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

Mickaël Salaün
2017-02-22T01:26:28Z

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);
+ } 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