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Userspace can map the page read-only and then upgrade the mapping to writable with mprotect(), after which it can write to the shared RTC registers. Clear VM_MAYWRITE on the read-only path, 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: fbd8ae106850b ("char: add SGI UV mmtimer driver") Cc: stable@vger.kernel.org Signed-off-by: Abdifatah Suruur --- diff --git a/drivers/char/uv_mmtimer.c b/drivers/char/uv_mmtimer.c index 956ebe2080a58..c8b6040f7a14c 100644 --- a/drivers/char/uv_mmtimer.c +++ b/drivers/char/uv_mmtimer.c @@ -154,6 +154,13 @@ static int uv_mmtimer_mmap(struct file *file, struct vm_area_struct *vma) if (vma->vm_flags & VM_WRITE) return -EPERM; + /* + * The mmtimer page is a system-wide read-only register page. + * Prevent the mapping from being upgraded to writable with + * mprotect(). + */ + vm_flags_clear(vma, VM_MAYWRITE); + if (PAGE_SIZE > (1 << 16)) return -ENOSYS;