linux-kernel - Re: [patch 2/8] allow unprivileged umount

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Message-Id: <E1HfVr9-000431-00@dorka.pomaz.szeredi.hu>
Date:	Sun, 22 Apr 2007 08:47:31 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	ebiederm@...ssion.com
CC:	akpm@...ux-foundation.org, miklos@...redi.hu, serue@...ibm.com,
	viro@....linux.org.uk, linuxram@...ibm.com,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	containers@...ts.osdl.org
Subject: Re: [patch 2/8] allow unprivileged umount

> > On Sat, 21 Apr 2007 10:09:42 +0200 Miklos Szeredi <miklos@...redi.hu> wrote:
> >
> >> > > +static bool permit_umount(struct vfsmount *mnt, int flags)
> >> > > +{
> >> > >
> >> > > ...
> >> > >
> >> > > +	return mnt->mnt_uid == current->uid;
> >> > > +}
> >> > 
> >> > Yes, this seems very wrong.  I'd have thought that comparing user_struct*'s
> >> > would get us a heck of a lot closer to being able to support aliasing of
> >> > UIDs between different namespaces.
> >> > 
> >> 
> >> OK, I'll fix this up.
> >> 
> >> Actually an earlier version of this patch did use user_struct's but
> >> I'd changed it to uids, because it's simpler.
> >
> > OK..
> >
> >>  I didn't think about
> >> this being contrary to the id namespaces thing.
> >
> > Well I was madly assuming that when serarate UID namespaces are in use, UID
> > 42 in container A will have a different user_struct from UID 42 in
> > container B.  I'd suggest that we provoke an opinion from Eric & co before
> > you do work on this.
> 
> That is what I what I have been thinking as well,

Does this mean, that containers will need this?  Or that you don't
know yet?

> storing a user struct on each mount point seems sane, plus it allows
> per user mount rlimits which is major plus.  Especially since we
> seem to be doing accounting only for user mounts a per user rlimit
> seems good.

I'm not against per-user rlimits for mounts, but I'd rather do this
later...

> To get the user we should be user fs_uid as HPA suggested.

fsuid is exclusively used for checking file permissions, which we
don't do here anymore.  So while it could be argued, that mount() _is_
a filesystem operation, it is really a different sort of filesystem
operation than the rest.

OTOH it wouldn't hurt to use fsuid instead of ruid...

> Finally I'm pretty certain the capability we should care about in
> this context is CAP_SETUID.  Instead of CAP_SYS_ADMIN.
> 
> If we have CAP_SETUID we can become which ever user owns the mount,
> and the root user in a container needs this, so he can run login
> programs.  So changing the appropriate super user checks from
> CAP_SYS_ADMIN to CAP_SETUID I think is the right thing todo.

That's a flawed logic.  If you want to mount as a specific user, and
you have CAP_SETUID, then just use set*uid() and then mount().

Changing the capability check for mount() would break the userspace
ABI.

Miklos
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