linux-kernel - Re: [PATCH] bpf: use FS_USERNS

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Message-ID: <20260206-gelenk-gierig-82ad15ec8080@brauner>
Date: Fri, 6 Feb 2026 13:32:59 +0100
From: Christian Brauner <brauner@...nel.org>
To: Alexander Mikhalitsyn <alexander@...alicyn.com>
Cc: ast@...nel.org, Daniel Borkmann <daniel@...earbox.net>, 
	Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau <martin.lau@...ux.dev>, 
	Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, John Fastabend <john.fastabend@...il.com>, 
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>, 
	Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>, Jeff Layton <jlayton@...nel.org>, 
	bpf@...r.kernel.org, linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...urfusion.io>
Subject: Re: [PATCH] bpf: use FS_USERNS_DELEGATABLE for bpffs

On Thu, Feb 05, 2026 at 11:45:41AM +0100, Alexander Mikhalitsyn wrote:
> From: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...urfusion.io>
> 
> Instead of FS_USERNS_MOUNT we should use recently introduced
> FS_USERNS_DELEGATABLE cause it better expresses what we
> really want to get there. Filesystem should not be allowed
> to be mounted by an unprivileged user, but at the same time
> we want to have sb->s_user_ns to point to the container's
> user namespace, at the same time superblock can only
> be created if capable(CAP_SYS_ADMIN) check is successful.
> 
> Tested and no regressions noticed.
> 
> No functional change intended.
> 
> Link: https://lore.kernel.org/linux-fsdevel/6dd181bf9f6371339a6c31f58f582a9aac3bc36a.camel@kernel.org [1]
> Fixes: 6fe01d3cbb92 ("bpf: Add BPF token delegation mount options to BPF FS")
> Cc: Alexei Starovoitov <ast@...nel.org>
> Cc: Daniel Borkmann <daniel@...earbox.net>
> Cc: Andrii Nakryiko <andrii@...nel.org>
> Cc: Martin KaFai Lau <martin.lau@...ux.dev>
> Cc: Eduard Zingerman <eddyz87@...il.com>
> Cc: Song Liu <song@...nel.org>
> Cc: Yonghong Song <yonghong.song@...ux.dev>
> Cc: John Fastabend <john.fastabend@...il.com>
> Cc: KP Singh <kpsingh@...nel.org>
> Cc: Stanislav Fomichev <sdf@...ichev.me>
> Cc: Hao Luo <haoluo@...gle.com>
> Cc: Jiri Olsa <jolsa@...nel.org>
> Cc: Jeff Layton <jlayton@...nel.org>
> Cc: Christian Brauner <brauner@...nel.org>
> Cc: bpf@...r.kernel.org
> Cc: linux-fsdevel@...r.kernel.org
> Cc: linux-kernel@...r.kernel.org
> Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...urfusion.io>
> - RWB-tag from Jeff [1]
> Reviewed-by: Jeff Layton <jlayton@...nel.org>
> ---
>  kernel/bpf/inode.c | 6 +-----
>  1 file changed, 1 insertion(+), 5 deletions(-)
> 
> diff --git a/kernel/bpf/inode.c b/kernel/bpf/inode.c
> index 9f866a010dad..d8dfdc846bd0 100644
> --- a/kernel/bpf/inode.c
> +++ b/kernel/bpf/inode.c
> @@ -1009,10 +1009,6 @@ static int bpf_fill_super(struct super_block *sb, struct fs_context *fc)
>  	struct inode *inode;
>  	int ret;
>  
> -	/* Mounting an instance of BPF FS requires privileges */
> -	if (fc->user_ns != &init_user_ns && !capable(CAP_SYS_ADMIN))
> -		return -EPERM;

Jeff's patch does:

        if (user_ns != &init_user_ns &&
            !(fc->fs_type->fs_flags & (FS_USERNS_MOUNT | FS_USERNS_DELEGATABLE))) {
                errorfc(fc, "VFS: Mounting from non-initial user namespace is not allowed");
                return ERR_PTR(-EPERM);
        }

IOW, it only restricts the ability to end up in bpf_fill_super() if
FS_USERNS_DELEGATABLE is set. You still need to perform the permission
check in bpf_fill_super() though otherwise anyone can mount bpffs in an
unprivileged container now.

So either Jeff's patch needs to be changed to require
capable(CAP_SYS_ADMIN) when FS_USERNS_DELEGATABLE is set (which makes
sense to me in general) or the check needs to remain n bpf_fill_super().

@Jeff do you require capable(CAP_SYS_ADMIN) from within nfs? I think you
somehow must because otherwise what prevents a container from mounting
arbitrary servers?

> -
>  	ret = simple_fill_super(sb, BPF_FS_MAGIC, bpf_rfiles);
>  	if (ret)
>  		return ret;
> @@ -1085,7 +1081,7 @@ static struct file_system_type bpf_fs_type = {
>  	.init_fs_context = bpf_init_fs_context,
>  	.parameters	= bpf_fs_parameters,
>  	.kill_sb	= bpf_kill_super,
> -	.fs_flags	= FS_USERNS_MOUNT,
> +	.fs_flags	= FS_USERNS_DELEGATABLE,
>  };
>  
>  static int __init bpf_init(void)
> -- 
> 2.47.3
>