[PATCH v21 15/28] mm: Introduce vm_ops->may_mprotect()

[Jarkko Sakkinen]

From: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>

SGX will use ->may_mprotect() to invoke and enforce SGX variations on
the existing file_mprotect() and mmap_file() LSM hooks.

The name may_mprotect() is intended to reflect the hook's purpose as a
way to restrict mprotect() as opposed to a wholesale replacement.

Due to the nature of SGX and its Enclave Page Cache (EPC), all enclave
VMAs are backed by a single file, i.e. /dev/sgx/enclave, that must be
MAP_SHARED.  Furthermore, all enclaves need read, write and execute
VMAs.  As a result, applying W^X restrictions on /dev/sgx/enclave using
existing LSM hooks is for all intents and purposes impossible, e.g.
denying either W or X would deny access to *any* enclave.

By hooking mprotect(), SGX can enforce maximal PTE protections that are
checked by LSMs at enclave load time (when the source file is available)
and can also invoke LSM hooks at the time of mapping, e.g. an enclave
specific hook or a modified call to file_mprotect(), e.g. to check
W+X mappings.

Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
---
 include/linux/mm.h |  2 ++
 mm/mprotect.c      | 13 ++++++++++---
 2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index dd0b5f4e1e45..0099b638836b 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -469,6 +469,8 @@ struct vm_operations_struct {
 	void (*close)(struct vm_area_struct * area);
 	int (*split)(struct vm_area_struct * area, unsigned long addr);
 	int (*mremap)(struct vm_area_struct * area);
+	int (*may_mprotect)(struct vm_area_struct *vma, unsigned long start,
+			    unsigned long end, unsigned long prot);
 	vm_fault_t (*fault)(struct vm_fault *vmf);
 	vm_fault_t (*huge_fault)(struct vm_fault *vmf,
 			enum page_entry_size pe_size);
diff --git a/mm/mprotect.c b/mm/mprotect.c
index bf38dfbbb4b4..18732543b295 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -547,13 +547,20 @@ static int do_mprotect_pkey(unsigned long start, size_t len,
 			goto out;
 		}
 
+		tmp = vma->vm_end;
+		if (tmp > end)
+			tmp = end;
+
+		if (vma->vm_ops && vma->vm_ops->may_mprotect) {
+			error = vma->vm_ops->may_mprotect(vma, nstart, tmp, prot);
+			if (error)
+				goto out;
+		}
+
 		error = security_file_mprotect(vma, reqprot, prot);
 		if (error)
 			goto out;
 
-		tmp = vma->vm_end;
-		if (tmp > end)
-			tmp = end;
 		error = mprotect_fixup(vma, &prev, nstart, tmp, newflags);
 		if (error)
 			goto out;
-- 
2.20.1