Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759881Ab3HNMMF (ORCPT ); Wed, 14 Aug 2013 08:12:05 -0400 Received: from mail-qc0-f174.google.com ([209.85.216.174]:33335 "EHLO mail-qc0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759738Ab3HNMMB (ORCPT ); Wed, 14 Aug 2013 08:12:01 -0400 From: Jeff Layton To: Al Viro Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Matthew Wilcox , Bruce Fields , David Howells Subject: [PATCH v3] locks: close potential race between setlease and open Date: Wed, 14 Aug 2013 08:11:50 -0400 Message-Id: <1376482310-7348-1-git-send-email-jlayton@redhat.com> X-Mailer: git-send-email 1.8.3.1 In-Reply-To: <1373310444-25862-1-git-send-email-jlayton@redhat.com> References: <1373310444-25862-1-git-send-email-jlayton@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5115 Lines: 156 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. 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 Cc: David Howells Reported-by: Al Viro Signed-off-by: Jeff Layton --- fs/locks.c | 71 ++++++++++++++++++++++++++++++++++++++++++++++++++------------ 1 file changed, 58 insertions(+), 13 deletions(-) diff --git a/fs/locks.c b/fs/locks.c index b27a300..a99adec 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 +1551,16 @@ 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. + */ + error = check_conflicting_open(dentry, arg); + if (error) + locks_unlink_lock(flp); out: return error; } -- 1.8.3.1 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/