Bug 1224998 (CVE-2023-52767)

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

tls: fix NULL deref on tls_sw_splice_eof() with empty record

syzkaller discovered that if tls_sw_splice_eof() is executed as part of
sendfile() when the plaintext/ciphertext sk_msg are empty, the send path
gets confused because the empty ciphertext buffer does not have enough
space for the encryption overhead. This causes tls_push_record() to go on
the `split = true` path (which is only supposed to be used when interacting
with an attached BPF program), and then get further confused and hit the
tls_merge_open_record() path, which then assumes that there must be at
least one populated buffer element, leading to a NULL deref.

It is possible to have empty plaintext/ciphertext buffers if we previously
bailed from tls_sw_sendmsg_locked() via the tls_trim_both_msgs() path.
tls_sw_push_pending_record() already handles this case correctly; let's do
the same check in tls_sw_splice_eof().

References:
http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2023-52767
https://git.kernel.org/pub/scm/linux/security/vulns.git/plain/cve/published/2023/CVE-2023-52767.mbox
https://git.kernel.org/stable/c/944900fe2736c07288efe2d9394db4d3ca23f2c9
https://git.kernel.org/stable/c/2214e2bb5489145aba944874d0ee1652a0a63dc8
https://git.kernel.org/stable/c/53f2cb491b500897a619ff6abd72f565933760f0
https://www.cve.org/CVERecord?id=CVE-2023-52767