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

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Message-ID: <20251130092731.726288-1-kartikey406@gmail.com>
Date: Sun, 30 Nov 2025 14:57:31 +0530
From: Deepanshu Kartikey <kartikey406@...il.com>
To: brauner@...nel.org,
	viro@...iv.linux.org.uk,
	neil@...wn.name,
	amir73il@...il.com,
	jlayton@...nel.org
Cc: linux-kernel@...r.kernel.org,
	Deepanshu Kartikey <kartikey406@...il.com>,
	syzbot+b74150fd2ef40e716ca2@...kaller.appspot.com
Subject: [PATCH] ipc/mqueue: fix dentry refcount imbalance in prepare_open()

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@...kaller.appspot.com
Closes: https://syzkaller.appspot.com/bug?extid=b74150fd2ef40e716ca2
Signed-off-by: Deepanshu Kartikey <kartikey406@...il.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