tls: don't leave a full plaintext sk_msg ring unpushed

Summary

In the Linux kernel, the following vulnerability has been resolved: tls: don't leave a full plaintext sk_msg ring unpushed When the copy path in tls_sw_sendmsg_locked() adds the fragment that fills the plaintext sk_msg ring, it does not set full_record, so the record is left full and unpushed. A later splice() then adds to an already full ring: sk_msg_page_add() has no fullness check of its own, so sg.end wraps onto sg.start and the ring appears empty. Fragments added after that overwrite live entries, and sg.size no longer matches what is reachable between sg.start and sg.end, so pushing the record runs the scatterwalk off the end of the scatterlist. An unprivileged user can trigger this on a loopback TCP socket with the "tls" ULP attached: BUG: kernel NULL pointer dereference, address: 0000000000000008 RIP: 0010:memcpy_from_scatterwalk+0x32/0xc0 Call Trace: skcipher_walk_next+0x1d1/0x2c0 gcm_encrypt_aesni_avx+0x1e9/0x220 bpf_exec_tx_verdict+0x3bb/0x860 tls_sw_sendmsg+0xa1a/0xca0 __sys_sendto+0x1da/0x1f0 Set full_record in the copy path when the ring becomes full, and push a record that is already full on entry to the sendmsg loop.