v2:
- fix potential double-free of lease if second check finds conflict
- add smp_mb's to ensure that other CPUs see i_flock changes
v3:
- remove smp_mb calls. Partial ordering is unlikely to help here.
v4:
- add back smp_mb calls. While we have implicit barriers in place
that enforce this today, it's best to be explicit about it as a
defensive coding measure.
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@xxxxxxxxxxxx>
Cc: David Howells <dhowells@xxxxxxxxxx>
Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
Reported-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
---
fs/locks.c | 75 ++++++++++++++++++++++++++++++++++++++++++++----------
include/linux/fs.h | 6 +++++
2 files changed, 68 insertions(+), 13 deletions(-)
diff --git a/fs/locks.c b/fs/locks.c
index b27a300..8dddcb5 100644
--- a/fs/locks.c
+++ b/fs/locks.c
@@ -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);
}
@@ -1455,6 +1470,32 @@ 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;
@@ -1464,12 +1505,8 @@ static int generic_add_lease(struct file *filp, long arg, struct file_lock **flp
lease = *flp;
- 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;
/*
@@ -1514,8 +1551,20 @@ static int generic_add_lease(struct file *filp, long arg, struct file_lock **flp
goto out;
locks_insert_lock(before, lease);
- return 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:
return error;
}
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 9818747..165bf41 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1955,6 +1955,12 @@ 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);
return 0;
--
1.8.3.1
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