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

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Date:   Thu, 15 Feb 2018 09:46:51 +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 Wed, Feb 14, 2018 at 2:44 PM, Miklos Szeredi <mszeredi@...hat.com> wrote:
> 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.

I'm talking rubbish, /dev/fuse is opened without privs in fusermount as well.

So there's not way to differentiate user_ns unpriv mounts from suid
fusermount unpriv mounts.

Maybe that's just as well...

Thanks,
Miklos