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Don't depend on extension ordering to avoid an empty final extension.

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In order to work around server bugs (see https://crbug.com/363583) we
need to ensure that the final extension is not empty. Doing this by
fixing the order of extensions is a little error-prone. Instead, insert
a padding extension to ensure this as neeeded.

Change-Id: I90760f2e6735082386c484c956a470aef38ed109
Reviewed-on: https://boringssl-review.googlesource.com/31284
Commit-Queue: Adam Langley <agl@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
Reviewed-by: David Benjamin <davidben@google.com>
This commit is contained in:
Adam Langley 2018-08-23 08:58:49 -07:00 committed by CQ bot account: commit-bot@chromium.org
parent 23849f09af
commit 7f4f41fa81
1 changed files with 31 additions and 9 deletions
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40
ssl/t1_lib.cc
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@ -2928,9 +2928,6 @@ static const struct tls_extension kExtensions[] = {
ext_quic_transport_params_parse_clienthello, ext_quic_transport_params_parse_clienthello,
ext_quic_transport_params_add_serverhello, ext_quic_transport_params_add_serverhello,
}, },
// The final extension must be non-empty. WebSphere Application Server 7.0 is
// intolerant to the last extension being zero-length. See
// https://crbug.com/363583.
{ {
TLSEXT_TYPE_supported_groups, TLSEXT_TYPE_supported_groups,
NULL, NULL,
@ -3007,6 +3004,7 @@ bool ssl_add_clienthello_tlsext(SSL_HANDSHAKE *hs, CBB *out,
} }
} }
bool last_was_empty = false;
for (size_t i = 0; i < kNumExtensions; i++) { for (size_t i = 0; i < kNumExtensions; i++) {
const size_t len_before = CBB_len(&extensions); const size_t len_before = CBB_len(&extensions);
if (!kExtensions[i].add_clienthello(hs, &extensions)) { if (!kExtensions[i].add_clienthello(hs, &extensions)) {
@ -3015,9 +3013,13 @@ bool ssl_add_clienthello_tlsext(SSL_HANDSHAKE *hs, CBB *out,
return false; return false;
} }
if (CBB_len(&extensions) != len_before) { const size_t bytes_written = CBB_len(&extensions) - len_before;
if (bytes_written != 0) {
hs->extensions.sent |= (1u << i); hs->extensions.sent |= (1u << i);
} }
// If the difference in lengths is only four bytes then the extension had
// an empty body.
last_was_empty = (bytes_written == 4);
} }
if (ssl->ctx->grease_enabled) { if (ssl->ctx->grease_enabled) {
@ -3037,17 +3039,35 @@ bool ssl_add_clienthello_tlsext(SSL_HANDSHAKE *hs, CBB *out,
OPENSSL_PUT_ERROR(SSL, ERR_R_INTERNAL_ERROR); OPENSSL_PUT_ERROR(SSL, ERR_R_INTERNAL_ERROR);
return false; return false;
} }
last_was_empty = false;
} }
if (!SSL_is_dtls(ssl)) { if (!SSL_is_dtls(ssl)) {
size_t psk_extension_len = ext_pre_shared_key_clienthello_length(hs); size_t psk_extension_len = ext_pre_shared_key_clienthello_length(hs);
header_len += 2 + CBB_len(&extensions) + psk_extension_len; header_len += 2 + CBB_len(&extensions) + psk_extension_len;
size_t padding_len = 0;
// The final extension must be non-empty. WebSphere Application
// Server 7.0 is intolerant to the last extension being zero-length. See
// https://crbug.com/363583.
if (last_was_empty && psk_extension_len == 0) {
padding_len = 1;
// The addition of the padding extension may push us into the F5 bug.
header_len += 4 + padding_len;
}
// Add padding to workaround bugs in F5 terminators. See RFC 7685.
//
// NB: because this code works out the length of all existing extensions
// it MUST always appear last (save for any PSK extension).
if (header_len > 0xff && header_len < 0x200) { if (header_len > 0xff && header_len < 0x200) {
// Add padding to workaround bugs in F5 terminators. See RFC 7685. // If our calculations already included a padding extension, remove that
// // factor because we're about to change its length.
// NB: because this code works out the length of all existing extensions if (padding_len != 0) {
// it MUST always appear last. header_len -= 4 + padding_len;
size_t padding_len = 0x200 - header_len; }
padding_len = 0x200 - header_len;
// Extensions take at least four bytes to encode. Always include at least // Extensions take at least four bytes to encode. Always include at least
// one byte of data if including the extension. WebSphere Application // one byte of data if including the extension. WebSphere Application
// Server 7.0 is intolerant to the last extension being zero-length. See // Server 7.0 is intolerant to the last extension being zero-length. See
@ -3057,7 +3077,9 @@ bool ssl_add_clienthello_tlsext(SSL_HANDSHAKE *hs, CBB *out,
} else { } else {
padding_len = 1; padding_len = 1;
} }
}
if (padding_len != 0) {
uint8_t *padding_bytes; uint8_t *padding_bytes;
if (!CBB_add_u16(&extensions, TLSEXT_TYPE_padding) || if (!CBB_add_u16(&extensions, TLSEXT_TYPE_padding) ||
!CBB_add_u16(&extensions, padding_len) || !CBB_add_u16(&extensions, padding_len) ||