Re: [RFC PATCH RESEND] vfs: Move security_inode_killpriv() after permission checks

[Stephen Smalley <sds () tycho nsa gov>]

On 01/20/2015 06:17 PM, James Morris wrote:
> On Sat, 17 Jan 2015, Ben Hutchings wrote:
>
> > chown() and write() should clear all privilege attributes on
> > a file - setuid, setgid, setcap and any other extended
> > privilege attributes.
> >
> > However, any attributes beyond setuid and setgid are managed by the
> > LSM and not directly by the filesystem, so they cannot be set along
> > with the other attributes.
> >
> > Currently we call security_inode_killpriv() in notify_change(),
> > but in case of a chown() this is too early - we have not called
> > inode_change_ok() or made any filesystem-specific permission/sanity
> > checks.
> >
> > Add a new function setattr_killpriv() which calls
> > security_inode_killpriv() if necessary, and change the setattr()
> > implementation to call this in each filesystem that supports xattrs.
> > This assumes that extended privilege attributes are always stored in
> > xattrs.
>
> It'd be useful to get some input from LSM module maintainers on this. 
>
> e.g. doesn't SELinux already handle this via policy directives?

There have been a couple postings of a similar patch set [1] by Jan
Kara, although I don't believe that series addressed chown().

If I am reading the patches correctly, they (correctly) don't affect
SELinux or Smack labels; they are just calling the existing
security_inode_killpriv() hook, which is only implemented for the
capability module to remove the security.capability xattr.

[1] http://marc.info/?l=linux-security-module&m=141890696325054&w=2