CVE-2026-72025: s390/monwriter: Reject buffer reuse with different data length

In the Linux kernel, the following vulnerability has been resolved:

s390/monwriter: Reject buffer reuse with different data length

When data buffers are reused, e.g. for interval sample records, the first record determines the data length, and the size of the buffer for user copy. Current monwriter code does not check if the data length was changed for subsequent records, which also would never happen for valid user programs.

However, a malicious user could change the data length, resulting in out of bounds user copy to the kernel buffer, and memory corruption. By default, the monwriter misc device is created with root-only permissions, so practical impact is typically low.

Fix this by checking for changed data length and rejecting such records.