[RFC PATCH 2/9] x86/sgx: Do not naturally align MAP_FIXED address

[Xing, Cedric]

> From: linux-sgx-owner at vger.kernel.org [mailto:linux-sgx-
> owner at vger.kernel.org] On Behalf Of Andy Lutomirski
> Sent: Tuesday, June 04, 2019 1:16 PM
> 
> On Tue, Jun 4, 2019 at 4:50 AM Jarkko Sakkinen
> <jarkko.sakkinen at linux.intel.com> wrote:
> >
> > On Fri, May 31, 2019 at 04:31:52PM -0700, Sean Christopherson wrote:
> > > SGX enclaves have an associated Enclave Linear Range (ELRANGE) that
> > > is tracked and enforced by the CPU using a base+mask approach,
> > > similar to how hardware range registers such as the variable MTRRs.
> > > As a result, the ELRANGE must be naturally sized and aligned.
> > >
> > > To reduce boilerplate code that would be needed in every userspace
> > > enclave loader, the SGX driver naturally aligns the mmap() address
> > > and also requires the range to be naturally sized.  Unfortunately,
> > > SGX fails to grant a waiver to the MAP_FIXED case, e.g. incorrectly
> > > rejects mmap() if userspace is attempting to map a small slice of an
> existing enclave.
> > >
> > > Signed-off-by: Sean Christopherson <sean.j.christopherson at intel.com>
> >
> > Why you want to allow mmap() to be called multiple times? mmap() could
> > be allowed only once with PROT_NONE and denied afterwards. Is this for
> > sending fd to another process that would map already existing enclave?
> >
> > I don't see any checks for whether the is enclave underneath. Also, I
> > think that in all cases mmap() callback should allow only PROT_NONE as
> > permissions for clarity even if it could called multiple times.
> >
> 
> What's the advantage to only allowing PROT_NONE?  The idea here is to
> allow a PROT_NONE map followed by some replacemets that overlay it for
> the individual segments.  Admittedly, mprotect() can do the same thing,
> but disallowing mmap() seems at least a bit surprising.

Disallowing mmap() is not only surprising but also unnecessary.

A bit off topic here. This mmap()/mprotect() discussion reminds me a question (guess for Jarkko): Now that vma->vm_file->private_data keeps a pointer to the enclave, why do we store it again in vma->vm_private? It isn't a big deal but non-NULL vm_private does prevent mprotect() from merging adjacent VMAs.