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Subject[PATCH 04/22] [PATCH] fs/locks.c: Fix sys_flock() race
-stable review patch.  If anyone has any objections, please let us know.
------------------

sys_flock() currently has a race which can result in a double free in the
multi-thread case.

Thread 1 Thread 2

sys_flock(file, LOCK_EX)
sys_flock(file, LOCK_UN)

If Thread 2 removes the lock from inode->i_lock before Thread 1 tests for
list_empty(&lock->fl_link) at the end of sys_flock, then both threads will
end up calling locks_free_lock for the same lock.

Fix is to make flock_lock_file() do the same as posix_lock_file(), namely
to make a copy of the request, so that the caller can always free the lock.

This also has the side-effect of fixing up a reference problem in the
lockd handling of flock.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>

---
fs/locks.c | 30 ++++++++++++++++--------------
1 file changed, 16 insertions(+), 14 deletions(-)

--- linux-2.6.16.16.orig/fs/locks.c
+++ linux-2.6.16.16/fs/locks.c
@@ -714,8 +714,9 @@ EXPORT_SYMBOL(posix_locks_deadlock);
* at the head of the list, but that's secret knowledge known only to
* flock_lock_file and posix_lock_file.
*/
-static int flock_lock_file(struct file *filp, struct file_lock *new_fl)
+static int flock_lock_file(struct file *filp, struct file_lock *request)
{
+ struct file_lock *new_fl = NULL;
struct file_lock **before;
struct inode * inode = filp->f_dentry->d_inode;
int error = 0;
@@ -730,17 +731,19 @@ static int flock_lock_file(struct file *
continue;
if (filp != fl->fl_file)
continue;
- if (new_fl->fl_type == fl->fl_type)
+ if (request->fl_type == fl->fl_type)
goto out;
found = 1;
locks_delete_lock(before);
break;
}
- unlock_kernel();

- if (new_fl->fl_type == F_UNLCK)
- return 0;
+ if (request->fl_type == F_UNLCK)
+ goto out;

+ new_fl = locks_alloc_lock();
+ if (new_fl == NULL)
+ goto out;
/*
* If a higher-priority process was blocked on the old file lock,
* give it the opportunity to lock the file.
@@ -748,26 +751,27 @@ static int flock_lock_file(struct file *
if (found)
cond_resched();

- lock_kernel();
for_each_lock(inode, before) {
struct file_lock *fl = *before;
if (IS_POSIX(fl))
break;
if (IS_LEASE(fl))
continue;
- if (!flock_locks_conflict(new_fl, fl))
+ if (!flock_locks_conflict(request, fl))
continue;
error = -EAGAIN;
- if (new_fl->fl_flags & FL_SLEEP) {
- locks_insert_block(fl, new_fl);
- }
+ if (request->fl_flags & FL_SLEEP)
+ locks_insert_block(fl, request);
goto out;
}
+ locks_copy_lock(new_fl, request);
locks_insert_lock(&inode->i_flock, new_fl);
- error = 0;
+ new_fl = NULL;

out:
unlock_kernel();
+ if (new_fl)
+ locks_free_lock(new_fl);
return error;
}

@@ -1532,9 +1536,7 @@ asmlinkage long sys_flock(unsigned int f
error = flock_lock_file_wait(filp, lock);

out_free:
- if (list_empty(&lock->fl_link)) {
- locks_free_lock(lock);
- }
+ locks_free_lock(lock);

out_putf:
fput(filp);
--
-
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