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locks: close potential race between setlease and open

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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: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@fieldses.org>
This commit is contained in:
Jeff Layton 2014-02-03 12:13:06 -05:00
parent 18156e7e66
commit 24cbe7845e
2 changed files with 68 additions and 13 deletions
Download patch file Download diff file Expand all files Collapse all files

6
include/linux/fs.h
View file

@ -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;