ptp: vmclock: prevent read-only mappings from becoming writable

vmclock_miscdev_mmap() rejects writable mappings of the shared vmclock
ABI page with -EROFS, but leaves VM_MAYWRITE set.  Userspace can map the
page read-only and then upgrade it to writable with mprotect(), after
which the guest can corrupt the host-written timekeeping data (sequence
counter, UTC time, TSC offset) that the vmclock ABI defines as read-only.

Clear VM_MAYWRITE on the read-only path so the mapping cannot be
upgraded, as i915 does for its read-only objects and as fixed in drm/vc4
(CVE-2026-68445) and drm/panthor (CVE-2024-53071).

Cc: [email protected]
Fixes: 2050327242 ("ptp: Add support for the AMZNC10C 'vmclock' device")
Signed-off-by: Abdifatah Suruur <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

drivers/ptp/ptp_vmclock.c

@@ -372,6 +372,12 @@ static int vmclock_miscdev_mmap(struct file *fp, struct vm_area_struct *vma)
 	if ((vma->vm_flags & (VM_READ|VM_WRITE)) != VM_READ)
 		return -EROFS;
 
+	/*
+	 * Restrict the read-only mapping so it cannot be upgraded to
+	 * writable later with mprotect().
+	 */
+	vm_flags_clear(vma, VM_MAYWRITE);
+
 	if (vma->vm_end - vma->vm_start != PAGE_SIZE || vma->vm_pgoff)
 		return -EINVAL;