[RFC PATCH v4 05/12] x86/sgx: Enforce noexec filesystem restriction for enclaves

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

Do not allow an enclave page to be mapped with PROT_EXEC if the source
vma does not have VM_MAYEXEC.  This effectively enforces noexec as
do_mmap() clears VM_MAYEXEC if the vma is being loaded from a noexec
path, i.e. prevents executing a file by loading it into an enclave.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
---
 arch/x86/kernel/cpu/sgx/driver/ioctl.c | 42 +++++++++++++++++++++++---
 1 file changed, 37 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/cpu/sgx/driver/ioctl.c b/arch/x86/kernel/cpu/sgx/driver/ioctl.c
index e18d2afd2aad..1fca70a36ce3 100644
--- a/arch/x86/kernel/cpu/sgx/driver/ioctl.c
+++ b/arch/x86/kernel/cpu/sgx/driver/ioctl.c
@@ -564,6 +564,39 @@ static int sgx_encl_add_page(struct sgx_encl *encl, unsigned long addr,
 	return ret;
 }
 
+static int sgx_encl_page_copy(void *dst, unsigned long src, unsigned long prot)
+{
+	struct vm_area_struct *vma;
+	int ret;
+
+	/* Hold mmap_sem across copy_from_user() to avoid a TOCTOU race. */
+	down_read(&current->mm->mmap_sem);
+
+	/* Query vma's VM_MAYEXEC as an indirect path_noexec() check. */
+	if (prot & PROT_EXEC) {
+		vma = find_vma(current->mm, src);
+		if (!vma) {
+			ret = -EFAULT;
+			goto out;
+		}
+
+		if (!(vma->vm_flags & VM_MAYEXEC)) {
+			ret = -EACCES;
+			goto out;
+		}
+	}
+
+	if (copy_from_user(dst, (void __user *)src, PAGE_SIZE))
+		ret = -EFAULT;
+	else
+		ret = 0;
+
+out:
+	up_read(&current->mm->mmap_sem);
+
+	return ret;
+}
+
 /**
  * sgx_ioc_enclave_add_page - handler for %SGX_IOC_ENCLAVE_ADD_PAGE
  *
@@ -604,13 +637,12 @@ static long sgx_ioc_enclave_add_page(struct file *filep, void __user *arg)
 
 	data = kmap(data_page);
 
-	if (copy_from_user((void *)data, (void __user *)addp.src, PAGE_SIZE)) {
-		ret = -EFAULT;
-		goto out;
-	}
-
 	prot = addp.prot & (PROT_READ | PROT_WRITE | PROT_EXEC);
 
+	ret = sgx_encl_page_copy(data, addp.src, prot);
+	if (ret)
+		goto out;
+
 	ret = sgx_encl_add_page(encl, addp.addr, data, &secinfo, addp.mrmask,
 				prot);
 	if (ret)
-- 
2.21.0