CVE-2025-21881 is a vulnerability in the Linux kernel's uprobes subsystem where the shared zeropage is not properly rejected in the uprobe_write_opcode() function. This flaw can cause kernel crashes due to incorrect reference counting and page state errors, leading to system instability. The issue arises from improper handling of zero page frame numbers (PFNs) and reference counts, which can trigger bad page state and bad RSS counter errors during process exit. The vulnerability was discovered through syzkaller fuzz testing and involves rare conditions related to zero folio pages.