linux-kernel - Re: [syzbot] WARNING in mntput_no

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Date:   Mon, 5 Apr 2021 22:07:37 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Jens Axboe <axboe@...nel.dk>,
        syzbot <syzbot+c88a7030da47945a3cc3@...kaller.appspotmail.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        syzkaller-bugs@...glegroups.com, io-uring@...r.kernel.org
Subject: Re: [syzbot] WARNING in mntput_no_expire (2)

On Mon, Apr 05, 2021 at 06:28:54PM +0000, Al Viro wrote:
> On Mon, Apr 05, 2021 at 06:23:49PM +0000, Al Viro wrote:
> > On Mon, Apr 05, 2021 at 07:08:01PM +0200, Christian Brauner wrote:
> > 
> > > Ah dentry count of -127 looks... odd.
> > 
> > dead + 1...
> > 
> > void lockref_mark_dead(struct lockref *lockref)
> > {
> >         assert_spin_locked(&lockref->lock);
> > 	lockref->count = -128;
> > }
> > 
> > IOW, a leaked (uncounted) reference to dentry, that got dget() called on
> > it after dentry had been freed.
> > 
> > 	IOW, current->fs->pwd.dentry happens to point to an already freed
> > struct dentry here.  Joy...
> > 
> > 	Could you slap
> > 
> > spin_lock(&current->fs->lock);
> > WARN_ON(d_count(current->fs->pwd.dentry) < 0);
> > spin_unlock(&current->fs->lock);
> > 
> > before and after calls of io_issue_sqe() and see if it triggers?  We definitely
> > are seeing buggered dentry refcounting here.
> 
> Check if this helps, please.
> 
> diff --git a/fs/namei.c b/fs/namei.c
> index 216f16e74351..82344f1139ff 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -2289,6 +2289,9 @@ static const char *path_init(struct nameidata *nd, unsigned flags)
>  	int error;
>  	const char *s = nd->name->name;
>  
> +	nd->path.mnt = NULL;
> +	nd->path.dentry = NULL;
> +
>  	/* LOOKUP_CACHED requires RCU, ask caller to retry */
>  	if ((flags & (LOOKUP_RCU | LOOKUP_CACHED)) == LOOKUP_CACHED)
>  		return ERR_PTR(-EAGAIN);
> @@ -2322,8 +2325,6 @@ static const char *path_init(struct nameidata *nd, unsigned flags)
>  	}
>  
>  	nd->root.mnt = NULL;
> -	nd->path.mnt = NULL;
> -	nd->path.dentry = NULL;
>  
>  	/* Absolute pathname -- fetch the root (LOOKUP_IN_ROOT uses nd->dfd). */
>  	if (*s == '/' && !(flags & LOOKUP_IN_ROOT)) {

Bingo. That fixes it.