Re: general protection fault in __list_del_entry_valid (2)

[Michal Hocko]

On Tue 21-11-17 15:05:00, Michal Hocko wrote:
> [Cc Al and Dave - email thread starts http://lkml.kernel.org/r/001a113f996099503a055e793dd3@xxxxxxxxxx]
> 
> On Tue 21-11-17 20:11:26, Tetsuo Handa wrote:
> > On 2017/11/21 16:35, syzbot wrote:
> > > Hello,
> > > 
> > > syzkaller hit the following crash on ca91659962303d4fd5211a5e4e13df5cbb11e744
> > > git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/master
> > > compiler: gcc (GCC) 7.1.1 20170620
> > > .config is attached
> > > Raw console output is attached.
> > > 
> > > Unfortunately, I don't have any reproducer for this bug yet.
> > 
> > Fault injection found an unchecked register_shrinker() return code.
> > Wow, register_shrinker()/unregister_shinker() is possibly frequently called path?
> > 
> > 
> > struct super_block *sget_userns(struct file_system_type *type,
> > 				int (*test)(struct super_block *,void *),
> > 				int (*set)(struct super_block *,void *),
> > 				int flags, struct user_namespace *user_ns,
> > 				void *data)
> > {
> > (...snipped...)
> > 	spin_unlock(&sb_lock);
> > 	get_filesystem(type);
> > 	register_shrinker(&s->s_shrink); // Error check required.
> > 	return s;
> 
> Yes, this is the case since numa aware shrinkers were introduced.

I meant 1d3d4437eae1 ("vmscan: per-node deferred work")

> I have
> a bit hard time to follow the code flow but why cannot we simply
> register the shrinker when we allocate the new super block? We
> still have the s_umount held so the shrinker cannot race with the
> registration code.
> 
> Something like the totally untested and possibly wrong
> ---
> diff --git a/fs/super.c b/fs/super.c
> index 994db21f59bf..1eb850413fdf 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -506,6 +506,11 @@ struct super_block *sget_userns(struct file_system_type *type,
>  		s = alloc_super(type, (flags & ~SB_SUBMOUNT), user_ns);
>  		if (!s)
>  			return ERR_PTR(-ENOMEM);
> +		if (register_shrinker(&s->s_shrink)) {
> +			up_write(&s->s_umount);
> +			destroy_super(s);
> +			return ERR_PTR(-ENOMEM);
> +		}
>  		goto retry;
>  	}
>  
> @@ -522,7 +527,6 @@ struct super_block *sget_userns(struct file_system_type *type,
>  	hlist_add_head(&s->s_instances, &type->fs_supers);
>  	spin_unlock(&sb_lock);
>  	get_filesystem(type);
> -	register_shrinker(&s->s_shrink);
>  	return s;
>  }
>  
> -- 
> Michal Hocko
> SUSE Labs

-- 
Michal Hocko
SUSE Labs