High [CVE-2026-74611] restore msg_iter before TLS 1.3 optimistic retry

In the Linux kernel, the following vulnerability has been resolved: tls: rx: restore msg_iter before TLS 1.3 optimistic retry tls_decrypt_sg() advances msg->msg_iter when it maps user pages for the optimistic TLS 1.3 zero-copy path.

If the decrypted record turns out not to be unpadded application data, tls_decrypt_sw() retries into a kernel skb, but leaves the iterator advanced. The subsequent copy from the skb then writes decrypted bytes again at a later point in the caller iovecs while recvmsg() reports only the post-retry length.

A TLS peer can trigger this after the receiver enables TLS_RX_EXPECT_NO_PAD. Revert the iterator by the number of bytes consumed by the optimistic mapping before retrying without zero-copy.

Add a selftest which sends a TLS 1.3 control record with TLS_RX_EXPECT_NO_PAD enabled and verifies that recvmsg() does not overwrite later iovecs beyond the returned length. A flaw was found in the Linux kernel's Transport Layer Security (TLS) 1.3 implementation.

When processing TLS 1.3 records with optimistic zero-copy enabled, the tls_decrypt_sg() function fails to properly restore the message iterator during a retry. A remote attacker, by sending a specially crafted TLS 1.3 control record, can exploit this to cause the kernel to write decrypted data to unintended memory locations.

This can lead to data corruption or potentially information disclosure.