[PATCH] fs/locks.c: Fix sys_flock() race

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>

1 files changed, 16 insertions, 14 deletions

diff --git a/fs/locks.c b/fs/locks.c
index 8fcfeb177a2a..dda83d6cd48b 100644
--- a/fs/locks.c
+++ b/fs/locks.c
@@ -726,8 +726,9 @@ 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;
@@ -742,17 +743,19 @@ static int flock_lock_file(struct file *filp, struct file_lock *new_fl)
                         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.
@@ -760,26 +763,27 @@ static int flock_lock_file(struct file *filp, struct file_lock *new_fl)
         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;
 }
 
@@ -1560,9 +1564,7 @@ asmlinkage long sys_flock(unsigned int fd, unsigned int cmd)
                 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);