[RFC PATCH v3 02/12] x86/sgx: Do not naturally align MAP_FIXED address

[Sean Christopherson <sean.j.christopherson@intel.com>]

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@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 03eca4bccee1..d7de4d9aea87 100644
--- a/arch/x86/kernel/cpu/sgx/driver/main.c
+++ b/arch/x86/kernel/cpu/sgx/driver/main.c
@@ -105,7 +105,13 @@ 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;
 
 	addr = current->mm->get_unmapped_area(file, addr, 2 * len, pgoff,
-- 
2.21.0