Messages in this thread Patch in this message | | | From | Andrew Vagin <> | Subject | [PATCH] pidns: remove recursion from free_pid_ns (v3) | Date | Sat, 6 Oct 2012 23:56:33 +0400 |
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Here is a stack trace of recursion: free_pid_ns(parent) put_pid_ns(parent) kref_put(&ns->kref, free_pid_ns); free_pid_ns
This patch turns recursion into loops.
pidns can be nested many times, so in case of recursion a simple user space program can provoke a kernel panic due to exceed of a kernel stack.
v2: * don't check parent on NULL * use atomic_dec_and_test(&kref->refcount) v3: Fix coding style issue
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org> Reviewed-by: Oleg Nesterov <oleg@redhat.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: "Eric W. Biederman" <ebiederm@xmission.com> Cc: Pavel Emelyanov <xemul@parallels.com> Signed-off-by: Andrew Vagin <avagin@openvz.org> --- include/linux/kref.h | 12 ++++++++++++ kernel/pid_namespace.c | 16 ++++++++++++---- 2 files changed, 24 insertions(+), 4 deletions(-)
diff --git a/include/linux/kref.h b/include/linux/kref.h index 65af688..2844262 100644 --- a/include/linux/kref.h +++ b/include/linux/kref.h @@ -95,6 +95,18 @@ static inline int kref_put(struct kref *kref, void (*release)(struct kref *kref) return kref_sub(kref, 1, release); } +/** + * kref_put - decrement refcount for object. + * @kref: object. + * + * Decrement the refcount. + * Return 1 if refcount is zero. + */ +static inline int __kref_put(struct kref *kref) +{ + return atomic_dec_and_test(&kref->refcount); +} + static inline int kref_put_mutex(struct kref *kref, void (*release)(struct kref *kref), struct mutex *lock) diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c index 6144bab..b051fa6 100644 --- a/kernel/pid_namespace.c +++ b/kernel/pid_namespace.c @@ -138,11 +138,19 @@ void free_pid_ns(struct kref *kref) ns = container_of(kref, struct pid_namespace, kref); - parent = ns->parent; - destroy_pid_namespace(ns); + while (1) { + parent = ns->parent; + destroy_pid_namespace(ns); - if (parent != NULL) - put_pid_ns(parent); + if (parent == &init_pid_ns) + break; + + /* kref_put cannot be used for avoiding recursion */ + if (__kref_put(&parent->kref) == 0) + break; + + ns = parent; + } } void zap_pid_ns_processes(struct pid_namespace *pid_ns) -- 1.7.1
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