From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 99C6631F9B4; Mon, 8 Jun 2026 20:08:13 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780949294; cv=none; b=IkTyEppxWVOh5RpRNVT3MKTjROLO2HIBbBDRroNPneXNEU8h57qIapaY9Jucs9pVAZnfQp7TJMtmEzxinJePG0EhcOPBbSv2XqSaVEpKejsvQSp6iO7NbalEB3poAzVFZMMTXlv8R50/a/z1e0k/20BFw4nHebTW6+oAGbCQWOs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780949294; c=relaxed/simple; bh=waUur4CucJO6qOjwL6GlEcvOwDg5YiIp/PXOTLj9UKk=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=rglVWnrf73BPO+/PFWvlpdHwlSTBLQcFJN0wf/7rfRyiyqFgo3EpsuhjuYFOy+raGtrdCjt5Mboix0TIy9l106FiMUWjZ4t8Jg2VZo6XbD3wEtGaHMxB1aotg/iuScYbvY31pd84vvjdVLttbm+pIUAj488QqXn4f3BH1nMxK8o= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=AnLt7C1N; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="AnLt7C1N" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 53D9D1F00893; Mon, 8 Jun 2026 20:08:12 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1780949293; bh=PKV9ey2fkieIe18ckC+tbTRLTOWdiRb4JQ7xkIPvkok=; h=Subject:From:To:Cc:Date:In-Reply-To:References; b=AnLt7C1NTyLwJoldYhVdAOvUyWPbe0AewR1ubMhHVDLwmBzzlXF44Wmn67tkdwUJ7 obvZ9HnyYCkkgSQMe5omyRRnrDp47r26Z0QaUExMF5kENaCYjyYKrxNJCBSr3zGjok dZw+kOq4aeS5sujORLPXC+a4E2EuqdEOFrdyRc7sgm6m4nipuy8jlB55IkkaU12FTp 7nhm5WYnA5HmMq/nRk+4wIi8TJDX7mp3jh1HNiWSil4XV+YD6w/RkHqM3mKOtX7dzs gUBgh5hDne2d/v4Ly48F9boAQIXPIkZk+woQqYByGHo4Ub4gaLcZL9dB85qn32SceL jtgEb/rulZcZw== Message-ID: Subject: Re: [PATCH] vfs: add FS_USERNS_DELEGATABLE flag and set it for NFS From: Jeff Layton To: Trond Myklebust , Anna Schumaker , Alexander Viro , Christian Brauner , Jan Kara , "Seth Forshee (DigitalOcean)" , Alexander Mikhalitsyn Cc: linux-nfs@vger.kernel.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org Date: Mon, 08 Jun 2026 16:08:10 -0400 In-Reply-To: <20260129-twmount-v1-1-4874ed2a15c4@kernel.org> References: <20260129-twmount-v1-1-4874ed2a15c4@kernel.org> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV 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Thu, 2026-01-29 at 16:47 -0500, Jeff Layton wrote: > Commit e1c5ae59c0f2 ("fs: don't allow non-init s_user_ns for filesystems > without FS_USERNS_MOUNT") prevents the mount of any filesystem inside a > container that doesn't have FS_USERNS_MOUNT set. >=20 > This broke NFS mounts in our containerized environment. We have a daemon > somewhat like systemd-mountfsd running in the init_ns. A process does a > fsopen() inside the container and passes it to the daemon via unix > socket. >=20 > The daemon then vets that the request is for an allowed NFS server and > performs the mount. This now fails because the fc->user_ns is set to the > value in the container and NFS doesn't set FS_USERNS_MOUNT. We don't > want to add FS_USERNS_MOUNT to NFS since that would allow the container > to mount any NFS server (even malicious ones). >=20 > Add a new FS_USERNS_DELEGATABLE flag, and enable it on NFS. >=20 > Fixes: e1c5ae59c0f2 ("fs: don't allow non-init s_user_ns for filesystems = without FS_USERNS_MOUNT") > Signed-off-by: Jeff Layton > --- > fs/nfs/fs_context.c | 8 ++++++-- > fs/super.c | 11 ++++++----- > include/linux/fs.h | 1 + > 3 files changed, 13 insertions(+), 7 deletions(-) >=20 > diff --git a/fs/nfs/fs_context.c b/fs/nfs/fs_context.c > index b4679b7161b0968810e13f57c889052ea015bf56..128ebd48b4f4ba1c17e8b5b1b= 9dcefbd7a97db1a 100644 > --- a/fs/nfs/fs_context.c > +++ b/fs/nfs/fs_context.c > @@ -1768,7 +1768,9 @@ struct file_system_type nfs_fs_type =3D { > .init_fs_context =3D nfs_init_fs_context, > .parameters =3D nfs_fs_parameters, > .kill_sb =3D nfs_kill_super, > - .fs_flags =3D FS_RENAME_DOES_D_MOVE|FS_BINARY_MOUNTDATA, > + .fs_flags =3D FS_RENAME_DOES_D_MOVE | > + FS_BINARY_MOUNTDATA | > + FS_USERNS_DELEGATABLE, > }; > MODULE_ALIAS_FS("nfs"); > EXPORT_SYMBOL_GPL(nfs_fs_type); > @@ -1780,7 +1782,9 @@ struct file_system_type nfs4_fs_type =3D { > .init_fs_context =3D nfs_init_fs_context, > .parameters =3D nfs_fs_parameters, > .kill_sb =3D nfs_kill_super, > - .fs_flags =3D FS_RENAME_DOES_D_MOVE|FS_BINARY_MOUNTDATA, > + .fs_flags =3D FS_RENAME_DOES_D_MOVE | > + FS_BINARY_MOUNTDATA | > + FS_USERNS_DELEGATABLE, > }; > MODULE_ALIAS_FS("nfs4"); > MODULE_ALIAS("nfs4"); > diff --git a/fs/super.c b/fs/super.c > index 3d85265d14001d51524dbaec0778af8f12c048ac..b7f1bb2b679b43261fbdcd586= 971c551b85e8372 100644 > --- a/fs/super.c > +++ b/fs/super.c > @@ -738,12 +738,13 @@ struct super_block *sget_fc(struct fs_context *fc, > int err; > =20 > /* > - * Never allow s_user_ns !=3D &init_user_ns when FS_USERNS_MOUNT is > - * not set, as the filesystem is likely unprepared to handle it. > - * This can happen when fsconfig() is called from init_user_ns with > - * an fs_fd opened in another user namespace. > + * Never allow s_user_ns !=3D &init_user_ns when FS_USERNS_MOUNT or > + * FS_USERNS_DELEGATABLE is not set, as the filesystem is likely > + * unprepared to handle it. This can happen when fsconfig() is called > + * from init_user_ns with an fs_fd opened in another user namespace. > */ > - if (user_ns !=3D &init_user_ns && !(fc->fs_type->fs_flags & FS_USERNS_M= OUNT)) { > + if (user_ns !=3D &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 allo= wed"); > return ERR_PTR(-EPERM); > } > diff --git a/include/linux/fs.h b/include/linux/fs.h > index a01621fa636a60764e1dfe83f2260caf50c4037e..94695ce5e25b5fbe4f321d547= 8172b8cb24e00d1 100644 > --- a/include/linux/fs.h > +++ b/include/linux/fs.h > @@ -2273,6 +2273,7 @@ struct file_system_type { > #define FS_MGTIME 64 /* FS uses multigrain timestamps */ > #define FS_LBS 128 /* FS supports LBS */ > #define FS_POWER_FREEZE 256 /* Always freeze on suspend/hibernate */ > +#define FS_USERNS_DELEGATABLE 512 /* Can be mounted inside userns from o= utside */ > #define FS_RENAME_DOES_D_MOVE 32768 /* FS will handle d_move() during re= name() internally. */ > int (*init_fs_context)(struct fs_context *); > const struct fs_parameter_spec *parameters; >=20 > --- > base-commit: 8dfce8991b95d8625d0a1d2896e42f93b9d7f68d > change-id: 20260129-twmount-114ddfd43420 >=20 > Best regards, Ping? I just realized that this patch never made it into -next or a release. Any chance we can get this into the coming v7.3 merge window? (although it looks like we'll need to renumber the bit since FS_USERNS_MOUNT_RESTRICTED has gone in since then. Thanks, --=20 Jeff Layton