kernel-ark/include/linux/namespace.h
Miklos Szeredi 1ce88cf466 [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 <miklos@szeredi.hu>
Acked-by: Al Viro <viro@parcelfarce.linux.theplanet.co.uk>
Acked-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
2005-07-07 18:23:51 -07:00

43 lines
858 B
C

#ifndef _NAMESPACE_H_
#define _NAMESPACE_H_
#ifdef __KERNEL__
#include <linux/mount.h>
#include <linux/sched.h>
struct namespace {
atomic_t count;
struct vfsmount * root;
struct list_head list;
struct rw_semaphore sem;
};
extern int copy_namespace(int, struct task_struct *);
extern void __put_namespace(struct namespace *namespace);
static inline void put_namespace(struct namespace *namespace)
{
if (atomic_dec_and_lock(&namespace->count, &vfsmount_lock))
/* releases vfsmount_lock */
__put_namespace(namespace);
}
static inline void exit_namespace(struct task_struct *p)
{
struct namespace *namespace = p->namespace;
if (namespace) {
task_lock(p);
p->namespace = NULL;
task_unlock(p);
put_namespace(namespace);
}
}
static inline void get_namespace(struct namespace *namespace)
{
atomic_inc(&namespace->count);
}
#endif
#endif