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Commit 3895dbf8 authored by Eric W. Biederman's avatar Eric W. Biederman
Browse files

mnt: Protect the mountpoint hashtable with mount_lock 



Protecting the mountpoint hashtable with namespace_sem was sufficient
until a call to umount_mnt was added to mntput_no_expire.  At which
point it became possible for multiple calls of put_mountpoint on
the same hash chain to happen on the same time.

Kristen Johansen <kjlx@templeofstupid.com> reported:
> This can cause a panic when simultaneous callers of put_mountpoint
> attempt to free the same mountpoint.  This occurs because some callers
> hold the mount_hash_lock, while others hold the namespace lock.  Some
> even hold both.
>
> In this submitter's case, the panic manifested itself as a GP fault in
> put_mountpoint() when it called hlist_del() and attempted to dereference
> a m_hash.pprev that had been poisioned by another thread.

Al Viro observed that the simple fix is to switch from using the namespace_sem
to the mount_lock to protect the mountpoint hash table.

I have taken Al's suggested patch moved put_mountpoint in pivot_root
(instead of taking mount_lock an additional time), and have replaced
new_mountpoint with get_mountpoint a function that does the hash table
lookup and addition under the mount_lock.   The introduction of get_mounptoint
ensures that only the mount_lock is needed to manipulate the mountpoint
hashtable.

d_set_mounted is modified to only set DCACHE_MOUNTED if it is not
already set.  This allows get_mountpoint to use the setting of
DCACHE_MOUNTED to ensure adding a struct mountpoint for a dentry
happens exactly once.

Cc: stable@vger.kernel.org
Fixes: ce07d891 ("mnt: Honor MNT_LOCKED when detaching mounts")
Reported-by: default avatarKrister Johansen <kjlx@templeofstupid.com>
Suggested-by: default avatarAl Viro <viro@ZenIV.linux.org.uk>
Acked-by: default avatarAl Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: default avatar"Eric W. Biederman" <ebiederm@xmission.com>
parent 0c744ea4

fs/dcache.c

+5 −2
Original line number Diff line number Diff line
@@ -1336,9 +1336,12 @@ int d_set_mounted(struct dentry *dentry) 
	}

	spin_lock(&dentry->d_lock);

	if (!d_unlinked(dentry)) {

		ret = -EBUSY;

		if (!d_mountpoint(dentry)) {

			dentry->d_flags |= DCACHE_MOUNTED;

			ret = 0;

		}

	}

 	spin_unlock(&dentry->d_lock);

out:

	write_sequnlock(&rename_lock);

fs/namespace.c

+45 −19
Original line number Diff line number Diff line
@@ -742,26 +742,50 @@ static struct mountpoint *lookup_mountpoint(struct dentry *dentry) 
	return NULL;

}



static struct mountpoint *new_mountpoint(struct dentry *dentry)

static struct mountpoint *get_mountpoint(struct dentry *dentry)

{

	struct hlist_head *chain = mp_hash(dentry);

	struct mountpoint *mp;

	struct mountpoint *mp, *new = NULL;

	int ret;



	mp = kmalloc(sizeof(struct mountpoint), GFP_KERNEL);

	if (!mp)

	if (d_mountpoint(dentry)) {

mountpoint:

		read_seqlock_excl(&mount_lock);

		mp = lookup_mountpoint(dentry);

		read_sequnlock_excl(&mount_lock);

		if (mp)

			goto done;

	}



	if (!new)

		new = kmalloc(sizeof(struct mountpoint), GFP_KERNEL);

	if (!new)

		return ERR_PTR(-ENOMEM);





	/* Exactly one processes may set d_mounted */

	ret = d_set_mounted(dentry);

	if (ret) {

		kfree(mp);

		return ERR_PTR(ret);

	}



	mp->m_dentry = dentry;

	mp->m_count = 1;

	hlist_add_head(&mp->m_hash, chain);

	INIT_HLIST_HEAD(&mp->m_list);

	/* Someone else set d_mounted? */

	if (ret == -EBUSY)

		goto mountpoint;



	/* The dentry is not available as a mountpoint? */

	mp = ERR_PTR(ret);

	if (ret)

		goto done;



	/* Add the new mountpoint to the hash table */

	read_seqlock_excl(&mount_lock);

	new->m_dentry = dentry;

	new->m_count = 1;

	hlist_add_head(&new->m_hash, mp_hash(dentry));

	INIT_HLIST_HEAD(&new->m_list);

	read_sequnlock_excl(&mount_lock);



	mp = new;

	new = NULL;

done:

	kfree(new);

	return mp;

}
@@ -1595,11 +1619,11 @@ void __detach_mounts(struct dentry *dentry) 
	struct mount *mnt;



	namespace_lock();

	lock_mount_hash();

	mp = lookup_mountpoint(dentry);

	if (IS_ERR_OR_NULL(mp))

		goto out_unlock;



	lock_mount_hash();

	event++;

	while (!hlist_empty(&mp->m_list)) {

		mnt = hlist_entry(mp->m_list.first, struct mount, mnt_mp_list);
@@ -1609,9 +1633,9 @@ void __detach_mounts(struct dentry *dentry) 
		}

		else umount_tree(mnt, UMOUNT_CONNECTED);

	}

	unlock_mount_hash();

	put_mountpoint(mp);

out_unlock:

	unlock_mount_hash();

	namespace_unlock();

}
@@ -2038,9 +2062,7 @@ static struct mountpoint *lock_mount(struct path *path) 
	namespace_lock();

	mnt = lookup_mnt(path);

	if (likely(!mnt)) {

		struct mountpoint *mp = lookup_mountpoint(dentry);

		if (!mp)

			mp = new_mountpoint(dentry);

		struct mountpoint *mp = get_mountpoint(dentry);

		if (IS_ERR(mp)) {

			namespace_unlock();

			inode_unlock(dentry->d_inode);
@@ -2059,7 +2081,11 @@ static struct mountpoint *lock_mount(struct path *path) 
static void unlock_mount(struct mountpoint *where)

{

	struct dentry *dentry = where->m_dentry;



	read_seqlock_excl(&mount_lock);

	put_mountpoint(where);

	read_sequnlock_excl(&mount_lock);



	namespace_unlock();

	inode_unlock(dentry->d_inode);

}
@@ -3135,9 +3161,9 @@ SYSCALL_DEFINE2(pivot_root, const char __user *, new_root, 
	touch_mnt_namespace(current->nsproxy->mnt_ns);

	/* A moved mount should not expire automatically */

	list_del_init(&new_mnt->mnt_expire);

	put_mountpoint(root_mp);

	unlock_mount_hash();

	chroot_fs_refs(&root, &new);

	put_mountpoint(root_mp);

	error = 0;

out4:

	unlock_mount(old_mp);