[PATCH 2/3] security: define security_page_mkwrite

[Serge E . Hallyn]

Define a security_page_mkwrite() hook which is called before making
a page writable.  This can be used by security modules after revoking
write access to a vma, in order to avoid reacquiring write access,
without having to manually change the file->f_mode.

The intended user of this hook is the SLIM security module, which
needs the ability to revoke write access to an open file when a
process' integrity level is lowered.

Signed-off-by: Serge E. Hallyn <serue@xxxxxxxxxx>
---
 include/linux/security.h |   21 +++++++++++++++++++++
 mm/memory.c              |   11 +++++++++++
 security/dummy.c         |    7 +++++++
 3 files changed, 39 insertions(+), 0 deletions(-)

diff --git a/include/linux/security.h b/include/linux/security.h
index f753038..5515188 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -410,6 +410,14 @@ #ifdef CONFIG_SECURITY
  *	the size of the buffer required.
  *	Returns number of bytes used/required on success.
  *
+ * Security hooks for page operations
+ *
+ * @page_mkwrite:
+ *	Check for permission to make a page table entry writable.
+ *	@vma is the vma to which the page belongs
+ *	@p is the struct page wanting to be made writable
+ *	Return 0 if permission is granted.
+ *
  * Security hooks for file operations
  *
  * @file_permission:
@@ -1198,6 +1206,7 @@ struct security_operations {
   	int (*inode_setsecurity)(struct inode *inode, const char *name, const void *value, size_t size, int flags);
   	int (*inode_listsecurity)(struct inode *inode, char *buffer, size_t buffer_size);
 
+	int (*page_mkwrite) (struct vm_area_struct *vma, struct page *p);
 	int (*file_permission) (struct file * file, int mask);
 	int (*file_alloc_security) (struct file * file);
 	void (*file_free_security) (struct file * file);
@@ -1738,6 +1747,12 @@ static inline int security_inode_listsec
 	return security_ops->inode_listsecurity(inode, buffer, buffer_size);
 }
 
+static inline int security_page_mkwrite(struct vm_area_struct *vma,
+			struct page *p)
+{
+	return security_ops->page_mkwrite(vma, p);
+}
+
 static inline int security_file_permission (struct file *file, int mask)
 {
 	return security_ops->file_permission (file, mask);
@@ -2405,6 +2420,12 @@ static inline int security_inode_listsec
 	return 0;
 }
 
+static inline int security_page_mkwrite(struct vm_area_struct *vma,
+			struct page *p)
+{
+	return 0;
+}
+
 static inline int security_file_permission (struct file *file, int mask)
 {
 	return 0;
diff --git a/mm/memory.c b/mm/memory.c
index 109e986..c4ed919 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -49,6 +49,7 @@ #include <linux/rmap.h>
 #include <linux/module.h>
 #include <linux/delayacct.h>
 #include <linux/init.h>
+#include <linux/security.h>
 
 #include <asm/pgalloc.h>
 #include <asm/uaccess.h>
@@ -1466,6 +1467,12 @@ static int do_wp_page(struct mm_struct *
 
 	if (unlikely((vma->vm_flags & (VM_SHARED|VM_WRITE)) ==
 				(VM_SHARED|VM_WRITE))) {
+		ret = security_page_mkwrite(vma, old_page);
+		if (ret) {
+			page_cache_release(old_page);
+			return VM_FAULT_SIGBUS;
+		}
+
 		if (vma->vm_ops && vma->vm_ops->page_mkwrite) {
 			/*
 			 * Notify the address space that the page is about to
@@ -2144,6 +2151,10 @@ retry:
 			/* if the page will be shareable, see if the backing
 			 * address space wants to know that the page is about
 			 * to become writable */
+			if (security_page_mkwrite(vma, new_page)) {
+				page_cache_release(new_page);
+				return VM_FAULT_SIGBUS;
+			}
 			if (vma->vm_ops->page_mkwrite &&
 			    vma->vm_ops->page_mkwrite(vma, new_page) < 0
 			    ) {
diff --git a/security/dummy.c b/security/dummy.c
index bbbfda7..20e838e 100644
--- a/security/dummy.c
+++ b/security/dummy.c
@@ -397,6 +397,12 @@ static const char *dummy_inode_xattr_get
 	return NULL;
 }
 
+static inline int dummy_page_mkwrite(struct vm_area_struct *vma,
+			struct page *p)
+{
+	return 0;
+}
+
 static int dummy_file_permission (struct file *file, int mask)
 {
 	return 0;
@@ -968,6 +974,7 @@ void security_fixup_ops (struct security
 	set_to_dummy_if_null(ops, inode_getsecurity);
 	set_to_dummy_if_null(ops, inode_setsecurity);
 	set_to_dummy_if_null(ops, inode_listsecurity);
+	set_to_dummy_if_null(ops, page_mkwrite);
 	set_to_dummy_if_null(ops, file_permission);
 	set_to_dummy_if_null(ops, file_alloc_security);
 	set_to_dummy_if_null(ops, file_free_security);
-- 
1.4.1.1

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