userns: Avoid recursion in put_user_ns
When freeing a deeply nested user namespace free_user_ns calls
put_user_ns on it's parent which may in turn call free_user_ns again.
When -fno-optimize-sibling-calls is passed to gcc one stack frame per
user namespace is left on the stack, potentially overflowing the
kernel stack. CONFIG_FRAME_POINTER forces -fno-optimize-sibling-calls
so we can't count on gcc to optimize this code.
Remove struct kref and use a plain atomic_t. Making the code more
flexible and easier to comprehend. Make the loop in free_user_ns
explict to guarantee that the stack does not overflow with
CONFIG_FRAME_POINTER enabled.
I have tested this fix with a simple program that uses unshare to
create a deeply nested user namespace structure and then calls exit.
With 1000 nesteuser namespaces before this change running my test
program causes the kernel to die a horrible death. With 10,000,000
nested user namespaces after this change my test program runs to
completion and causes no harm.
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Pointed-out-by: Vasily Kulikov <segoon@openwall.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
index b9bd2e6..4ce0093 100644
--- a/include/linux/user_namespace.h
+++ b/include/linux/user_namespace.h
@@ -21,7 +21,7 @@
struct uid_gid_map uid_map;
struct uid_gid_map gid_map;
struct uid_gid_map projid_map;
- struct kref kref;
+ atomic_t count;
struct user_namespace *parent;
kuid_t owner;
kgid_t group;
@@ -35,18 +35,18 @@
static inline struct user_namespace *get_user_ns(struct user_namespace *ns)
{
if (ns)
- kref_get(&ns->kref);
+ atomic_inc(&ns->count);
return ns;
}
extern int create_user_ns(struct cred *new);
extern int unshare_userns(unsigned long unshare_flags, struct cred **new_cred);
-extern void free_user_ns(struct kref *kref);
+extern void free_user_ns(struct user_namespace *ns);
static inline void put_user_ns(struct user_namespace *ns)
{
- if (ns)
- kref_put(&ns->kref, free_user_ns);
+ if (ns && atomic_dec_and_test(&ns->count))
+ free_user_ns(ns);
}
struct seq_operations;