From 1ce88cf466f7b6078b14d67d186a3d7c19dd5609 Mon Sep 17 00:00:00 2001 From: Miklos Szeredi Date: Thu, 7 Jul 2005 17:57:24 -0700 Subject: [PATCH] [PATCH] namespace.c: fix race in mark_mounts_for_expiry() This patch fixes a race found by Ram in mark_mounts_for_expiry() in fs/namespace.c. The bug can only be triggered with simultaneous exiting of a process having a private namespace, and expiry of a mount from within that namespace. It's practically impossible to trigger, and I haven't even tried. But still, a bug is a bug. The race happens when put_namespace() is called by another task, while mark_mounts_for_expiry() is between atomic_read() and get_namespace(). In that case get_namespace() will be called on an already dead namespace with unforeseeable results. The solution was suggested by Al Viro, with his own words: Instead of screwing with atomic_read() in there, why don't we simply do the following: a) atomic_dec_and_lock() in put_namespace() b) __put_namespace() called without dropping lock c) the first thing done by __put_namespace would be struct vfsmount *root = namespace->root; namespace->root = NULL; spin_unlock(...); .... umount_tree(root); ... d) check in mark_... would be simply namespace && namespace->root. And we are all set; no screwing around with atomic_read(), no magic at all. Dying namespace gets NULL ->root. All changes of ->root happen under spinlock. If under a spinlock we see non-NULL ->mnt_namespace, it won't be freed until we drop the lock (we will set ->mnt_namespace to NULL under that lock before we get to freeing namespace). If under a spinlock we see non-NULL ->mnt_namespace and ->mnt_namespace->root, we can grab a reference to namespace and be sure that it won't go away. Signed-off-by: Miklos Szeredi Acked-by: Al Viro Acked-by: Christoph Hellwig Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- fs/namespace.c | 7 +++++-- include/linux/namespace.h | 3 ++- 2 files changed, 7 insertions(+), 3 deletions(-) diff --git a/fs/namespace.c b/fs/namespace.c index a0d0ef1f1a48..9d17541ebafa 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -869,7 +869,7 @@ void mark_mounts_for_expiry(struct list_head *mounts) /* don't do anything if the namespace is dead - all the * vfsmounts from it are going away anyway */ namespace = mnt->mnt_namespace; - if (!namespace || atomic_read(&namespace->count) <= 0) + if (!namespace || !namespace->root) continue; get_namespace(namespace); @@ -1450,9 +1450,12 @@ void __init mnt_init(unsigned long mempages) void __put_namespace(struct namespace *namespace) { + struct vfsmount *root = namespace->root; + namespace->root = NULL; + spin_unlock(&vfsmount_lock); down_write(&namespace->sem); spin_lock(&vfsmount_lock); - umount_tree(namespace->root); + umount_tree(root); spin_unlock(&vfsmount_lock); up_write(&namespace->sem); kfree(namespace); diff --git a/include/linux/namespace.h b/include/linux/namespace.h index 697991b69f9b..0e5a86f13b2f 100644 --- a/include/linux/namespace.h +++ b/include/linux/namespace.h @@ -17,7 +17,8 @@ extern void __put_namespace(struct namespace *namespace); static inline void put_namespace(struct namespace *namespace) { - if (atomic_dec_and_test(&namespace->count)) + if (atomic_dec_and_lock(&namespace->count, &vfsmount_lock)) + /* releases vfsmount_lock */ __put_namespace(namespace); } -- 2.20.1