Re: [PATCH v2 0/3] fuse: Add support for mounts from pid/user namespaces

From: Seth Forshee
Date: Thu Sep 25 2014 - 14:44:18 EST

On Thu, Sep 25, 2014 at 11:05:36AM -0700, Eric W. Biederman wrote:
> Miklos Szeredi <miklos@xxxxxxxxxx> writes:
> > On Wed, Sep 24, 2014 at 7:10 PM, Eric W. Biederman
> > <ebiederm@xxxxxxxxxxxx> wrote:
> >
> >
> >> So in summary I see:
> >> - Low utility in being able to manipulate files with bad uids.
> >> - Bad uids are mostly likely malicious action.
> >> - make_bad_inode is trivial to analyze.
> >> - No impediments to change if I am wrong.
> >>
> >> So unless there is a compelling case, right now I would recommend
> >> returning -EIO initially. That allows us to concentrate on the easier
> >> parts of this and it leaves the changes only in fuse.
> >
> > The problem with marking the inode bad is that it will mark it bad for
> > all instances of this filesystem. Including ones which are in a
> > namespace where the UIDs make perfect sense.
> There are two cases:
> app <-> fuse
> fuse <-> server
> I proposed mark_bad_inode for "userspace server -> fuse".
> Where we have one superblock and one server so and one namespace that
> they decide to talk in when the filesystem was mounted.
> I think bad_inode is a reasonable response when the filesystem server
> starts spewing non-sense.
> > So that really doesn't look like a good solution.
> >
> > Doing the check in inode_permission() might be too heavyweight, but
> > it's still the only one that looks sane.
> For the "app <-> fuse" case we already have checks in inode_permision
> that are kuid based that handle that case. We use kuids not for
> performance (although there is a small advatnage) but to much more to
> keep the logic simple and maintainable.
> For the "app -> fuse" case in .setattr we do need a check to verify
> that the uid and gid are valid. However that check was added with
> the basic user namespace support and fuse current returns -EOVERFLOW
> when that happens.

Where does this happen? I haven't managed to track it down yet.

I've also added a check in fuse for this. If a uid/gid passed to
fuse_setattr doesn't map into the namespace it will return -EINVAL.
Sounds like maybe it should return -EOVERFLOW instead.

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