Re: [kernel-hardening] [PATCH v5 06/10] seccomp,landlock: Handle Landlock events per process hierarchy

Djalal Harouni
2017-03-02T10:22:50Z

On Wed, Feb 22, 2017 at 2:26 AM, Mickaël Salaün <mic@digikod.net> wrote:
> The seccomp(2) syscall can be use to apply a Landlock rule to the
> current process. As with a seccomp filter, the Landlock rule is enforced
> for all its future children. An inherited rule tree can be updated
> (append-only) by the owner of inherited Landlock nodes (e.g. a parent
> process that create a new rule). However, an intermediate task, which
> did not create a rule, will not be able to update its children's rules.
>
> Landlock rules can be tied to a Landlock event. When such an event is
> triggered, a tree of rules can be evaluated. Thisk kind of tree is
> created with a first node. This node reference a list of rules and an
> optional parent node. Each rule return a 32-bit value which can
> interrupt the evaluation with a non-zero value. If every rules returned
> zero, the evaluation continues with the rule list of the parent node,
> until the end of the tree.
>
> Changes since v4:
> * merge manager and seccomp patches
> * return -EFAULT in seccomp(2) when user_bpf_fd is null to easely check
> if Landlock is supported
> * only allow a process with the global CAP_SYS_ADMIN to use Landlock
> (will be lifted in the future)
> * add an early check to exit as soon as possible if the current process
> does not have Landlock rules
>
> Changes since v3:
> * remove the hard link with seccomp (suggested by Andy Lutomirski and
> Kees Cook):
> * remove the cookie which could imply multiple evaluation of Landlock
> rules
> * remove the origin field in struct landlock_data
> * remove documentation fix (merged upstream)
> * rename the new seccomp command to SECCOMP_ADD_LANDLOCK_RULE
> * internal renaming
> * split commit
> * new design to be able to inherit on the fly the parent rules
>
> Changes since v2:
> * Landlock programs can now be run without seccomp filter but for any
> syscall (from the process) or interruption
> * move Landlock related functions and structs into security/landlock/*
> (to manage cgroups as well)
> * fix seccomp filter handling: run Landlock programs for each of their
> legitimate seccomp filter
> * properly clean up all seccomp results
> * cosmetic changes to ease the understanding
> * fix some ifdef
>
> Signed-off-by: Mickaël Salaün <mic@digikod.net>
> Cc: Alexei Starovoitov <ast@kernel.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Andy Lutomirski <luto@amacapital.net>
> Cc: James Morris <james.l.morris@oracle.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Serge E. Hallyn <serge@hallyn.com>
> Cc: Will Drewry <wad@chromium.org>
> ---
> include/linux/seccomp.h | 8 ++
> include/uapi/linux/seccomp.h | 1 +
> kernel/fork.c | 14 +-
> kernel/seccomp.c | 8 ++
> security/landlock/Makefile | 2 +-
> security/landlock/hooks.c | 42 +++++-
> security/landlock/manager.c | 321 +++++++++++++++++++++++++++++++++++++++++++
> 7 files changed, 392 insertions(+), 4 deletions(-)
> create mode 100644 security/landlock/manager.c
>
> diff --git a/include/linux/seccomp.h b/include/linux/seccomp.h
> index e25aee2cdfc0..9a38de3c0e72 100644
> --- a/include/linux/seccomp.h
> +++ b/include/linux/seccomp.h
> @@ -10,6 +10,10 @@
> #include <linux/thread_info.h>
> #include <asm/seccomp.h>
>
> +#if defined(CONFIG_SECCOMP_FILTER) && defined(CONFIG_SECURITY_LANDLOCK)
> +struct landlock_events;
> +#endif /* CONFIG_SECCOMP_FILTER && CONFIG_SECURITY_LANDLOCK */
> +
> struct seccomp_filter;
> /**
> * struct seccomp - the state of a seccomp'ed process
> @@ -18,6 +22,7 @@ struct seccomp_filter;
> * system calls available to a process.
> * @filter: must always point to a valid seccomp-filter or NULL as it is
> * accessed without locking during system call entry.
> + * @landlock_events: contains an array of Landlock rules.
> *
> * @filter must only be accessed from the context of current as there
> * is no read locking.
> @@ -25,6 +30,9 @@ struct seccomp_filter;
> struct seccomp {
> int mode;
> struct seccomp_filter *filter;
> +#if defined(CONFIG_SECCOMP_FILTER) && defined(CONFIG_SECURITY_LANDLOCK)
> + struct landlock_events *landlock_events;
> +#endif /* CONFIG_SECCOMP_FILTER && CONFIG_SECURITY_LANDLOCK */
> };
>
> #ifdef CONFIG_HAVE_ARCH_SECCOMP_FILTER
> diff --git a/include/uapi/linux/seccomp.h b/include/uapi/linux/seccomp.h
> index 0f238a43ff1e..56dd692cddac 100644
> --- a/include/uapi/linux/seccomp.h
> +++ b/include/uapi/linux/seccomp.h
> @@ -13,6 +13,7 @@
> /* Valid operations for seccomp syscall. */
> #define SECCOMP_SET_MODE_STRICT 0
> #define SECCOMP_SET_MODE_FILTER 1
> +#define SECCOMP_ADD_LANDLOCK_RULE 2
>
> /* Valid flags for SECCOMP_SET_MODE_FILTER */
> #define SECCOMP_FILTER_FLAG_TSYNC 1
> diff --git a/kernel/fork.c b/kernel/fork.c
> index a4f0d0e8aeb2..bd5c72dffe60 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -37,6 +37,7 @@
> #include <linux/security.h>
> #include <linux/hugetlb.h>
> #include <linux/seccomp.h>
> +#include <linux/landlock.h>
> #include <linux/swap.h>
> #include <linux/syscalls.h>
> #include <linux/jiffies.h>
> @@ -515,7 +516,10 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node)
> * the usage counts on the error path calling free_task.
> */
> tsk->seccomp.filter = NULL;
> -#endif
> +#ifdef CONFIG_SECURITY_LANDLOCK
> + tsk->seccomp.landlock_events = NULL;
> +#endif /* CONFIG_SECURITY_LANDLOCK */
> +#endif /* CONFIG_SECCOMP */
>
> setup_thread_stack(tsk, orig);
> clear_user_return_notifier(tsk);
> @@ -1388,7 +1392,13 @@ static void copy_seccomp(struct task_struct *p)
>
> /* Ref-count the new filter user, and assign it. */
> get_seccomp_filter(current);
> - p->seccomp = current->seccomp;
> + p->seccomp.mode = current->seccomp.mode;
> + p->seccomp.filter = current->seccomp.filter;
> +#if defined(CONFIG_SECCOMP_FILTER) && defined(CONFIG_SECURITY_LANDLOCK)
> + p->seccomp.landlock_events = current->seccomp.landlock_events;
> + if (p->seccomp.landlock_events)
> + atomic_inc(&p->seccomp.landlock_events->usage);
> +#endif /* CONFIG_SECCOMP_FILTER && CONFIG_SECURITY_LANDLOCK */
>
> /*
> * Explicitly enable no_new_privs here in case it got set
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index 06f2f3ee454c..ef412d95ff5d 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -32,6 +32,7 @@
> #include <linux/security.h>
> #include <linux/tracehook.h>
> #include <linux/uaccess.h>
> +#include <linux/landlock.h>
>
> /**
> * struct seccomp_filter - container for seccomp BPF programs
> @@ -492,6 +493,9 @@ static void put_seccomp_filter(struct seccomp_filter *filter)
> void put_seccomp(struct task_struct *tsk)
> {
> put_seccomp_filter(tsk->seccomp.filter);
> +#ifdef CONFIG_SECURITY_LANDLOCK
> + put_landlock_events(tsk->seccomp.landlock_events);
> +#endif /* CONFIG_SECURITY_LANDLOCK */
> }
>
> /**
> @@ -796,6 +800,10 @@ static long do_seccomp(unsigned int op, unsigned int flags,
> return seccomp_set_mode_strict();
> case SECCOMP_SET_MODE_FILTER:
> return seccomp_set_mode_filter(flags, uargs);
> +#if defined(CONFIG_SECCOMP_FILTER) && defined(CONFIG_SECURITY_LANDLOCK)
> + case SECCOMP_ADD_LANDLOCK_RULE:
> + return landlock_seccomp_append_prog(flags, uargs);
> +#endif /* CONFIG_SECCOMP_FILTER && CONFIG_SECURITY_LANDLOCK */
> default:
> return -EINVAL;
> }
> diff --git a/security/landlock/Makefile b/security/landlock/Makefile
> index 8dc8bde660bd..6c1b0d8bd810 100644
> --- a/security/landlock/Makefile
> +++ b/security/landlock/Makefile
> @@ -2,4 +2,4 @@ ccflags-$(CONFIG_SECURITY_LANDLOCK) += -Werror=unused-function
>
> obj-$(CONFIG_SECURITY_LANDLOCK) := landlock.o
>
> -landlock-y := hooks.o
> +landlock-y := hooks.o manager.o
> diff --git a/security/landlock/hooks.c b/security/landlock/hooks.c
> index 88ebe3f01758..704ea18377d2 100644
> --- a/security/landlock/hooks.c
> +++ b/security/landlock/hooks.c
> @@ -290,7 +290,44 @@ static u64 mem_prot_to_access(unsigned long prot, bool private)
>
> static inline bool landlock_used(void)
> {
> +#ifdef CONFIG_SECCOMP_FILTER
> + return !!(current->seccomp.landlock_events);
> +#else
> return false;
> +#endif /* CONFIG_SECCOMP_FILTER */
> +}
> +
> +/**
> + * landlock_run_prog - run Landlock program for a syscall
> + *
> + * @event_idx: event index in the rules array
> + * @ctx: non-NULL eBPF context
> + * @events: Landlock events pointer
> + */
> +static int landlock_run_prog(u32 event_idx, const struct landlock_context *ctx,
> + struct landlock_events *events)
> +{
> + struct landlock_node *node;
> +
> + if (!events)
> + return 0;
> +
> + for (node = events->nodes[event_idx]; node; node = node->prev) {
> + struct landlock_rule *rule;
> +
> + for (rule = node->rule; rule; rule = rule->prev) {
> + u32 ret;
> +
> + if (WARN_ON(!rule->prog))
> + continue;
> + rcu_read_lock();
> + ret = BPF_PROG_RUN(rule->prog, (void *)ctx);
> + rcu_read_unlock();
> + if (ret)
> + return -EPERM;
> + }
> + }
> + return 0;
> }
>
> static int landlock_decide(enum landlock_subtype_event event,
> @@ -309,7 +346,10 @@ static int landlock_decide(enum landlock_subtype_event event,
> .arg2 = ctx_values[1],
> };
>
> - /* insert manager call here */
> +#ifdef CONFIG_SECCOMP_FILTER
> + ret = landlock_run_prog(event_idx, &ctx,
> + current->seccomp.landlock_events);
> +#endif /* CONFIG_SECCOMP_FILTER */
>
> return ret;
> }
> diff --git a/security/landlock/manager.c b/security/landlock/manager.c
> new file mode 100644
> index 000000000000..00bb2944c85e
> --- /dev/null
> +++ b/security/landlock/manager.c
> @@ -0,0 +1,321 @@
> +/*
> + * Landlock LSM - seccomp manager
> + *
> + * Copyright © 2017 Mickaël Salaün <mic@digikod.net>
> + *
> + * This program is free software; you can redistribute it and/or modify
> + * it under the terms of the GNU General Public License version 2, as
> + * published by the Free Software Foundation.
> + */
> +
> +#include <asm/page.h> /* PAGE_SIZE */
> +#include <linux/atomic.h> /* atomic_*(), smp_store_release() */
> +#include <linux/bpf.h> /* bpf_prog_put() */
> +#include <linux/filter.h> /* struct bpf_prog */
> +#include <linux/kernel.h> /* round_up() */
> +#include <linux/landlock.h>
> +#include <linux/sched.h> /* current_cred(), task_no_new_privs() */
> +#include <linux/security.h> /* security_capable_noaudit() */
> +#include <linux/slab.h> /* alloc(), kfree() */
> +#include <linux/types.h> /* atomic_t */
> +#include <linux/uaccess.h> /* copy_from_user() */
> +
> +#include "common.h"
> +
> +static void put_landlock_rule(struct landlock_rule *rule)
> +{
> + struct landlock_rule *orig = rule;
> +
> + /* clean up single-reference branches iteratively */
> + while (orig && atomic_dec_and_test(&orig->usage)) {
> + struct landlock_rule *freeme = orig;
> +
> + bpf_prog_put(orig->prog);
> + orig = orig->prev;
> + kfree(freeme);
> + }
> +}
> +
> +static void put_landlock_node(struct landlock_node *node)
> +{
> + struct landlock_node *orig = node;
> +
> + /* clean up single-reference branches iteratively */
> + while (orig && atomic_dec_and_test(&orig->usage)) {
> + struct landlock_node *freeme = orig;
> +
> + put_landlock_rule(orig->rule);
> + orig = orig->prev;
> + kfree(freeme);
> + }
> +}
> +
> +void put_landlock_events(struct landlock_events *events)
> +{
> + if (events && atomic_dec_and_test(&events->usage)) {
> + size_t i;
> +
> + /* XXX: Do we need to use lockless_dereference() here? */
> + for (i = 0; i < ARRAY_SIZE(events->nodes); i++) {
> + if (!events->nodes[i])
> + continue;
> + /* Are we the owner of this node? */
> + if (events->nodes[i]->owner == &events->nodes[i])
> + events->nodes[i]->owner = NULL;
> + put_landlock_node(events->nodes[i]);
> + }
> + kfree(events);
> + }
> +}
> +
> +static struct landlock_events *new_raw_landlock_events(void)
> +{
> + struct landlock_events *ret;
> +
> + /* array filled with NULL values */
> + ret = kzalloc(sizeof(*ret), GFP_KERNEL);
> + if (!ret)
> + return ERR_PTR(-ENOMEM);
> + atomic_set(&ret->usage, 1);
> + return ret;
> +}
> +
> +static struct landlock_events *new_filled_landlock_events(void)
> +{
> + size_t i;
> + struct landlock_events *ret;
> +
> + ret = new_raw_landlock_events();
> + if (IS_ERR(ret))
> + return ret;
> + /*
> + * We need to initially allocate every nodes to be able to update the
> + * rules they are pointing to, across every (future) children of the
> + * current task.
> + */
> + for (i = 0; i < ARRAY_SIZE(ret->nodes); i++) {
> + struct landlock_node *node;
> +
> + node = kzalloc(sizeof(*node), GFP_KERNEL);
> + if (!node)
> + goto put_events;
> + atomic_set(&node->usage, 1);
> + /* we are the owner of this node */
> + node->owner = &ret->nodes[i];
> + ret->nodes[i] = node;
> + }
> + return ret;
> +
> +put_events:
> + put_landlock_events(ret);
> + return ERR_PTR(-ENOMEM);
> +}
> +
> +static void add_landlock_rule(struct landlock_events *events,
> + struct landlock_rule *rule)
> +{
> + /* subtype.landlock_rule.event > 0 for loaded programs */
> + u32 event_idx = get_index(rule->prog->subtype.landlock_rule.event);
> +
> + rule->prev = events->nodes[event_idx]->rule;
> + WARN_ON(atomic_read(&rule->usage));
> + atomic_set(&rule->usage, 1);
> + /* do not increment the previous rule usage */
> + smp_store_release(&events->nodes[event_idx]->rule, rule);
> +}
> +
> +/* Limit Landlock events to 256KB. */
> +#define LANDLOCK_EVENTS_MAX_PAGES (1 << 6)
> +
> +/**
> + * landlock_append_prog - attach a Landlock rule to @current_events
> + *
> + * @current_events: landlock_events pointer, must be locked (if needed) to
> + * prevent a concurrent put/free. This pointer must not be
> + * freed after the call.
> + * @prog: non-NULL Landlock rule to append to @current_events. @prog will be
> + * owned by landlock_append_prog() and freed if an error happened.
> + *
> + * Return @current_events or a new pointer when OK. Return a pointer error
> + * otherwise.
> + */
> +static struct landlock_events *landlock_append_prog(
> + struct landlock_events *current_events, struct bpf_prog *prog)
> +{
> + struct landlock_events *new_events = current_events;
> + unsigned long pages;
> + struct landlock_rule *rule;
> + u32 event_idx;
> +
> + if (prog->type != BPF_PROG_TYPE_LANDLOCK) {
> + new_events = ERR_PTR(-EINVAL);
> + goto put_prog;
> + }
> +
> + /* validate memory size allocation */
> + pages = prog->pages;
> + if (current_events) {
> + size_t i;
> +
> + for (i = 0; i < ARRAY_SIZE(current_events->nodes); i++) {
> + struct landlock_node *walker_n;
> +
> + for (walker_n = current_events->nodes[i];
> + walker_n;
> + walker_n = walker_n->prev) {
> + struct landlock_rule *walker_r;
> +
> + for (walker_r = walker_n->rule;
> + walker_r;
> + walker_r = walker_r->prev)
> + pages += walker_r->prog->pages;
> + }
> + }
> + /* count a struct landlock_events if we need to allocate one */
> + if (atomic_read(&current_events->usage) != 1)
> + pages += round_up(sizeof(*current_events), PAGE_SIZE) /
> + PAGE_SIZE;
> + }
> + if (pages > LANDLOCK_EVENTS_MAX_PAGES) {
> + new_events = ERR_PTR(-E2BIG);
> + goto put_prog;
> + }
> +
> + rule = kzalloc(sizeof(*rule), GFP_KERNEL);
> + if (!rule) {
> + new_events = ERR_PTR(-ENOMEM);
> + goto put_prog;
> + }
> + rule->prog = prog;
> +
> + /* subtype.landlock_rule.event > 0 for loaded programs */
> + event_idx = get_index(rule->prog->subtype.landlock_rule.event);
> +
> + if (!current_events) {
> + /* add a new landlock_events, if needed */
> + new_events = new_filled_landlock_events();
> + if (IS_ERR(new_events))
> + goto put_rule;
> + add_landlock_rule(new_events, rule);
> + } else {
> + if (new_events->nodes[event_idx]->owner ==
> + &new_events->nodes[event_idx]) {
> + /* We are the owner, we can then update the node. */
> + add_landlock_rule(new_events, rule);
> + } else if (atomic_read(&current_events->usage) == 1) {
> + WARN_ON(new_events->nodes[event_idx]->owner);
> + /*
> + * We can become the new owner if no other task use it.
> + * This avoid an unnecessary allocation.
> + */
> + new_events->nodes[event_idx]->owner =
> + &new_events->nodes[event_idx];
> + add_landlock_rule(new_events, rule);

I still don't get it all, but maybe here to make it simple you have to
always allocate, since the WARN_ON() suggests it should be scheduled
to be removed... and avoid to care about whether you are using/freeing
old rules of the dead task... ?

> + } else {
> + /*
> + * We are not the owner, we need to fork current_events
> + * and then add a new node.
> + */
> + struct landlock_node *node;
> + size_t i;
> +
> + node = kmalloc(sizeof(*node), GFP_KERNEL);
> + if (!node) {
> + new_events = ERR_PTR(-ENOMEM);
> + goto put_rule;
> + }
> + atomic_set(&node->usage, 1);
> + /* set the previous node after the new_events
> + * allocation */
> + node->prev = NULL;
> + /* do not increment the previous node usage */
> + node->owner = &new_events->nodes[event_idx];
> + /* rule->prev is already NULL */
> + atomic_set(&rule->usage, 1);
> + node->rule = rule;
> +
> + new_events = new_raw_landlock_events();
> + if (IS_ERR(new_events)) {
> + /* put the rule as well */
> + put_landlock_node(node);
> + return ERR_PTR(-ENOMEM);
> + }
> + for (i = 0; i < ARRAY_SIZE(new_events->nodes); i++) {
> + new_events->nodes[i] =
> + lockless_dereference(
> + current_events->nodes[i]);
> + if (i == event_idx)
> + node->prev = new_events->nodes[i];
> + if (!WARN_ON(!new_events->nodes[i]))
> + atomic_inc(&new_events->nodes[i]->usage);
> + }
> + new_events->nodes[event_idx] = node;
> +
> + /*
> + * @current_events will not be freed here because it's usage
> + * field is > 1. It is only prevented to be freed by another
> + * subject thanks to the caller of landlock_append_prog() which
> + * should be locked if needed.
> + */
> + put_landlock_events(current_events);
> + }
> + }
> + return new_events;
> +
> +put_prog:
> + bpf_prog_put(prog);
> + return new_events;
> +
> +put_rule:
> + put_landlock_rule(rule);
> + return new_events;
> +}
> +
> +/**
> + * landlock_seccomp_append_prog - attach a Landlock rule to the current process
> + *
> + * current->seccomp.landlock_events is lazily allocated. When a process fork,
> + * only a pointer is copied. When a new event is added by a process, if there
> + * is other references to this process' landlock_events, then a new allocation
> + * is made to contains an array pointing to Landlock rule lists. This design
> + * has low-performance impact and is memory efficient while keeping the
> + * property of append-only rules.
> + *
> + * @flags: not used for now, but could be used for TSYNC
> + * @user_bpf_fd: file descriptor pointing to a loaded Landlock rule
> + */
> +#ifdef CONFIG_SECCOMP_FILTER
> +int landlock_seccomp_append_prog(unsigned int flags, const char __user *user_bpf_fd)
> +{
> + struct landlock_events *new_events;
> + struct bpf_prog *prog;
> + int bpf_fd;
> +
> + /* force no_new_privs to limit privilege escalation */
> + if (!task_no_new_privs(current))
> + return -EPERM;
> + /* will be removed in the future to allow unprivileged tasks */
> + if (!capable(CAP_SYS_ADMIN))
> + return -EPERM;
> + if (!user_bpf_fd)
> + return -EFAULT;
> + if (flags)
> + return -EINVAL;
> + if (copy_from_user(&bpf_fd, user_bpf_fd, sizeof(bpf_fd)))
> + return -EFAULT;
> + prog = bpf_prog_get(bpf_fd);
> + if (IS_ERR(prog))
> + return PTR_ERR(prog);
> +
> + /*
> + * We don't need to lock anything for the current process hierarchy,
> + * everything is guarded by the atomic counters.
> + */
> + new_events = landlock_append_prog(current->seccomp.landlock_events, prog);
> + /* @prog is managed/freed by landlock_append_prog() */
> + if (IS_ERR(new_events))
> + return PTR_ERR(new_events);
> + current->seccomp.landlock_events = new_events;
> + return 0;
> +}
> +#endif /* CONFIG_SECCOMP_FILTER */
> --
> 2.11.0
>

--
tixxdz