From: Miklos Szeredi <miklos@szeredi.hu>
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 <miklos@szeredi.hu>
Acked-by: Al Viro <viro@parcelfarce.linux.theplanet.co.uk>
Signed-off-by: Andrew Morton <akpm@osdl.org>
---
fs/namespace.c | 7 +++++--
include/linux/namespace.h | 4 +++-
2 files changed, 8 insertions(+), 3 deletions(-)
diff -puN fs/namespace.c~namespacec-fix-race-in-mark_mounts_for_expiry fs/namespace.c
--- 25/fs/namespace.c~namespacec-fix-race-in-mark_mounts_for_expiry Mon May 23 16:05:11 2005
+++ 25-akpm/fs/namespace.c Mon May 23 16:05:11 2005
@@ -869,7 +869,7 @@ void mark_mounts_for_expiry(struct list_
/* 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 mempa
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 -puN include/linux/namespace.h~namespacec-fix-race-in-mark_mounts_for_expiry include/linux/namespace.h
--- 25/include/linux/namespace.h~namespacec-fix-race-in-mark_mounts_for_expiry Mon May 23 16:05:11 2005
+++ 25-akpm/include/linux/namespace.h Mon May 23 16:05:11 2005
@@ -15,10 +15,12 @@ struct namespace {
extern void umount_tree(struct vfsmount *);
extern int copy_namespace(int, struct task_struct *);
extern void __put_namespace(struct namespace *namespace);
+extern spinlock_t vfsmount_lock;
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);
}
_