[PATCH] xen/gntdev: prevent private mappings from becoming writable

[Abdifatah Suruur <suruurism@gmail.com>]

gntdev_mmap() rejects writable private (non-shared) mappings of foreign
grant pages, because a private writable mapping takes the COW path on
pages that have no proper struct-page backing.  However, VM_MAYWRITE is
left set, so userspace can map the grant read-only and then upgrade the
mapping to writable with mprotect(), hitting the same COW path the
check was written to prevent.

Clear VM_MAYWRITE for private mappings, as i915 does for its read-only
objects and as fixed in drm/vc4 (CVE-2026-68445) and drm/panthor
(CVE-2024-53071) and ptp: vmclock (commit
a5edadbae57e2298a56cf7a4e774a027905a331f).

Fixes: ab31523c2fcac ("xen/gntdev: allow usermode to map granted pages")
Cc: stable@vger.kernel.org
Signed-off-by: Abdifatah Suruur <suruurism@gmail.com>

---
diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
index 1dcc4675580ed..b71f5bae25b63 100644
--- a/drivers/xen/gntdev.c
+++ b/drivers/xen/gntdev.c
@@ -1066,6 +1066,13 @@ static int gntdev_mmap(struct file *flip, struct vm_area_struct *vma)
 	if ((vma->vm_flags & VM_WRITE) && !(vma->vm_flags & VM_SHARED))
 		return -EINVAL;
 
+	/*
+	 * Private mappings of foreign grant pages must never become
+	 * writable: they would take the COW path on granted pages.
+	 */
+	if (!(vma->vm_flags & VM_SHARED))
+		vm_flags_clear(vma, VM_MAYWRITE);
+
 	pr_debug("map %d+%d at %lx (pgoff %lx)\n",
 		 index, count, vma->vm_start, vma->vm_pgoff);