[PATCH net] tls: fix NULL deref on tls_sw_splice_eof() with empty record

[Jann Horn <jannh@google.com>]

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().

Fixes: df720d288dbb ("tls/sw: Use splice_eof() to flush")
Cc: stable@vger.kernel.org
Reported-by: syzbot+40d43509a099ea756317@syzkaller.appspotmail.com
Signed-off-by: Jann Horn <jannh@google.com>
---
Note: syzkaller did not find a reliable reproducer for this; I wrote
my own reproducer that reliably hits the crash on an unpatched kernel,
and I've verified that the crash goes away with the patch applied.

 net/tls/tls_sw.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/tls/tls_sw.c b/net/tls/tls_sw.c
index a78e8e722409..316f76187962 100644
--- a/net/tls/tls_sw.c
+++ b/net/tls/tls_sw.c
@@ -1232,11 +1232,14 @@ void tls_sw_splice_eof(struct socket *sock)
 	lock_sock(sk);
 
 retry:
+	/* same checks as in tls_sw_push_pending_record() */
 	rec = ctx->open_rec;
 	if (!rec)
 		goto unlock;
 
 	msg_pl = &rec->msg_plaintext;
+	if (msg_pl->sg.size == 0)
+		goto unlock;
 
 	/* Check the BPF advisor and perform transmission. */
 	ret = bpf_exec_tx_verdict(msg_pl, sk, false, TLS_RECORD_TYPE_DATA,

base-commit: 98b1cc82c4affc16f5598d4fa14b1858671b2263
-- 
2.43.0.rc1.413.gea7ed67945-goog