Re: [PATCH] epoll: Fix UAF dentry name access in wakeup source setup

[Al Viro <viro@zeniv.linux.org.uk>]

On Wed, Apr 29, 2020 at 04:31:04AM +0200, Jann Horn wrote:

> I'm guessing this will go through akpm's tree?
> 
>  fs/eventpoll.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/eventpoll.c b/fs/eventpoll.c
> index 8c596641a72b0..5052a41670479 100644
> --- a/fs/eventpoll.c
> +++ b/fs/eventpoll.c
> @@ -1450,7 +1450,7 @@ static int reverse_path_check(void)
>  
>  static int ep_create_wakeup_source(struct epitem *epi)
>  {
> -	const char *name;
> +	struct name_snapshot name;
>  	struct wakeup_source *ws;
>  
>  	if (!epi->ep->ws) {
> @@ -1459,8 +1459,9 @@ static int ep_create_wakeup_source(struct epitem *epi)
>  			return -ENOMEM;
>  	}
>  
> -	name = epi->ffd.file->f_path.dentry->d_name.name;
> -	ws = wakeup_source_register(NULL, name);
> +	take_dentry_name_snapshot(&name, epi->ffd.file->f_path.dentry);
> +	ws = wakeup_source_register(NULL, name.name.name);
> +	release_dentry_name_snapshot(&name);

I'm not sure I like it.  Sure, it won't get freed under you that way; it still
can go absolutely stale by the time you return from wakeup_source_register().
What is it being used for?