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

[Andy Lutomirski]

On Tue, Jun 4, 2019 at 4:50 AM Jarkko Sakkinen
<jarkko.sakkinen@xxxxxxxxxxxxxxx> 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@xxxxxxxxx>
>
> 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.