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MINOR: quic: centralize padding for HP sampling on packet building

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The below patch has simplified INITIAL padding on emission. Now,
qc_prep_pkts() is responsible to activate padding for this case, and
there is no more special case in qc_do_build_pkt() needed.

  commit 8bc339a6ad
  BUG/MAJOR: quic: fix INITIAL padding with probing packet only

However, qc_do_build_pkt() may still activate padding on its own, to
ensure that a packet is big enough so that header protection decryption
can be performed by the peer. HP decryption is performed by extracting a
sample from the ciphered packet, starting 4 bytes after PN offset.
Sample length is 16 bytes as defined by TLS algos used by QUIC. Thus, a
QUIC sender must ensures that length of packet number plus payload
fields to be at least 4 bytes long. This is enough given that each
packet is completed by a 16 bytes AEAD tag which can be part of the HP
sample.

This patch simplifies qc_do_build_pkt() by centralizing padding for this
case in a single location. This is performed at the end of the function
after payload is completed. The code is thus simpler.

This is not a bug. However, it may be interesting to backport this patch
up to 2.6, as qc_do_build_pkt() is a tedious function, in particular
when dealing with padding generation, thus it may benefit greatly from
simplification.
This commit is contained in:
Amaury Denoyelle 2025-08-12 10:40:06 +02:00
parent 7d554ca629
commit 1529ec1a25
2 changed files with 26 additions and 15 deletions
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3
include/haproxy/quic_conn-t.h
View file

@ -145,6 +145,9 @@ enum quic_pkt_type {
#define QUIC_PACKET_PNL_BITMASK 0x03
#define QUIC_PACKET_PN_MAXLEN 4
/* TLS algo supported by QUIC uses a 16-bytes sample for HP. */
#define QUIC_HP_SAMPLE_LEN 16
/*
* 0 1 2 3
* 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

38
src/quic_tx.c
View file

@ -1950,13 +1950,6 @@ static int qc_do_build_pkt(unsigned char *pos, const unsigned char *end,
add_ping_frm = 1;
len += 1;
dglen += 1;
/* Ensure packet is big enough so that header protection sample
* decryption can be performed. Note that +1 is for the PING
* frame.
*/
if (!padding && *pn_len + 1 < QUIC_PACKET_PN_MAXLEN)
len += padding_len = QUIC_PACKET_PN_MAXLEN - *pn_len - 1;
}
/* Handle Initial packet padding if necessary. */
@ -1974,14 +1967,29 @@ static int qc_do_build_pkt(unsigned char *pos, const unsigned char *end,
}
}
}
else if (len_frms && len_frms < QUIC_PACKET_PN_MAXLEN) {
len += padding_len = QUIC_PACKET_PN_MAXLEN - len_frms;
}
/* TODO qc_do_build_pkt() must rely on its <probe> argument instead of using QEL <pto_probe> field. */
else if (LIST_ISEMPTY(&frm_list) && !cc && !qel->pktns->tx.pto_probe) {
/* If there is no frame at all to follow, add at least a PADDING frame. */
if (!ack_frm_len)
len += padding_len = QUIC_PACKET_PN_MAXLEN - *pn_len;
/* RFC 9001 5.4.2. Header Protection Sample
*
* To ensure that sufficient data is available for sampling, packets are
* padded so that the combined lengths of the encoded packet number and
* protected payload is at least 4 bytes longer than the sample required
* for header protection. The cipher suites defined in [TLS13] -- other
* than TLS_AES_128_CCM_8_SHA256, for which a header protection scheme
* is not defined in this document -- have 16-byte expansions and 16-
* byte header protection samples. This results in needing at least 3
* bytes of frames in the unprotected payload if the packet number is
* encoded on a single byte, or 2 bytes of frames for a 2-byte packet
* number encoding.
*/
/* Add padding if packet is too small for HP sampling as specified
* above. QUIC TLS algos relies on 16 bytes sample extracted 4 bytes
* after PN offset. Thus, pn and payload must be at least 4 bytes long,
* so that the sample will be extracted as the AEAD tag.
*/
if (*pn_len + len < QUIC_PACKET_PN_MAXLEN + QUIC_HP_SAMPLE_LEN) {
padding_len = QUIC_PACKET_PN_MAXLEN + QUIC_HP_SAMPLE_LEN - (*pn_len + len);
len += padding_len;
}
if (pkt->type != QUIC_PACKET_TYPE_SHORT && !quic_enc_int(&pos, end, len))