linux-kernel - Re: [PATCH] ipc/mqueue: fix dentry refcount imbalance in prepare

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAOQ4uxiANGPugt+puZb7V62yAECBAB=fNQ=09+-QJoYXcyCwng@mail.gmail.com>
Date: Sun, 30 Nov 2025 10:57:23 +0100
From: Amir Goldstein <amir73il@...il.com>
To: Deepanshu Kartikey <kartikey406@...il.com>, brauner@...nel.org
Cc: viro@...iv.linux.org.uk, neil@...wn.name, jlayton@...nel.org, 
	linux-kernel@...r.kernel.org, 
	syzbot+b74150fd2ef40e716ca2@...kaller.appspot.com
Subject: Re: [PATCH] ipc/mqueue: fix dentry refcount imbalance in prepare_open()

On Sun, Nov 30, 2025 at 10:27 AM Deepanshu Kartikey
<kartikey406@...il.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@...kaller.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.