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Date: Mon, 27 Nov 2023 17:05:31 +0100
From: Christian Brauner <brauner@kernel.org>
To: Andrii Nakryiko <andrii@kernel.org>
Cc: bpf@vger.kernel.org, netdev@vger.kernel.org, paul@paul-moore.com,
	linux-fsdevel@vger.kernel.org,
	linux-security-module@vger.kernel.org, keescook@chromium.org,
	kernel-team@meta.com, sargun@sargun.me
Subject: Re: [PATCH v10 bpf-next 03/17] bpf: introduce BPF token object
Message-ID: <20231127-anvertrauen-geldhahn-08f009fe1af1@brauner>
References: <20231110034838.1295764-1-andrii@kernel.org>
 <20231110034838.1295764-4-andrii@kernel.org>
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In-Reply-To: <20231110034838.1295764-4-andrii@kernel.org>

> +	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.