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

[Mimi Zohar]

On Wed, 2019-04-03 at 10:33 -0400, Stephen Smalley wrote:
> On 4/3/19 10:33 AM, Mimi Zohar wrote:
> > On Wed, 2019-04-03 at 09:10 -0400, Stephen Smalley wrote:
> >> On 4/3/19 7:57 AM, Mimi Zohar wrote:
> > 
> >>> 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.
> >>
> >> I guess I don't understand why IMA isn't already being called from
> >> security_file_mprotect(). security_file_mprotect() could just call
> >> ima_file_mmap(vma->vm_file, prot) if all of the security hooks pass.
> >>
> >> SELinux can be used to prevent unauthorized mprotect PROT_EXEC but it
> >> won't perform a measurement of the file if it is allowed by policy.
> > 
> >  From a measurement perspective, this will at least measure the file,
> > but the call to ima_file_mmap() will verify the file signature against
> > the file, not what is currently in memory, right?
> 
> Yes, but you can use SELinux to prevent that (don't allow execmem or 
> execmod permissions for that domain).

Ok, let's start with at least adding the ima_file_mmap() call as you
suggested.  For LSMs which don't have this capability, they might need
to be stacked with a minor LSM such as S.A.R.A, once it is upstreamed,
or similar.

Mimi