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userns: Add a limit on the number of user namespaces

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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>
This commit is contained in:
Eric W. Biederman
2016-08-08 13:41:24 -05:00
parent dbec28460a
commit b376c3e1b6
4 changed files with 81 additions and 11 deletions
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31
kernel/user_namespace.c
View File

@@ -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));
}