netdev - Re: [PATCH v10 bpf-next 03/17] bpf: introduce BPF token object

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Date: Mon, 27 Nov 2023 10:18:16 -0800
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Christian Brauner <brauner@...nel.org>
Cc: Andrii Nakryiko <andrii@...nel.org>, bpf@...r.kernel.org, netdev@...r.kernel.org, 
	paul@...l-moore.com, linux-fsdevel@...r.kernel.org, 
	linux-security-module@...r.kernel.org, keescook@...omium.org, 
	kernel-team@...a.com, sargun@...gun.me
Subject: Re: [PATCH v10 bpf-next 03/17] bpf: introduce BPF token object

On Mon, Nov 27, 2023 at 8:05 AM Christian Brauner <brauner@...nel.org> wrote:
>
> > +     if (path.mnt->mnt_root != path.dentry) {
>
> You want to verify that you can only create tokens from the root of the
> bpffs mount. So for
>
>   sudo mount -t bpf bpf /mnt
>
> you want bpf tokens to be creatable from:
>
>   fd = open("/mnt")
>
> or from bind-mounts of the fs root:
>
>   sudo mount --bind /mnt /srv
>   fd = open("/srv")
>
> but not from
>
>   sudo mount --bind /mnt/foo /opt
>   fd = open("/opt")
>
> But I think your current check allows for that because if you bind-mount
> /mnt/foo to /opt then fd = open("/opt")
>
>   path.mnt->mnt_root == foo and path.dentry == foo
>
> I think
>
> path.dentry != path.mnt->mnt_sb->s_root
>
> should give you what you want.

Ah, subtle difference (for me :)). Yes, I'd like the actual root of
bpffs to be used, will adjust the check, thanks!