fix double-free in tipc_buf_append()

Summary

In the Linux kernel, the following vulnerability has been resolved: tipc: fix double-free in tipc_buf_append() tipc_msg_validate() can potentially reallocate the skb it is validating, freeing the old one. In tipc_buf_append(), it was being called with a pointer to a local variable which was a copy of the caller's skb pointer. If the skb was reallocated and validation subsequently failed, the error handling path would free the original skb pointer, which had already been freed, leading to double-free. Fix this by checking if head now points to a newly allocated reassembled skb. If it does, reassign *headbuf for later freeing operations. A flaw was found in the Linux kernel's Transparent Inter-Process Communication (TIPC) module. This vulnerability, a double-free, occurs when the `tipc_buf_append()` function incorrectly handles memory after a socket buffer (skb) reallocation. An attacker could potentially exploit this to cause system instability or a denial of service. In some scenarios, this type of memory corruption could also lead to arbitrary code execution. Red Hat severity: Important — CVSS 7 (CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H). Weakness: CWE-763. No fixing RHSA erratum has published yet; monitor the Red Hat CVE page and patch when it ships.