>> + */
>> + u32 access;
>> +};
>> +
>> +/**
>> + * struct landlock_hierarchy - Node in a ruleset hierarchy
>> + */
>> +struct landlock_hierarchy {
>> + /**
>> + * @parent: Pointer to the parent node, or NULL if it is a root Lanlock
>
> nit: Landlock

Thanks :)

>
>> + * domain.
>> + */
>> + struct landlock_hierarchy *parent;
>> + /**
>> + * @usage: Number of potential children domains plus their parent
>> + * domain.
>> + */
>> + refcount_t usage;
>> +};
>> +
>> +/**
>> + * struct landlock_ruleset - Landlock ruleset
>> + *
>> + * This data structure must contains unique entries, be updatable, and quick to
>
> s/contains/contain/

OK

>
>> + * match an object.
>> + */
>> +struct landlock_ruleset {
>> + /**
>> + * @root: Root of a red-black tree containing &struct landlock_rule
>> + * nodes.
>
> Maybe add: "Once the ruleset is installed on a process, this tree is
> immutable until @usage reaches zero."

Right.

>
>> + */
>> + struct rb_root root;
>> + /**
>> + * @hierarchy: Enables hierarchy identification even when a parent
>> + * domain vanishes. This is needed for the ptrace protection.
>> + */
>> + struct landlock_hierarchy *hierarchy;
>> + union {
>> + /**
>> + * @work_free: Enables to free a ruleset within a lockless
>> + * section. This is only used by
>> + * landlock_put_ruleset_deferred() when @usage reaches zero.
>> + * The fields @lock, @usage, @nb_layers, @nb_rules and
>> + * @fs_access_mask are then unused.
>> + */
>> + struct work_struct work_free;
>> + struct {
>> + /**
>> + * @lock: Guards against concurrent modifications of
>> + * @root, if @usage is greater than zero.
>> + */
>> + struct mutex lock;
>> + /**
>> + * @usage: Number of processes (i.e. domains) or file
>> + * descriptors referencing this ruleset.
>> + */
>> + refcount_t usage;
>> + /**
>> + * @nb_rules: Number of non-overlapping (i.e. not for
>> + * the same object) rules in this ruleset.
>> + */
>> + u32 nb_rules;
>> + /**
>> + * @nb_layers: Number of layers which are used in this
>> + * ruleset. This enables to check that all the layers
>> + * allow an access request. A value of 0 identify a
>
> s/identify/identifies/

OK

>
>
>
>> + * non-merged ruleset (i.e. not a domain).
>> + */
>> + u32 nb_layers;
>> + /**
>> + * @fs_access_mask: Contains the subset of filesystem
>> + * actions which are restricted by a ruleset. This is
>> + * used when merging rulesets and for user space
>> + * backward compatibility (i.e. future-proof). Set
>> + * once and never changed for the lifetime of the
>> + * ruleset.
>> + */
>> + u32 fs_access_mask;
>> + };
>> + };
>> +};