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authorEric W. Biederman <ebiederm@xmission.com>2012-12-28 18:58:39 -0800
committerEric W. Biederman <ebiederm@xmission.com>2013-01-26 22:11:41 -0800
commitc61a2810a2161986353705b44d9503e6bb079f4f (patch)
tree1f949f359d0344c2a47c8d0c597815aceb8091c0 /include
parent923c7538236564c46ee80c253a416705321f13e3 (diff)
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>
Diffstat (limited to 'include')
-rw-r--r--include/linux/user_namespace.h10
1 files changed, 5 insertions, 5 deletions
diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
index b9bd2e6c73cc..4ce009324933 100644
--- a/include/linux/user_namespace.h
+++ b/include/linux/user_namespace.h
@@ -21,7 +21,7 @@ struct user_namespace {
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 @@ extern struct user_namespace init_user_ns;
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;