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: 2050327242 ("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>
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