[RFC PATCH v2 3/5] x86/sgx: Enforce noexec filesystem restriction for enclaves

[Sean Christopherson <sean.j.christopherson@xxxxxxxxx>]

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.
Checking noexec indirectly by way of VM_MAYEXEC naturally handles any
other cases that clear VM_MAYEXEC to deny execute permissions.

Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
---
 arch/x86/kernel/cpu/sgx/driver/ioctl.c | 47 +++++++++++++++++++++++---
 1 file changed, 42 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/cpu/sgx/driver/ioctl.c b/arch/x86/kernel/cpu/sgx/driver/ioctl.c
index ef5c2ce0f37b..44b2d73de7c3 100644
--- a/arch/x86/kernel/cpu/sgx/driver/ioctl.c
+++ b/arch/x86/kernel/cpu/sgx/driver/ioctl.c
@@ -577,6 +577,44 @@ 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;
+
+	if (!(prot & VM_EXEC))
+		return 0;
+
+	/* Hold mmap_sem across copy_from_user() to avoid a TOCTOU race. */
+	down_read(&current->mm->mmap_sem);
+
+	vma = find_vma(current->mm, src);
+	if (!vma) {
+		ret = -EFAULT;
+		goto out;
+	}
+
+	/*
+	 * Query VM_MAYEXEC as an indirect path_noexec() check (see do_mmap()),
+	 * but with some future proofing against other cases that may deny
+	 * execute permissions.
+	 */
+	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
  *
@@ -616,13 +654,12 @@ static long sgx_ioc_enclave_add_page(struct file *filep, unsigned int cmd,
 
 	data = kmap(data_page);
 
-	if (copy_from_user((void *)data, (void __user *)addp->src, PAGE_SIZE)) {
-		ret = -EFAULT;
-		goto out;
-	}
-
 	prot = addp->flags & (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