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

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Message-ID: <b884396677f65e9801255d38e5e5c544b70244c3.camel@kernel.org>
Date: Fri, 06 Feb 2026 07:45:37 -0500
From: Jeff Layton <jlayton@...nel.org>
To: Christian Brauner <brauner@...nel.org>, 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>, 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 Fri, 2026-02-06 at 13:32 +0100, Christian Brauner wrote:
> 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?
> 

As Alexander pointed out, the mount_capable() check will still fail
without CAP_SYS_ADMIN.

-- 
Jeff Layton <jlayton@...nel.org>