Ensure that the largest_pn values are migrated to our channel qrx

Recently, our overnight QUIC interop runs began failing in CI when an
openssl server was tested against an ngtcp2 client:
https://github.com/openssl/openssl/actions/runs/16739736813

The underlying cause bears some explination for historical purposes

The problem began happening with a recent update to ngtcp2 in which
ngtcp2 updated its wolfssl tls backend to support ML-KEM, which caused
ngtcp to emit a client hello message that offered several groups
(including X25519MLKEM768) but only provided a keyshare for x25519.
This in turn triggered the openssl server to respond with a hello retry
request (HRR), requesting an ML-KEM keyshare instead, which ngtcp2
obliged. However all subsequent frames from the client were discarded by
the server, due to failing packet body decryption.

The problem was tracked down to a mismatch in the initial vectors used
by the client and server, leading to an AEAD tag mismatch.

Packet protection keys generate their IV's in QUIC by xoring the packet
number of the received frame with the base IV as derived via HKDF in the
tls layer.

The underlying problem was that openssl hit a very odd corner case with
how we compute the packet number of the received frame.  To save space,
QUIC encodes packet numbers using a variable length integer, and only
sends the changed bits in the packet number.  This requires that the
receiver (openssl) store the largest received pn of the connection,
which we nominally do.

However, in default_port_packet_handler (where QUIC frames are processed
prior to having an established channel allocated) we use a temporary qrx
to validate the packet protection of those frames.  This temporary qrx
may be incorporated into the channel in some cases, but is not in the
case of a valid frame that generates an HRR at the TLS layer.  In this
case, the channel allocates its own qrx independently.  When this
occurs, the largest_pn value of the temporary qrx is lost, and
subsequent frames are unable to be received, as the newly allocated qrx
belives that the larges_pn for a given pn_space is 0, rather than the
value received in the initial frame (which was a complete 32 bit value,
rather than just the changed lower 8 bits).  As a result the IV
construction produced the wrong value, and the decrypt failed on those
subsequent frames.

Up to this point, that wasn't even a problem, as most quic
implementations start their packet numbering at 0, so the next packet
could still have its packet number computed properly.  The combination
of ngtcp using large random values for initial packet numbers, along
with the HRR triggering a separate qrx creation on a channel led to the
discovery of this discrepancy.

The fix seems pretty straightforward.  When we detect in
port_default_packet_handler, that we have a separate qrx in the new
channel, we migrate processed packets from the temporary qrx to the
canonical channel qrx.  In addition to doing that, we also need to
migrate the largest_pn array from the temporary qrx to the channel_qrx
so that subsequent frame reception is guaranteed to compute the received
frame packet number properly, and as such, compute the proper IV for
packet protection decryption.

Fixes openssl/project#1296

Reviewed-by: Saša Nedvědický <sashan@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/28189)

include/internal/quic_record_rx.h

@@ -167,6 +167,17 @@ int ossl_qrx_provide_secret(OSSL_QRX              *qrx,
                             const unsigned char   *secret,
                             size_t                 secret_len);
 
+/*
+ * Utility function to update the pn space from a src to a dst qrx.
+ * Occasionally we use a temporary qrx to do packet validation on quic frames
+ * that are not yet associated with a channel, and in the event a validation is
+ * successful AND we allocate a new qrx for the newly created channel, we need
+ * to migrate the largest_pn values recorded in the tmp qrx to the channel qrx.
+ * If we don't then PN decoding fails in cases where the initial PN is a large value.
+ * This function does that migration for us
+ */
+void ossl_qrx_update_pn_space(OSSL_QRX *src, OSSL_QRX *dst);
+
 /*
  * Informs the QRX that it can now discard key material for a given EL. The QRX
  * will no longer be able to process incoming packets received at that

ssl/quic/quic_port.c

@@ -1691,6 +1691,7 @@ static void port_default_packet_handler(QUIC_URXE *e, void *arg,
          */
         while (ossl_qrx_read_pkt(qrx_src, &qrx_pkt) == 1)
             ossl_quic_channel_inject_pkt(new_ch, qrx_pkt);
+        ossl_qrx_update_pn_space(qrx_src, new_ch->qrx);
     }
 
     /*

ssl/quic/quic_record_rx.c

@@ -237,6 +237,16 @@ static void qrx_cleanup_urxl(OSSL_QRX *qrx, QUIC_URXE_LIST *l)
     }
 }
 
+void ossl_qrx_update_pn_space(OSSL_QRX *src, OSSL_QRX *dst)
+{
+    size_t i;
+
+    for (i = 0; i < QUIC_PN_SPACE_NUM; i++)
+        dst->largest_pn[i] = src->largest_pn[i];
+
+    return;
+}
+
 void ossl_qrx_free(OSSL_QRX *qrx)
 {
     uint32_t i;