linux-kernel - Re: [PATCH v4 5/5] fuse: Allow user namespace mounts

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Date:	Wed, 15 Oct 2014 16:08:54 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Andy Lutomirski <luto@...capital.net>,
	Miklos Szeredi <miklos@...redi.hu>,
	fuse-devel@...ts.sourceforge.net,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"Serge H. Hallyn" <serge.hallyn@...ntu.com>
Subject: Re: [PATCH v4 5/5] fuse: Allow user namespace mounts

On Wed, Oct 15, 2014 at 8:20 AM, Seth Forshee
<seth.forshee@...onical.com> wrote:
> On Wed, Oct 15, 2014 at 07:58:53AM -0700, Andy Lutomirski wrote:
>> On 10/14/2014 07:25 AM, Seth Forshee wrote:
>> > Cc: Eric W. Biederman <ebiederm@...ssion.com>
>> > Cc: Serge H. Hallyn <serge.hallyn@...ntu.com>
>> > Signed-off-by: Seth Forshee <seth.forshee@...onical.com>
>> > ---
>> >  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 5e00a6a76049..6522926b14e4 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,
>> >  };
>> > @@ -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,
>>
>> I think it's decision time -- if these patches are applied, then FUSE
>> will be the first filesystem for which userns nodev behavior matters for
>> security, so applying this patch will enshrine an API decision.
>>
>> I would very much prefer to make this patch depend on this:
>>
>> http://lkml.kernel.org/g/2686c32f00b14148379e8cfee9c028c794d4aa1a.1407974494.git.luto@amacapital.net
>>
>> That change will require that anyone who tries to mount one of these
>> things explicitly requests MS_NODEV instead of keeping the current
>> behavior of implicitly setting MS_NODEV and possibly confusing user code
>> that tries to remount.
>>
>> If you like my patch, feel free to fold it in to your series, or Eric
>> can apply it directly (pretty please).
>>
>> For background, with your patches as is, if you mount a FUSE fs and then
>> remount it with identical flags, the remount is likely to fail.
>
> I discussed this with Eric during LinuxCon NA ... as I recall he was
> undecided about whether or not to use your patch at the time. I do
> prefer an explicit failure over implicitly adding MS_NODEV, but it's not
> up to me. I do agree though that we should make a decision before
> merging the fuse patches, I was just assuming that the decision was
> already made.

As far as I know, no decision has been made.  I discussed it with Eric
at LinuxCon NA, too.  Too bad we didn't meet there.

Hopefully your patches will convince him to ack my patch :)

--Andy
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