Re: [PATCH RESEND v2 11/18] fs: Ensure the mounter of a filesystem is privileged towards its inodes

From: Seth Forshee
Date: Wed Mar 30 2016 - 10:58:54 EST

On Tue, Mar 29, 2016 at 08:36:09PM -0500, Eric W. Biederman wrote:
> Seth Forshee <seth.forshee@xxxxxxxxxxxxx> writes:
> > On Fri, Mar 04, 2016 at 04:43:06PM -0600, Eric W. Biederman wrote:
> >> In general this is only an issue if uids and gids on the filesystem
> >> do not map into the user namespace.
> >>
> >> Therefore the general fix is to limit the logic of checking for
> >> capabilities in s_user_ns if we are dealing with INVALID_UID and
> >> INVALID_GID. For proc and kernfs that should never be the case
> >> so the problem becomes a non-issue.
> >>
> >> Further I would look at limiting that relaxation to just
> >> inode_change_ok.
> >
> > Finally got around to implementing this today; is the patch below what
> > you had in mind?
> Pretty much.
> For the same reason that capble_wrt_inode_uidgid(inode) had to look
> at both inode->i_uid and inode->i_gid I think we need to look at
> both inode->i_uid and inode->i_gid in those case.
> I am worried about chgrp_ok in cases such as inode->i_uid is valid
> but unmapped. I have a similiar worry about chown_ok where
> inode->i_gid is valid but unmapped (although that worry is less
> serious).

That makes sense.

So then what is wanted is to check that the other id is either invalid,
or else it maps into s_user_ns. So for chown_ok() something like this:

if (!uid_valid(inode->i_uid) &&
(!gid_valid(inode->i_gid) || kgid_has_mapping(inode->i_sb->s_user_ns, inode->i_gid)) &&
ns_capable(inode->i_sb->s_user_ns, CAP_CHOWN))
return true;

and likewise for chgrp_ok(). Does that satisfy your concerns?

> >> So that we can easily wrap that check per filesystem
> >> and deny the relaxation for proc and kernfs. proc and kernfs already
> >> have wrappers for .setattr so denying changes when !uid_vaid and
> >> !gid_valid would be a trivial addition, and ensure calamity does
> >> not ensure.
> >
> > I'm confused about this part though. As you say above, proc and kernfs
> > will never have inodes with invalid ids, so it's not an issue. Do you
> > just mean this to be extra insurance against problems?
> I meant two things.
> 1) As filesystems explicitly have to call inode_change_ok they can
> over ride the default if it is possible.
> 2) Because being paranoid about backward compatibility matters, it
> almost certainly workth add adding a check:
> "if (!uid_valid(inode->i_uid) ||!gid_valid(inode->i_gid)) return -EPERM"
> To proc and sysfs just before they call inode_change_ok just so we
> don't need to analyze them and confirm that they don't use
> That just makes the patch more robust.
> The we could leave removing that code for a follow on patch where
> someone takes the time to read through and audit all of the proc and
> sysfs code to ensure that the case does not arise, instead of just
> implicitily assuming it.
> That is the usual pattern when pushing down changes. Do something
> that is easily guaranteed to work, and leave the careful looking for
> a patch all of it's own.

Okay, I'll add checks.