CVE-2026-63830: net: skmsg: preserve sg.copy across SG transforms

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

net: skmsg: preserve sg.copy across SG transforms

The skmsg sg.copy bitmap is part of the scatterlist entry ownership state. A set bit tells skmsgcomputedatapointers() not to expose the entry through writable BPF ctx->data. This protects entries backed by pages that are not private to the skmsg, such as splice-backed file page-cache pages.

Several skmsg transform paths move, copy, split, or compact msg->sg.data[] entries without moving the matching sg.copy bit. This can make an externally backed entry arrive at a new slot with a clear copy bit. A later SKMSG verdict can then expose sgvirt(sge) as writable ctx->data and BPF stores can modify the original page cache.

Keep sg.copy synchronized with sg.data[] whenever entries are transferred, shifted, split, or copied into a new skmsg. Clear the bit when an entry is replaced by a newly allocated private page or freed. This covers the BPF pull/push/pop helpers, skmsgshiftleft/right(), skmsgxfer(), and tlssplitopenrecord(), including the partial tail entry created during TLS open-record splitting.