linux-kernel - Re: [PATCH 10/11] fuse: Allow user namespace mounts

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]

Date:   Wed, 14 Feb 2018 14:44:34 +0100
From:   Miklos Szeredi <mszeredi@...hat.com>
To:     Dongsu Park <dongsu@...volk.io>
Cc:     lkml <linux-kernel@...r.kernel.org>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Alban Crequy <alban@...volk.io>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Seth Forshee <seth.forshee@...onical.com>,
        Sargun Dhillon <sargun@...gun.me>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 10/11] fuse: Allow user namespace mounts

On Fri, Dec 22, 2017 at 3:32 PM, Dongsu Park <dongsu@...volk.io> wrote:
> From: Seth Forshee <seth.forshee@...onical.com>
>
> To be able to mount fuse from non-init user namespaces, it's necessary
> to set FS_USERNS_MOUNT flag to fs_flags.
>
> Patch v4 is available: https://patchwork.kernel.org/patch/8944681/
>
> Cc: linux-fsdevel@...r.kernel.org
> Cc: linux-kernel@...r.kernel.org
> Cc: Miklos Szeredi <mszeredi@...hat.com>
> Signed-off-by: Seth Forshee <seth.forshee@...onical.com>
> [dongsu: add a simple commit messasge]
> Signed-off-by: Dongsu Park <dongsu@...volk.io>
> ---
>  fs/fuse/inode.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
> index 7f6b2e55..8c98edee 100644
> --- a/fs/fuse/inode.c
> +++ b/fs/fuse/inode.c
> @@ -1212,7 +1212,7 @@ static void fuse_kill_sb_anon(struct super_block *sb)
>  static struct file_system_type fuse_fs_type = {
>         .owner          = THIS_MODULE,
>         .name           = "fuse",
> -       .fs_flags       = FS_HAS_SUBTYPE,
> +       .fs_flags       = FS_HAS_SUBTYPE | FS_USERNS_MOUNT,
>         .mount          = fuse_mount,
>         .kill_sb        = fuse_kill_sb_anon,
>  };

I think enabling FS_USERNS_MOUNT should be pretty safe.

I was thinking opting out should be as simple as "chmod o-rw
/dev/fuse".  But that breaks libfuse, even though fusermount opens
/dev/fuse in privileged mode, so it shouldn't.  That can be fixed in
libfuse, but it's an unfortunate bug and it also means /dev/fuse is
configured with "crw-rw-rw-" in most cases.  Which means it will be
opting out, not opting in, which is the less safe version.

> @@ -1244,7 +1244,7 @@ static struct file_system_type fuseblk_fs_type = {
>         .name           = "fuseblk",
>         .mount          = fuse_mount_blk,
>         .kill_sb        = fuse_kill_sb_blk,
> -       .fs_flags       = FS_REQUIRES_DEV | FS_HAS_SUBTYPE,
> +       .fs_flags       = FS_REQUIRES_DEV | FS_HAS_SUBTYPE | FS_USERNS_MOUNT,
>  };
>  MODULE_ALIAS_FS("fuseblk");

As I said, this hunk should be dropped from the first version, because
it's possibly unsafe.

Thanks,
Miklos