linux-kernel - Re: general protection fault in kernfs_kill

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Date:   Mon, 14 May 2018 05:04:15 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     syzbot <syzbot+481ad819c717f6b78df9@...kaller.appspotmail.com>,
        linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com,
        gregkh@...uxfoundation.org, tj@...nel.org
Subject: Re: general protection fault in kernfs_kill_sb (2)

On Mon, May 14, 2018 at 12:20:16PM +0900, Tetsuo Handa wrote:

> But there remains a refcount bug because deactivate_locked_super() from
> kernfs_mount_ns() triggers kobj_ns_drop() from sysfs_kill_sb() via
> sb->kill_sb() when kobj_ns_drop() is always called by sysfs_mount()
> if kernfs_mount_ns() returned an error.

Trivial:

unfuck sysfs_mount()

Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
---
diff --git a/fs/sysfs/mount.c b/fs/sysfs/mount.c
index b428d317ae92..92682fcc41f6 100644
--- a/fs/sysfs/mount.c
+++ b/fs/sysfs/mount.c
@@ -25,7 +25,7 @@ static struct dentry *sysfs_mount(struct file_system_type *fs_type,
 {
 	struct dentry *root;
 	void *ns;
-	bool new_sb;
+	bool new_sb = false;
 
 	if (!(flags & SB_KERNMOUNT)) {
 		if (!kobj_ns_current_may_mount(KOBJ_NS_TYPE_NET))
@@ -35,9 +35,9 @@ static struct dentry *sysfs_mount(struct file_system_type *fs_type,
 	ns = kobj_ns_grab_current(KOBJ_NS_TYPE_NET);
 	root = kernfs_mount_ns(fs_type, flags, sysfs_root,
 				SYSFS_MAGIC, &new_sb, ns);
-	if (IS_ERR(root) || !new_sb)
+	if (!new_sb)
 		kobj_ns_drop(KOBJ_NS_TYPE_NET, ns);
-	else if (new_sb)
+	else if (!IS_ERR(root))
 		root->d_sb->s_iflags |= SB_I_USERNS_VISIBLE;
 
 	return root;