提交 24cbe784 编写于 作者: J Jeff Layton

locks: close potential race between setlease and open

As Al Viro points out, there is an unlikely, but possible race between
opening a file and setting a lease on it. generic_add_lease is done with
the i_lock held, but the inode->i_flock check in break_lease is
lockless. It's possible for another task doing an open to do the entire
pathwalk and call break_lease between the point where generic_add_lease
checks for a conflicting open and adds the lease to the list. If this
occurs, we can end up with a lease set on the file with a conflicting
open.

To guard against that, check again for a conflicting open after adding
the lease to the i_flock list. If the above race occurs, then we can
simply unwind the lease setting and return -EAGAIN.

Because we take dentry references and acquire write access on the file
before calling break_lease, we know that if the i_flock list is empty
when the open caller goes to check it then the necessary refcounts have
already been incremented. Thus the additional check for a conflicting
open will see that there is one and the setlease call will fail.

Cc: Bruce Fields <bfields@fieldses.org>
Cc: David Howells <dhowells@redhat.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Reported-by: NAl Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: NJeff Layton <jlayton@redhat.com>
Signed-off-by: NJ. Bruce Fields <bfields@fieldses.org>
上级 18156e7e
......@@ -652,15 +652,18 @@ static void locks_insert_lock(struct file_lock **pos, struct file_lock *fl)
locks_insert_global_locks(fl);
}
/*
* Delete a lock and then free it.
* Wake up processes that are blocked waiting for this lock,
* notify the FS that the lock has been cleared and
* finally free the lock.
/**
* locks_delete_lock - Delete a lock and then free it.
* @thisfl_p: pointer that points to the fl_next field of the previous
* inode->i_flock list entry
*
* Unlink a lock from all lists and free the namespace reference, but don't
* free it yet. Wake up processes that are blocked waiting for this lock and
* notify the FS that the lock has been cleared.
*
* Must be called with the i_lock held!
*/
static void locks_delete_lock(struct file_lock **thisfl_p)
static void locks_unlink_lock(struct file_lock **thisfl_p)
{
struct file_lock *fl = *thisfl_p;
......@@ -675,6 +678,18 @@ static void locks_delete_lock(struct file_lock **thisfl_p)
}
locks_wake_up_blocks(fl);
}
/*
* Unlink a lock from all lists and free it.
*
* Must be called with i_lock held!
*/
static void locks_delete_lock(struct file_lock **thisfl_p)
{
struct file_lock *fl = *thisfl_p;
locks_unlink_lock(thisfl_p);
locks_free_lock(fl);
}
......@@ -1472,6 +1487,32 @@ int fcntl_getlease(struct file *filp)
return type;
}
/**
* check_conflicting_open - see if the given dentry points to a file that has
* an existing open that would conflict with the
* desired lease.
* @dentry: dentry to check
* @arg: type of lease that we're trying to acquire
*
* Check to see if there's an existing open fd on this file that would
* conflict with the lease we're trying to set.
*/
static int
check_conflicting_open(const struct dentry *dentry, const long arg)
{
int ret = 0;
struct inode *inode = dentry->d_inode;
if ((arg == F_RDLCK) && (atomic_read(&inode->i_writecount) > 0))
return -EAGAIN;
if ((arg == F_WRLCK) && ((d_count(dentry) > 1) ||
(atomic_read(&inode->i_count) > 1)))
ret = -EAGAIN;
return ret;
}
static int generic_add_lease(struct file *filp, long arg, struct file_lock **flp)
{
struct file_lock *fl, **before, **my_before = NULL, *lease;
......@@ -1499,12 +1540,8 @@ static int generic_add_lease(struct file *filp, long arg, struct file_lock **flp
return -EINVAL;
}
error = -EAGAIN;
if ((arg == F_RDLCK) && (atomic_read(&inode->i_writecount) > 0))
goto out;
if ((arg == F_WRLCK)
&& ((d_count(dentry) > 1)
|| (atomic_read(&inode->i_count) > 1)))
error = check_conflicting_open(dentry, arg);
if (error)
goto out;
/*
......@@ -1549,7 +1586,19 @@ static int generic_add_lease(struct file *filp, long arg, struct file_lock **flp
goto out;
locks_insert_lock(before, lease);
error = 0;
/*
* The check in break_lease() is lockless. It's possible for another
* open to race in after we did the earlier check for a conflicting
* open but before the lease was inserted. Check again for a
* conflicting open and cancel the lease if there is one.
*
* We also add a barrier here to ensure that the insertion of the lock
* precedes these checks.
*/
smp_mb();
error = check_conflicting_open(dentry, arg);
if (error)
locks_unlink_lock(flp);
out:
if (is_deleg)
mutex_unlock(&inode->i_mutex);
......
......@@ -1964,6 +1964,12 @@ static inline int locks_verify_truncate(struct inode *inode,
static inline int break_lease(struct inode *inode, unsigned int mode)
{
/*
* Since this check is lockless, we must ensure that any refcounts
* taken are done before checking inode->i_flock. Otherwise, we could
* end up racing with tasks trying to set a new lease on this file.
*/
smp_mb();
if (inode->i_flock)
return __break_lease(inode, mode, FL_LEASE);
return 0;
......
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