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authorEric W. Biederman <ebiederm@xmission.com>2016-08-08 13:41:24 -0500
committerEric W. Biederman <ebiederm@xmission.com>2016-08-08 13:41:24 -0500
commitb376c3e1b6770ddcb4f0782be16358095fcea0b6 (patch)
tree30789f9f1e84abe4d25a52e2fcad209504c181bf
parentdbec28460a89aa7c02c3301e9e108d98272549d2 (diff)
userns: Add a limit on the number of user namespaces
Export the export the maximum number of user namespaces as /proc/sys/userns/max_user_namespaces. Acked-by: Kees Cook <keescook@chromium.org> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
-rw-r--r--include/linux/user_namespace.h6
-rw-r--r--kernel/fork.c2
-rw-r--r--kernel/ucount.c53
-rw-r--r--kernel/user_namespace.c31
4 files changed, 81 insertions, 11 deletions
diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
index e5697eaf6bf9..6421cca2daa9 100644
--- a/include/linux/user_namespace.h
+++ b/include/linux/user_namespace.h
@@ -44,9 +44,15 @@ struct user_namespace {
struct ctl_table_set set;
struct ctl_table_header *sysctls;
#endif
+ int max_user_namespaces;
+ atomic_t user_namespaces;
};
extern struct user_namespace init_user_ns;
+extern bool setup_userns_sysctls(struct user_namespace *ns);
+extern void retire_userns_sysctls(struct user_namespace *ns);
+extern bool inc_user_namespaces(struct user_namespace *ns);
+extern void dec_user_namespaces(struct user_namespace *ns);
#ifdef CONFIG_USER_NS
diff --git a/kernel/fork.c b/kernel/fork.c
index 52e725d4a866..daa6a82b4900 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -321,6 +321,8 @@ void __init fork_init(void)
init_task.signal->rlim[RLIMIT_NPROC].rlim_max = max_threads/2;
init_task.signal->rlim[RLIMIT_SIGPENDING] =
init_task.signal->rlim[RLIMIT_NPROC];
+
+ init_user_ns.max_user_namespaces = max_threads;
}
int __weak arch_dup_task_struct(struct task_struct *dst,
diff --git a/kernel/ucount.c b/kernel/ucount.c
index cbde1dc87851..6c2205c0befd 100644
--- a/kernel/ucount.c
+++ b/kernel/ucount.c
@@ -43,7 +43,18 @@ static struct ctl_table_root set_root = {
.permissions = set_permissions,
};
+static int zero = 0;
+static int int_max = INT_MAX;
static struct ctl_table userns_table[] = {
+ {
+ .procname = "max_user_namespaces",
+ .data = &init_user_ns.max_user_namespaces,
+ .maxlen = sizeof(init_user_ns.max_user_namespaces),
+ .mode = 0644,
+ .proc_handler = proc_dointvec_minmax,
+ .extra1 = &zero,
+ .extra2 = &int_max,
+ },
{ }
};
#endif /* CONFIG_SYSCTL */
@@ -55,6 +66,8 @@ bool setup_userns_sysctls(struct user_namespace *ns)
setup_sysctl_set(&ns->set, &set_root, set_is_seen);
tbl = kmemdup(userns_table, sizeof(userns_table), GFP_KERNEL);
if (tbl) {
+ tbl[0].data = &ns->max_user_namespaces;
+
ns->sysctls = __register_sysctl_table(&ns->set, "userns", tbl);
}
if (!ns->sysctls) {
@@ -78,6 +91,46 @@ void retire_userns_sysctls(struct user_namespace *ns)
#endif
}
+static inline bool atomic_inc_below(atomic_t *v, int u)
+{
+ int c, old;
+ c = atomic_read(v);
+ for (;;) {
+ if (unlikely(c >= u))
+ return false;
+ old = atomic_cmpxchg(v, c, c+1);
+ if (likely(old == c))
+ return true;
+ c = old;
+ }
+}
+
+bool inc_user_namespaces(struct user_namespace *ns)
+{
+ struct user_namespace *pos, *bad;
+ for (pos = ns; pos; pos = pos->parent) {
+ int max = READ_ONCE(pos->max_user_namespaces);
+ if (!atomic_inc_below(&pos->user_namespaces, max))
+ goto fail;
+ }
+ return true;
+fail:
+ bad = pos;
+ for (pos = ns; pos != bad; pos = pos->parent)
+ atomic_dec(&pos->user_namespaces);
+
+ return false;
+}
+
+void dec_user_namespaces(struct user_namespace *ns)
+{
+ struct user_namespace *pos;
+ for (pos = ns; pos; pos = pos->parent) {
+ int dec = atomic_dec_if_positive(&pos->user_namespaces);
+ WARN_ON_ONCE(dec < 0);
+ }
+}
+
static __init int user_namespace_sysctl_init(void)
{
#ifdef CONFIG_SYSCTL
diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index a63332253c7e..7d87017a0040 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -23,9 +23,6 @@
#include <linux/projid.h>
#include <linux/fs_struct.h>
-extern bool setup_userns_sysctls(struct user_namespace *ns);
-extern void retire_userns_sysctls(struct user_namespace *ns);
-
static struct kmem_cache *user_ns_cachep __read_mostly;
static DEFINE_MUTEX(userns_state_mutex);
@@ -34,6 +31,7 @@ static bool new_idmap_permitted(const struct file *file,
struct uid_gid_map *map);
static void free_user_ns(struct work_struct *work);
+
static void set_cred_user_ns(struct cred *cred, struct user_namespace *user_ns)
{
/* Start with the same capabilities as init but useless for doing
@@ -68,8 +66,12 @@ int create_user_ns(struct cred *new)
kgid_t group = new->egid;
int ret;
+ ret = -EUSERS;
if (parent_ns->level > 32)
- return -EUSERS;
+ goto fail;
+
+ if (!inc_user_namespaces(parent_ns))
+ goto fail;
/*
* Verify that we can not violate the policy of which files
@@ -77,26 +79,27 @@ int create_user_ns(struct cred *new)
* by verifing that the root directory is at the root of the
* mount namespace which allows all files to be accessed.
*/
+ ret = -EPERM;
if (current_chrooted())
- return -EPERM;
+ goto fail_dec;
/* The creator needs a mapping in the parent user namespace
* or else we won't be able to reasonably tell userspace who
* created a user_namespace.
*/
+ ret = -EPERM;
if (!kuid_has_mapping(parent_ns, owner) ||
!kgid_has_mapping(parent_ns, group))
- return -EPERM;
+ goto fail_dec;
+ ret = -ENOMEM;
ns = kmem_cache_zalloc(user_ns_cachep, GFP_KERNEL);
if (!ns)
- return -ENOMEM;
+ goto fail_dec;
ret = ns_alloc_inum(&ns->ns);
- if (ret) {
- kmem_cache_free(user_ns_cachep, ns);
- return ret;
- }
+ if (ret)
+ goto fail_free;
ns->ns.ops = &userns_operations;
atomic_set(&ns->count, 1);
@@ -106,6 +109,7 @@ int create_user_ns(struct cred *new)
ns->owner = owner;
ns->group = group;
INIT_WORK(&ns->work, free_user_ns);
+ ns->max_user_namespaces = INT_MAX;
/* Inherit USERNS_SETGROUPS_ALLOWED from our parent */
mutex_lock(&userns_state_mutex);
@@ -126,7 +130,11 @@ fail_keyring:
key_put(ns->persistent_keyring_register);
#endif
ns_free_inum(&ns->ns);
+fail_free:
kmem_cache_free(user_ns_cachep, ns);
+fail_dec:
+ dec_user_namespaces(parent_ns);
+fail:
return ret;
}
@@ -163,6 +171,7 @@ static void free_user_ns(struct work_struct *work)
#endif
ns_free_inum(&ns->ns);
kmem_cache_free(user_ns_cachep, ns);
+ dec_user_namespaces(parent);
ns = parent;
} while (atomic_dec_and_test(&parent->count));
}