[RFC PATCH v4 02/12] x86/sgx: Do not naturally align MAP_FIXED address

[Jarkko Sakkinen]

On Wed, Jun 19, 2019 at 03:23:51PM -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>
> ---
>  arch/x86/kernel/cpu/sgx/driver/main.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/cpu/sgx/driver/main.c b/arch/x86/kernel/cpu/sgx/driver/main.c
> index 07aa5f91b2dd..29384cdd0842 100644
> --- a/arch/x86/kernel/cpu/sgx/driver/main.c
> +++ b/arch/x86/kernel/cpu/sgx/driver/main.c
> @@ -115,7 +115,13 @@ static unsigned long sgx_get_unmapped_area(struct file *file,
>  					   unsigned long pgoff,
>  					   unsigned long flags)
>  {
> -	if (len < 2 * PAGE_SIZE || len & (len - 1) || flags & MAP_PRIVATE)
> +	if (flags & MAP_PRIVATE)
> +		return -EINVAL;
> +
> +	if (flags & MAP_FIXED)
> +		return addr;
> +
> +	if (len < 2 * PAGE_SIZE || len & (len - 1))
>  		return -EINVAL;

Why the last check is needed given that mmap() no longer does not
associate with the layout of the enclave? I'd just wipe it away.

/Jarkko