From: Trond Myklebust <Trond.Myklebust@xxxxxxxxxx>
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@xxxxxxxxxx>
---
fs/locks.c | 30 ++++++++++++++++--------------
1 files changed, 16 insertions(+), 14 deletions(-)
diff --git a/fs/locks.c b/fs/locks.c
index 4d9e71d..6ba3756 100644
--- a/fs/locks.c
+++ b/fs/locks.c
@@ -735,8 +735,9 @@ 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;
@@ -751,17 +752,19 @@ 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.
@@ -769,26 +772,27 @@ 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;
}
@@ -1569,9 +1573,7 @@ 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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