Re: [PATCH 3/3] security: allow capable check to permit mmap or low vm space

[Eric Paris <eparis@redhat.com>]

On Sat, 2007-11-17 at 08:58 +1100, James Morris wrote:
> On Fri, 16 Nov 2007, Eric Paris wrote:
> 
> > On Sat, 2007-11-17 at 08:47 +1100, James Morris wrote:
> > > On Fri, 16 Nov 2007, Eric Paris wrote:
> > > 
> > > > On a kernel with CONFIG_SECURITY but without an LSM which implements
> > > > security_file_mmap it is impossible for an application to mmap addresses
> > > > lower than mmap_min_addr.
> > > 
> > > Actually, should we be doing any checking in the dummy module, given that 
> > > it is not done with !CONFIG_SECURITY ?
> > 
> > I'm not sure I understand the question.  We already do a number of
> > capable type security checks in dummy functions. See dummy_settime() as
> > just one example.
> 
> I mean just in this case.  If no mmap_min_addr check is done without 
> CONFIG_SECURITY, then perhaps none should be done in the dummy module, 
> i.e. preserving existing behavior.  LSM is theoretically supposed to be 
> unnoticable from a behavioral pov unless a non-dummy module is loaded.

When this protection was originally concieved it intentionally was
offing something even without an more 'full featured' LSM.  That was the
whole reason I had to drop the secondary stacking hook inside the
selinux code.

While I now understand the question, I think that this is the behavior
most people would want.  I'll revert the security enhancement for
non-LSM systems if others agree with James, but I think adding another
small bit of protection against kernel flaws for everyone who wants
security is a win.  (and remember, in kernel we still default this to
off so noone is going to 'accidentally' see and security checks in the
dummy hooks)

-Eric