Should mprotect(..., PROT_EXEC) be checked by IMA?

[Mimi Zohar]

On Fri, 2019-03-29 at 15:50 +0300, Igor Zhbanov wrote:
> On 29.03.2019 15:28, Stephen Smalley wrote:
> > On 3/29/19 6:59 AM, Mimi Zohar wrote:
> >> [Cc'ing the LSM mailing list and others]
> >>
> >> On Fri, 2019-03-29 at 13:00 +0300, Igor Zhbanov wrote:
> >>> Hi Mimi,On 28.03.2019 20:17, Mimi Zohar wrote:
> >>
> >>>> I just came across the grsecurity article on mprotect.[1]
> >>>>   Has anyone looked at it? Would it make sense to make it a minor LSM?
> >>>>
> >>>> [1]https://pax.grsecurity.net/docs/mprotect.txt
> >>>
> >>> Interesting article. It is almost exactly of what I wanted to be
> >>> implemented.
> >>>
> >>> If this minor LSM would be stackable to allow combining with e.g. SELinux
> >>> then why not.
> >>
> >> Stacking shouldn't be a problem.  Other LSMs are already on the
> >> mprotect hook.  Let's hear what others think.
> > 
> > SELinux already provides a set of controls over executable mappings;
> > see selinux_mmap_file and selinux_file_mprotect. Other major security
> > modules may do likewise but I can't speak to that. Is there some gap
> > you are trying to address that isn't already covered, or are you just
> > trying to provide such restrictions without requiring one of the
> > major modules?
> 
> I want to be sure that no unsigned code page could be executed. So exploits
> could only be of ROP kind and not being able to download any extra code
> from their servers. That's why I found that disabling of anonymous executable
> pages could be useful for that (as well as disabling of making executable
> pages writable to modify already mapped code). In conjunction with IMA it
> should guarantee that no untrusted code could be executed.

Let's separate the different types of attacks.  From an IMA
perspective, memory attacks are out of scope.  That leaves mmap'ed
files, possibly just mmap'ed shared files.  Currently IMA can be
configured to verify a file's integrity, based on signatures, being
mmap'ed execute.  Assuming that not all files opened require a file
signature, a file could be mmap'ed read/write and later changed to
execute to circumvent the mmap'ed execute signature requirement.  If
the existing LSMs are able to prevent this sort of attack, we could
just document this requirement.

Mimi