[PATCH net-next v4 1/6] tls: block decryption when a rekey is pending

[Sabrina Dubroca <sd@xxxxxxxxxxxxxxx>]

When a TLS handshake record carrying a KeyUpdate message is received,
all subsequent records will be encrypted with a new key. We need to
stop decrypting incoming records with the old key, and wait until
userspace provides a new key.

Make a note of this in the RX context just after decrypting that
record, and stop recvmsg/splice calls with EKEYEXPIRED until the new
key is available.

key_update_pending can't be combined with the existing bitfield,
because we will read it locklessly in ->poll.

v3:
 - move key_update_pending check into tls_rx_rec_wait (Jakub)
 - TLS_RECORD_TYPE_HANDSHAKE was added to include/net/tls_prot.h by
   the tls handshake series, drop that from this patch
 - move key_update_pending into an existing hole

v4:
 - flip TLS_RECORD_TYPE_HANDSHAKE test and use likely() (Jakub)
 - pass ctx rather than sk to tls_check_pending_rekey (Jakub)
 - use WRITE_ONCE to set key_update_pending to pair with ->poll's
   lockless read

Signed-off-by: Sabrina Dubroca <sd@xxxxxxxxxxxxxxx>
---
 include/net/tls.h |  3 +++
 net/tls/tls_sw.c  | 35 +++++++++++++++++++++++++++++++++++
 2 files changed, 38 insertions(+)

diff --git a/include/net/tls.h b/include/net/tls.h
index 3a33924db2bc..870e4421c599 100644
--- a/include/net/tls.h
+++ b/include/net/tls.h
@@ -59,6 +59,8 @@ struct tls_rec;
 
 #define TLS_CRYPTO_INFO_READY(info)	((info)->cipher_type)
 
+#define TLS_HANDSHAKE_KEYUPDATE		24	/* rfc8446 B.3: Key update */
+
 #define TLS_AAD_SPACE_SIZE		13
 
 #define TLS_MAX_IV_SIZE			16
@@ -130,6 +132,7 @@ struct tls_sw_context_rx {
 	u8 async_capable:1;
 	u8 zc_capable:1;
 	u8 reader_contended:1;
+	bool key_update_pending;
 
 	struct tls_strparser strp;
 
diff --git a/net/tls/tls_sw.c b/net/tls/tls_sw.c
index bbf26cc4f6ee..db98710c4810 100644
--- a/net/tls/tls_sw.c
+++ b/net/tls/tls_sw.c
@@ -1314,6 +1314,10 @@ tls_rx_rec_wait(struct sock *sk, struct sk_psock *psock, bool nonblock,
 	int ret = 0;
 	long timeo;
 
+	/* a rekey is pending, let userspace deal with it */
+	if (unlikely(ctx->key_update_pending))
+		return -EKEYEXPIRED;
+
 	timeo = sock_rcvtimeo(sk, nonblock);
 
 	while (!tls_strp_msg_ready(ctx)) {
@@ -1720,6 +1724,32 @@ tls_decrypt_device(struct sock *sk, struct msghdr *msg,
 	return 1;
 }
 
+static int tls_check_pending_rekey(struct tls_context *ctx, struct sk_buff *skb)
+{
+	const struct tls_msg *tlm = tls_msg(skb);
+	const struct strp_msg *rxm = strp_msg(skb);
+	char hs_type;
+	int err;
+
+	if (likely(tlm->control != TLS_RECORD_TYPE_HANDSHAKE))
+		return 0;
+
+	if (rxm->full_len < 1)
+		return -EINVAL;
+
+	err = skb_copy_bits(skb, rxm->offset, &hs_type, 1);
+	if (err < 0)
+		return err;
+
+	if (hs_type == TLS_HANDSHAKE_KEYUPDATE) {
+		struct tls_sw_context_rx *rx_ctx = ctx->priv_ctx_rx;
+
+		WRITE_ONCE(rx_ctx->key_update_pending, true);
+	}
+
+	return 0;
+}
+
 static int tls_rx_one_record(struct sock *sk, struct msghdr *msg,
 			     struct tls_decrypt_arg *darg)
 {
@@ -1739,6 +1769,10 @@ static int tls_rx_one_record(struct sock *sk, struct msghdr *msg,
 	rxm->full_len -= prot->overhead_size;
 	tls_advance_record_sn(sk, prot, &tls_ctx->rx);
 
+	err = tls_check_pending_rekey(tls_ctx, darg->skb);
+	if (err < 0)
+		return err;
+
 	return 0;
 }
 
@@ -2719,6 +2753,7 @@ int tls_set_sw_offload(struct sock *sk, int tx)
 		crypto_info = &ctx->crypto_recv.info;
 		cctx = &ctx->rx;
 		aead = &sw_ctx_rx->aead_recv;
+		sw_ctx_rx->key_update_pending = false;
 	}
 
 	cipher_desc = get_cipher_desc(crypto_info->cipher_type);
-- 
2.47.0