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).

Reported-by: Abdifatah Suruur <suruurism@gmail.com>

Fixes: 2050327242269 ("ptp: Add support for the AMZNC10C vmclock device")
Cc: stable@vger.kernel.org
Signed-off-by: Abdifatah Suruur <suruurism@gmail.com>

---
--- a/drivers/ptp/ptp_vmclock.c
+++ b/drivers/ptp/ptp_vmclock.c
@@ -372,6 +372,12 @@
 	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;