linux-kernel - Re: WARNING: lock held when returning to user space in grab

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Message-ID: <19260f8b-d831-9a9a-5315-1f4353dadc9b@i-love.sakura.ne.jp>
Date:   Thu, 3 Jan 2019 01:49:55 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Tejun Heo <tj@...nel.org>
Cc:     Zefan Li <lizefan@...wei.com>,
        syzbot <syzbot+87b93137e0280beaeba1@...kaller.appspotmail.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        syzkaller-bugs@...glegroups.com, viro@...iv.linux.org.uk
Subject: Re: WARNING: lock held when returning to user space in grab_super

On 2019/01/03 1:16, Tejun Heo wrote:
> Happy new year, Tetsuo.
> 
> On Wed, Jan 02, 2019 at 09:08:56PM +0900, Tetsuo Handa wrote:
>> According to commit 633feee310de6b6c ("cgroup: refactor mount path and
>> clearly distinguish v1 and v2 paths"), cgroup_do_mount() is failing to
>> do full teardown steps for kernfs_mount() (deactivate_locked_super() ?)
>> when kernfs_node_dentry() failed.
> 
> Hmm... that's basically dget()'ing the root dentry of the sb.  I'm not
> sure how that could fail.  Can it?

kernfs_node_dentry() calls lookup_one_len_unlocked() which involves
memory allocation, and memory allocation fault injection made
lookup_one_len_unlocked() fail, and thus kernfs_node_dentry() failed.
What's strange?