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

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

commit a5edadbae57e2298a56cf7a4e774a027905a331f upstream.

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: stable@vger.kernel.org
Fixes: 205032724226 ("ptp: Add support for the AMZNC10C 'vmclock' device")
Signed-off-by: Abdifatah Suruur <suruurism@gmail.com>
Link: https://patch.msgid.link/20260813174707.14809-1-suruurism@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>