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

[Jarkko Sakkinen]

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.

/Jarkko