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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <CAJqdLrrU-iJcSg-pA_L4iZLkt6Xd2Ju5=C3x71QZmB9X4zYtsQ@mail.gmail.com>
Date: Fri, 6 Feb 2026 13:44:14 +0100
From: Alexander Mikhalitsyn <alexander@...alicyn.com>
To: Christian Brauner <brauner@...nel.org>
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

Am Fr., 6. Feb. 2026 um 13:42 Uhr schrieb Alexander Mikhalitsyn
<alexander@...alicyn.com>:
>
> Am Fr., 6. Feb. 2026 um 13:33 Uhr schrieb Christian Brauner
> <brauner@...nel.org>:
> >
> > 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);
> >         }
>
> Hi Christian,
>
> >
> > 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.
>
> Yeah, this is what  mount_capable(struct fs_context *fc) does. I'm removing
> FS_USERNS_MOUNT so know it checks capable(CAP_SYS_ADMIN), instead of
> ns_capable(fc->user_ns, CAP_SYS_ADMIN).
>
> No functional changes here.
>
> >
> > 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?
>
> Same point here. We have this check in mount_capable(struct fs_context
> *fc) already.

And of course, I would never send a patch like this without testing it
for unprivileged
user and without ensuring that it doesn't cause any obvious privilege
escalation ;-)

>
> Kind regards,
> Alex
>
> >
> > > -
> > >       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
> > >