[PATCH 11/39] selinux: reject writable opens of policy file, drop mmap shared/write check

The policy file has no write method and is exposed read-only (S_IRUGO in
selinux_files[]), yet sel_open_policy() performs no open mode check, so a
CAP_DAC_OVERRIDE caller can open it O_RDWR. Reject FMODE_WRITE at open, as
kernfs does.

The file can then never be mapped with FMODE_WRITE, so do_mmap() always
clears VM_MAYWRITE and VM_SHARED for MAP_SHARED mappings and the VM_SHARED
check in sel_mmap_policy() cannot be reached. Remove it.

This also stops sel_mmap_policy() clearing VM_MAYWRITE on a mapping that is
neither a PFN map nor a mixed map, ahead of the core enforcing that only
such mappings may do so.

Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
---
 security/selinux/selinuxfs.c | 11 +++--------
 1 file changed, 3 insertions(+), 8 deletions(-)

diff --git a/security/selinux/selinuxfs.c b/security/selinux/selinuxfs.c
index c7d91476971c..545a6f89f9e7 100644
--- a/security/selinux/selinuxfs.c
+++ b/security/selinux/selinuxfs.c

@@ -340,6 +340,9 @@ static int sel_open_policy(struct inode *inode, struct file *filp)
 	struct policy_load_memory *plm = NULL;
 	int rc;
 
+	if (filp->f_mode & FMODE_WRITE)
+		return -EACCES;
+
 	rc = avc_has_perm(current_sid(), SECINITSID_SECURITY,
 			  SECCLASS_SECURITY, SECURITY__READ_POLICY, NULL);
 	if (rc)

@@ -424,14 +427,6 @@ static const struct vm_operations_struct sel_mmap_policy_ops = {
 
 static int sel_mmap_policy(struct file *filp, struct vm_area_struct *vma)
 {
-	if (vma->vm_flags & VM_SHARED) {
-		/* do not allow mprotect to make mapping writable */
-		vm_flags_clear(vma, VM_MAYWRITE);
-
-		if (vma->vm_flags & VM_WRITE)
-			return -EACCES;
-	}
-
 	vm_flags_set(vma, VM_DONTEXPAND | VM_DONTDUMP);
 	vma->vm_ops = &sel_mmap_policy_ops;
 

-- 
2.55.0