linux-kernel - Re: [syzbot] [btrfs?] VFS: Busy inodes after unmount (use-after-free) (2)

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Message-ID: <8bd7960a-540c-426f-8c47-620d66fbdfb4@I-love.SAKURA.ne.jp>
Date: Fri, 27 Jun 2025 22:52:20 +0900
From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To: syzbot <syzbot+1134d3a5b062e9665a7a@...kaller.appspotmail.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [syzbot] [btrfs?] VFS: Busy inodes after unmount (use-after-free)
 (2)

#syz test

Subject: [PATCH v2] ocfs2: update d_splice_alias() return code checking

When commit d3556babd7fa ("ocfs2: fix d_splice_alias() return code
checking") was merged into v3.18-rc3, d_splice_alias() was returning
one of a valid dentry, NULL or an ERR_PTR.

When commit b5ae6b15bd73 ("merge d_materialise_unique() into
d_splice_alias()") was merged into v3.19-rc1, d_splice_alias() started
returning -ELOOP as one of ERR_PTR values.

Now, when syzkaller mounts a crafted ocfs2 filesystem image that hits
d_splice_alias() == -ELOOP case from ocfs2_lookup(), ocfs2_lookup() fails
to handle -ELOOP case and generic_shutdown_super() hits "VFS: Busy inodes
after unmount" message.

Instead of calling ocfs2_dentry_attach_lock() or ocfs2_dentry_attach_gen()
when d_splice_alias() returned an ERR_PTR value, change ocfs2_lookup() to
bail out immediately.

Also, ocfs2_lookup() needs to call dupt() when ocfs2_dentry_attach_lock()
returned an ERR_PTR value.

Reported-by: syzbot <syzbot+1134d3a5b062e9665a7a@...kaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=1134d3a5b062e9665a7a
Suggested-by: Al Viro <viro@...iv.linux.org.uk>
Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
---
 fs/ocfs2/namei.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/fs/ocfs2/namei.c b/fs/ocfs2/namei.c
index 99278c8f0e24..f75fd19974bc 100644
--- a/fs/ocfs2/namei.c
+++ b/fs/ocfs2/namei.c
@@ -142,6 +142,8 @@ static struct dentry *ocfs2_lookup(struct inode *dir, struct dentry *dentry,

 bail_add:
 	ret = d_splice_alias(inode, dentry);
+	if (IS_ERR(ret))
+		goto bail_unlock;

 	if (inode) {
 		/*
@@ -154,13 +156,12 @@ static struct dentry *ocfs2_lookup(struct inode *dir, struct dentry *dentry,
 		 * NOTE: This dentry already has ->d_op set from
 		 * ocfs2_get_parent() and ocfs2_get_dentry()
 		 */
-		if (!IS_ERR_OR_NULL(ret))
-			dentry = ret;
-
-		status = ocfs2_dentry_attach_lock(dentry, inode,
+		status = ocfs2_dentry_attach_lock(ret ? ret : dentry, inode,
 						  OCFS2_I(dir)->ip_blkno);
 		if (status) {
 			mlog_errno(status);
+			if (ret)
+				dput(ret);
 			ret = ERR_PTR(status);
 			goto bail_unlock;
 		}
-- 
2.50.0