When opening an existing message queue, prepare_open() does not increment
the dentry refcount, but end_creating() always calls dput(). This causes
a refcount imbalance that triggers a WARN_ON_ONCE in fast_dput() when the
file is later closed.

The creation path via vfs_mkobj() correctly increments the refcount, but
the "already exists" path was missing the corresponding dget().

Add the missing dget() call when opening an existing queue to balance the
dput() in end_creating().

Reported-by: syzbot+b74150fd2ef40e716ca2@syzkaller.appspot.com
Closes: https://syzkaller.appspot.com/bug?extid=b74150fd2ef40e716ca2
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
---
 ipc/mqueue.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/ipc/mqueue.c b/ipc/mqueue.c
index 328bcc3ee3ad..63ff2c322549 100644
--- a/ipc/mqueue.c
+++ b/ipc/mqueue.c
@@ -883,6 +883,7 @@ static int prepare_open(struct dentry *dentry, int oflag, int ro,
 	if ((oflag & O_ACCMODE) == (O_RDWR | O_WRONLY))
 		return -EINVAL;
 	acc = oflag2acc[oflag & O_ACCMODE];
+	dget(dentry);
 	return inode_permission(&nop_mnt_idmap, d_inode(dentry), acc);
 }
 
-- 
2.43.0

On Sun, Nov 30, 2025 at 10:27 AM Deepanshu Kartikey
<kartikey406@gmail.com> wrote:
>
> When opening an existing message queue, prepare_open() does not increment
> the dentry refcount, but end_creating() always calls dput(). This causes
> a refcount imbalance that triggers a WARN_ON_ONCE in fast_dput() when the
> file is later closed.
>
> The creation path via vfs_mkobj() correctly increments the refcount, but
> the "already exists" path was missing the corresponding dget().
>
> Add the missing dget() call when opening an existing queue to balance the
> dput() in end_creating().

Sorry but this analysis looks wrong.

AFAIS, the bug was that end_creating() should have been before the out_putfd
label just as path_put() was before the commit.

>
> Reported-by: syzbot+b74150fd2ef40e716ca2@syzkaller.appspot.com
> Closes: https://syzkaller.appspot.com/bug?extid=b74150fd2ef40e716ca2

Fix should have
Fixes: c9ba789dad15b ("VFS: introduce start_creating_noperm() and
start_removing_noperm()")

But I see this is already fixed by the FD_ADD() work and  mqueue_file_open()
helper by Christian.

Specifically, the bug was fixed by the merge conflict resolution
d6ea5537c1a66 Merge tag 'vfs-6.19-rc1.fd_prepare' of
gitolite.kernel.org:pub/scm/linux/kernel/git/vfs/vfs into vfs.all

Which should be in linux-next from yesterday.

Christian,

Do you think we need a fix mid-way before the merge of FD_ADD()?

Thanks,
Amir.