linux-kernel - Re: [PATCH 3/4] watch_queue: Implement mount topology and attribute change notifications

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Date:   Fri, 24 Jul 2020 12:14:18 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     David Howells <dhowells@...hat.com>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        Nicolas Dichtel <nicolas.dichtel@...nd.com>,
        Ian Kent <raven@...maw.net>,
        Christian Brauner <christian@...uner.io>,
        Jeff Layton <jlayton@...hat.com>, Karel Zak <kzak@...hat.com>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Linux API <linux-api@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/4] watch_queue: Implement mount topology and attribute
 change notifications

This just can't be right.

On Fri, Jul 24, 2020 at 6:12 AM David Howells <dhowells@...hat.com> wrote:
>
> +
> +/**
> + * sys_watch_mount - Watch for mount topology/attribute changes
> + * @dfd: Base directory to pathwalk from or fd referring to mount.
> + * @filename: Path to mount to place the watch upon
> + * @at_flags: Pathwalk control flags
> + * @watch_fd: The watch queue to send notifications to.
> + * @watch_id: The watch ID to be placed in the notification (-1 to remove watch)
> + */
> +SYSCALL_DEFINE5(watch_mount, [...]
> +               int, watch_id)
...
> +       if (watch_id < -1 || watch_id > 0xff)
> +               return -EINVAL;
...
> +       ret = inode_permission(path.dentry->d_inode, MAY_EXEC);
> +       if (ret)
> +               goto err_path;
...
> +       if (watch_id >= 0) {
...
> +               watch = kzalloc(sizeof(*watch), GFP_KERNEL);
> +               if (!watch)
> +                       goto err_wlist;

So now you can basically allocate as much kernel memory as you want as
a regular user, as long as you have a mounted directory you can walk
(ie everybody).

Is there any limiting of watches anywhere? I don't see it.

I notice we already have this pattern elsewhere. I think we need to
fix this before we add more watch types.

Watch allocation shouldn't just be a kzalloc(). I think you should
have a "watch_allocate()" that does the initialization of id etc, but
also does some basic per-user watch resource tracking or something.

              Linus