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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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53
kernel/ucount.c
View File

@@ -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