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

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Date: Mon, 27 Nov 2023 17:05:31 +0100
From: Christian Brauner <brauner@...nel.org>
To: Andrii Nakryiko <andrii@...nel.org>
Cc: 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

> +	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.