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internal: GetMaxPlainTextSize: precise pad size when adjusting for MTU

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This commit is contained in:
Marco Oliverio
2025-12-22 12:46:20 +01:00
parent 1200efdeb3
commit aa4fb5d3e5
1 changed files with 31 additions and 5 deletions
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36
src/internal.c
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@@ -41862,13 +41862,39 @@ int wolfSSL_GetMaxPlaintextSize(WOLFSSL *ssl)
#else
mtu = MAX_MTU;
#endif
if (recordSz > mtu) {
maxFrag -= (recordSz - mtu);
recordSz = wolfssl_i_GetRecordSize(ssl, maxFrag, IsEncryptionOn(ssl, 1));
/* record size of maxFrag fits in MTU */
if (recordSz <= mtu) {
return maxFrag;
}
/* adjust plaintext size to fit in MTU */
maxFrag -= (recordSz - mtu);
if (maxFrag <= 0) {
WOLFSSL_MSG("MTU too small for any plaintext");
return DTLS_SIZE_ERROR;
}
#ifndef WOLFSSL_AEAD_ONLY
/* account for padding if using block cipher*/
if (ssl->specs.cipher_type == block)
maxFrag -= ssl->specs.block_size;
/* For block ciphers, reducing maxFrag may change padding alignment,
* causing the record to still exceed MTU. Iterate to find exact fit.
* Converges in at most 2 iterations due to bounded padding variance. */
if (ssl->specs.cipher_type == block) {
int iter;
for (iter = 0; iter < 2; iter++) {
recordSz = wolfssl_i_GetRecordSize(ssl, maxFrag,
IsEncryptionOn(ssl, 1));
if (recordSz <= mtu)
break;
maxFrag -= (recordSz - mtu);
}
if (recordSz > mtu) {
/* this should never happen */
WOLFSSL_MSG("Failed to fit record in MTU after padding adjust");
return DTLS_SIZE_ERROR;
}
}
#endif
}
#endif /* WOLFSSL_DTLS */