Medium [CVE-2026-64289] Set upper bounds on cache invalidation entry_num and entry_len

In the Linux kernel, the following vulnerability has been resolved: iommufd: Set upper bounds on cache invalidation entry_num and entry_len iommufd_hwpt_invalidate() takes a user-controlled entry_num and entry_len, each bounded only by U32_MAX.

An entry_len beyond the kernel's struct size makes the copy helper verify the extra bytes are zero, scanning that excess in one uninterruptible pass; a multi-gigabyte value over zeroed user memory trips the soft-lockup watchdog. A large entry_num is the other half, driving the backend invalidation loop with no reschedule.

The VT-d nested handler, for one, copies each entry and flushes caches per iteration, pinning the CPU on a non-preemptible kernel. Cap both in the ioctl. entry_len is held under PAGE_SIZE, above any request struct, and entry_num under 1 << 19, the order of a hardware invalidation queue and well beyond any real batch, bounding the per-call loop length.

In the `iommufd` component, the `iommufd_hwpt_invalidate()` function allows a local user to provide excessively large values for `entry_num` and `entry_len` during cache invalidation. This can lead to a Denial of Service (DoS) by causing the system to become unresponsive.

Specifically, large `entry_len` values can trigger a soft-lockup watchdog due to extensive memory scanning, while large `entry_num` values can cause an uninterruptible loop, pinning the CPU.