[PATCH mm-unstable v1 3/3] drivers/misc/open-dice: don't touch VM_MAYSHARE

[David Hildenbrand <david@redhat.com>]

A MAP_SHARED mapping always has VM_MAYSHARE set, and writable
(VM_MAYWRITE) MAP_SHARED mappings have VM_SHARED set as well. To
identify a MAP_SHARED mapping, it's sufficient to look at VM_MAYSHARE.

We cannot have VM_MAYSHARE|VM_WRITE mappings without having VM_SHARED
set. Consequently, current code will never actually end up clearing
VM_MAYSHARE and that code is confusing, because nobody is supposed to
mess with VM_MAYWRITE.

Let's clean it up and restructure the code. No functional change intended.

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 drivers/misc/open-dice.c | 14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/drivers/misc/open-dice.c b/drivers/misc/open-dice.c
index c61be3404c6f..9dda47b3fd70 100644
--- a/drivers/misc/open-dice.c
+++ b/drivers/misc/open-dice.c
@@ -90,15 +90,13 @@ static int open_dice_mmap(struct file *filp, struct vm_area_struct *vma)
 {
 	struct open_dice_drvdata *drvdata = to_open_dice_drvdata(filp);
 
-	/* Do not allow userspace to modify the underlying data. */
-	if ((vma->vm_flags & VM_WRITE) && (vma->vm_flags & VM_SHARED))
-		return -EPERM;
-
-	/* Ensure userspace cannot acquire VM_WRITE + VM_SHARED later. */
-	if (vma->vm_flags & VM_WRITE)
-		vma->vm_flags &= ~VM_MAYSHARE;
-	else if (vma->vm_flags & VM_SHARED)
+	if (vma->vm_flags & VM_MAYSHARE) {
+		/* Do not allow userspace to modify the underlying data. */
+		if (vma->vm_flags & VM_WRITE)
+			return -EPERM;
+		/* Ensure userspace cannot acquire VM_WRITE later. */
 		vma->vm_flags &= ~VM_MAYWRITE;
+	}
 
 	/* Create write-combine mapping so all clients observe a wipe. */
 	vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot);
-- 
2.39.0