Re: [PATCH bpf-next v11 2/7] landlock: Add the management of domains

["Serge E. Hallyn" <serge@hallyn.com>]

On Tue, Oct 29, 2019 at 06:15:00PM +0100, Mickaël Salaün wrote:
> A Landlock domain is a set of eBPF programs.  There is a list for each
> different program types that can be run on a specific Landlock hook
> (e.g. ptrace).  A domain is tied to a set of subjects (i.e. tasks).  A
> Landlock program should not try (nor be able) to infer which subject is
> currently enforced, but to have a unique security policy for all
> subjects tied to the same domain.  This make the reasoning much easier
> and help avoid pitfalls.
> 
> The next commits tie a domain to a task's credentials thanks to
> seccomp(2), but we could use cgroups or a security file-system to
> enforce a sysadmin-defined policy .
> 
> Signed-off-by: Mickaël Salaün <mic@digikod.net>
> Cc: Alexei Starovoitov <ast@kernel.org>
> Cc: Andy Lutomirski <luto@amacapital.net>
> Cc: Daniel Borkmann <daniel@iogearbox.net>
> Cc: James Morris <jmorris@namei.org>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Serge E. Hallyn <serge@hallyn.com>
> Cc: Will Drewry <wad@chromium.org>
> ---
> 
> Changes since v10:
> * rename files and names to clearly define a domain
> * create a standalone patch to ease review
> ---

[...]

> +/**
> + * store_landlock_prog - prepend and deduplicate a Landlock prog_list
> + *
> + * Prepend @prog to @init_domain while ignoring @prog if they are already in
> + * @ref_domain.  Whatever is the result of this function call, you can call
> + * bpf_prog_put(@prog) after.
> + *
> + * @init_domain: empty domain to prepend to
> + * @ref_domain: domain to check for duplicate programs
> + * @prog: program to prepend
> + *
> + * Return -errno on error or 0 if @prog was successfully stored.
> + */
> +static int store_landlock_prog(struct landlock_domain *init_domain,
> +		const struct landlock_domain *ref_domain,
> +		struct bpf_prog *prog)
> +{
> +	struct landlock_prog_list *tmp_list = NULL;
> +	int err;
> +	size_t hook;
> +	enum landlock_hook_type last_type;
> +	struct bpf_prog *new = prog;
> +
> +	/* allocate all the memory we need */
> +	struct landlock_prog_list *new_list;
> +
> +	last_type = get_hook_type(new);
> +
> +	/* ignore duplicate programs */

This comment should be "don't allow" rather than "ignore", right?

> +	if (ref_domain) {
> +		struct landlock_prog_list *ref;
> +
> +		hook = get_hook_index(get_hook_type(new));
> +		for (ref = ref_domain->programs[hook]; ref;
> +				ref = ref->prev) {
> +			if (ref->prog == new)
> +				return -EINVAL;
> +		}
> +	}
> +
> +	new = bpf_prog_inc(new);
> +	if (IS_ERR(new)) {
> +		err = PTR_ERR(new);
> +		goto put_tmp_list;
> +	}
> +	new_list = kzalloc(sizeof(*new_list), GFP_KERNEL);
> +	if (!new_list) {
> +		bpf_prog_put(new);
> +		err = -ENOMEM;
> +		goto put_tmp_list;
> +	}
> +	/* ignore Landlock types in this tmp_list */
> +	new_list->prog = new;
> +	new_list->prev = tmp_list;
> +	refcount_set(&new_list->usage, 1);
> +	tmp_list = new_list;
> +
> +	if (!tmp_list)
> +		/* inform user space that this program was already added */

I'm not following this.  You just kzalloc'd new_list, pointed
tmp_list to new_list, so how could tmp_list be NULL?  Was there
a bad code reorg here, or am i being dense?

> +		return -EEXIST;
> +
> +	/* properly store the list (without error cases) */
> +	while (tmp_list) {
> +		struct landlock_prog_list *new_list;
> +
> +		new_list = tmp_list;
> +		tmp_list = tmp_list->prev;
> +		/* do not increment the previous prog list usage */
> +		hook = get_hook_index(get_hook_type(new_list->prog));
> +		new_list->prev = init_domain->programs[hook];
> +		/* no need to add from the last program to the first because
> +		 * each of them are a different Landlock type */
> +		smp_store_release(&init_domain->programs[hook], new_list);
> +	}
> +	return 0;
> +
> +put_tmp_list:
> +	put_landlock_prog_list(tmp_list);
> +	return err;
> +}
> +
> +/* limit Landlock programs set to 256KB */
> +#define LANDLOCK_PROGRAMS_MAX_PAGES (1 << 6)
> +
> +/**
> + * landlock_prepend_prog - attach a Landlock prog_list to @current_domain
> + *
> + * Whatever is the result of this function call, you can call
> + * bpf_prog_put(@prog) after.
> + *
> + * @current_domain: landlock_domain pointer, must be (RCU-)locked (if needed)
> + *                  to prevent a concurrent put/free. This pointer must not be
> + *                  freed after the call.
> + * @prog: non-NULL Landlock prog_list to prepend to @current_domain. @prog will
> + *        be owned by landlock_prepend_prog() and freed if an error happened.
> + *
> + * Return @current_domain or a new pointer when OK. Return a pointer error
> + * otherwise.
> + */
> +struct landlock_domain *landlock_prepend_prog(
> +		struct landlock_domain *current_domain,
> +		struct bpf_prog *prog)
> +{
> +	struct landlock_domain *new_domain = current_domain;
> +	unsigned long pages;
> +	int err;
> +	size_t i;
> +	struct landlock_domain tmp_domain = {};
> +
> +	if (prog->type != BPF_PROG_TYPE_LANDLOCK_HOOK)
> +		return ERR_PTR(-EINVAL);
> +
> +	/* validate memory size allocation */
> +	pages = prog->pages;
> +	if (current_domain) {
> +		size_t i;
> +
> +		for (i = 0; i < ARRAY_SIZE(current_domain->programs); i++) {
> +			struct landlock_prog_list *walker_p;
> +
> +			for (walker_p = current_domain->programs[i];
> +					walker_p; walker_p = walker_p->prev)
> +				pages += walker_p->prog->pages;
> +		}
> +		/* count a struct landlock_domain if we need to allocate one */
> +		if (refcount_read(&current_domain->usage) != 1)
> +			pages += round_up(sizeof(*current_domain), PAGE_SIZE)
> +				/ PAGE_SIZE;
> +	}
> +	if (pages > LANDLOCK_PROGRAMS_MAX_PAGES)
> +		return ERR_PTR(-E2BIG);
> +
> +	/* ensure early that we can allocate enough memory for the new
> +	 * prog_lists */
> +	err = store_landlock_prog(&tmp_domain, current_domain, prog);
> +	if (err)
> +		return ERR_PTR(err);
> +
> +	/*
> +	 * Each task_struct points to an array of prog list pointers.  These
> +	 * tables are duplicated when additions are made (which means each
> +	 * table needs to be refcounted for the processes using it). When a new
> +	 * table is created, all the refcounters on the prog_list are bumped
> +	 * (to track each table that references the prog). When a new prog is
> +	 * added, it's just prepended to the list for the new table to point
> +	 * at.
> +	 *
> +	 * Manage all the possible errors before this step to not uselessly
> +	 * duplicate current_domain and avoid a rollback.
> +	 */
> +	if (!new_domain) {
> +		/*
> +		 * If there is no Landlock domain used by the current task,
> +		 * then create a new one.
> +		 */
> +		new_domain = new_landlock_domain();
> +		if (IS_ERR(new_domain))
> +			goto put_tmp_lists;
> +	} else if (refcount_read(&current_domain->usage) > 1) {
> +		/*
> +		 * If the current task is not the sole user of its Landlock
> +		 * domain, then duplicate it.
> +		 */
> +		new_domain = new_landlock_domain();
> +		if (IS_ERR(new_domain))
> +			goto put_tmp_lists;
> +		for (i = 0; i < ARRAY_SIZE(new_domain->programs); i++) {
> +			new_domain->programs[i] =
> +				READ_ONCE(current_domain->programs[i]);
> +			if (new_domain->programs[i])
> +				refcount_inc(&new_domain->programs[i]->usage);
> +		}
> +
> +		/*
> +		 * Landlock domain from the current task will not be freed here
> +		 * because the usage is strictly greater than 1. It is only
> +		 * prevented to be freed by another task thanks to the caller
> +		 * of landlock_prepend_prog() which should be locked if needed.
> +		 */
> +		landlock_put_domain(current_domain);
> +	}
> +
> +	/* prepend tmp_domain to new_domain */
> +	for (i = 0; i < ARRAY_SIZE(tmp_domain.programs); i++) {
> +		/* get the last new list */
> +		struct landlock_prog_list *last_list =
> +			tmp_domain.programs[i];
> +
> +		if (last_list) {
> +			while (last_list->prev)
> +				last_list = last_list->prev;
> +			/* no need to increment usage (pointer replacement) */
> +			last_list->prev = new_domain->programs[i];
> +			new_domain->programs[i] = tmp_domain.programs[i];
> +		}
> +	}
> +	return new_domain;
> +
> +put_tmp_lists:
> +	for (i = 0; i < ARRAY_SIZE(tmp_domain.programs); i++)
> +		put_landlock_prog_list(tmp_domain.programs[i]);
> +	return new_domain;
> +}
> diff --git a/security/landlock/domain_manage.h b/security/landlock/domain_manage.h
> new file mode 100644
> index 000000000000..5b5b49f6e3e8
> --- /dev/null
> +++ b/security/landlock/domain_manage.h
> @@ -0,0 +1,23 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +/*
> + * Landlock LSM - domain management headers
> + *
> + * Copyright © 2016-2019 Mickaël Salaün <mic@digikod.net>
> + * Copyright © 2018-2019 ANSSI
> + */
> +
> +#ifndef _SECURITY_LANDLOCK_DOMAIN_MANAGE_H
> +#define _SECURITY_LANDLOCK_DOMAIN_MANAGE_H
> +
> +#include <linux/filter.h>
> +
> +#include "common.h"
> +
> +void landlock_get_domain(struct landlock_domain *dom);
> +void landlock_put_domain(struct landlock_domain *dom);
> +
> +struct landlock_domain *landlock_prepend_prog(
> +		struct landlock_domain *current_domain,
> +		struct bpf_prog *prog);
> +
> +#endif /* _SECURITY_LANDLOCK_DOMAIN_MANAGE_H */
> -- 
> 2.23.0