linux-kernel - Re: [PATCH RESEND v2 16/18] fuse: Support fuse filesystems outside of init_user

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Message-ID: <20160309152505.GA28779@ubuntu-hedt>
Date:	Wed, 9 Mar 2016 09:25:05 -0600
From:	Seth Forshee <seth.forshee@...onical.com>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Richard Weinberger <richard.weinberger@...il.com>,
	Austin S Hemmelgarn <ahferroin7@...il.com>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-bcache@...r.kernel.org, dm-devel@...hat.com,
	linux-raid@...r.kernel.org, linux-mtd@...ts.infradead.org,
	Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
	fuse-devel <fuse-devel@...ts.sourceforge.net>,
	LSM <linux-security-module@...r.kernel.org>,
	selinux@...ho.nsa.gov
Subject: Re: [PATCH RESEND v2 16/18] fuse: Support fuse filesystems outside
 of init_user_ns

On Wed, Mar 09, 2016 at 03:48:22PM +0100, Miklos Szeredi wrote:
> On Wed, Mar 9, 2016 at 3:18 PM, Seth Forshee <seth.forshee@...onical.com> wrote:
> > On Wed, Mar 09, 2016 at 12:29:23PM +0100, Miklos Szeredi wrote:
> >> On Mon, Jan 04, 2016 at 12:03:55PM -0600, Seth Forshee wrote:
> 
> >> > -static int parse_fuse_opt(char *opt, struct fuse_mount_data *d, int is_bdev)
> >> > +static int parse_fuse_opt(char *opt, struct fuse_mount_data *d, int is_bdev,
> >> > +                     struct user_namespace *user_ns)
> >> >  {
> >> >     char *p;
> >> >     memset(d, 0, sizeof(struct fuse_mount_data));
> >> >     d->max_read = ~0;
> >> >     d->blksize = FUSE_DEFAULT_BLKSIZE;
> >> > +   d->user_id = make_kuid(user_ns, 0);
> >> > +   d->group_id = make_kgid(user_ns, 0);
> >>
> >> It is true that if "user_id=" or "group_id" options were omitted we used the
> >> zero uid/gid values.  However, this isn't actually used by anybody AFAIK, and
> >> generalizing it for userns doesn't seem to make much sense.
> >>
> >> So I suggest we that we instead return an error if mounting from a userns AND
> >> neither "allow_other" nor both "user_id" and "group_id" are specified.
> >
> > But those are also used for ownership of the connection files in
> > fusectl. In an allow_other mount shouldn't those files by owned by
> > namespace root and not global root?
> 
> Yes.
> 
> Can't we use current_cred()->uid/gid? Or fsuid/fsgid maybe?

That would be a departure from the current behavior in the !allow_other
case for unprivileged users. Since those mounts are done by an suid
helper all of those ids would be root in the userns, wouldn't they?

> When we have true unprivileged mounts, the user_id/group_id options
> become redundant anyway and we can just use the current credentials.

True, but we don't yet have that.

Thanks,
Seth