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Workaround TLS fragmented records #32

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merged 1 commit into from
Nov 21, 2023
Merged

Workaround TLS fragmented records #32

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1 change: 1 addition & 0 deletions .gitignore
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Original file line number Diff line number Diff line change
Expand Up @@ -4,3 +4,4 @@
CMakeCache.txt
src/.vscode/
*.o
*test_tls
2 changes: 1 addition & 1 deletion src/netguard/include/util.h
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Original file line number Diff line number Diff line change
Expand Up @@ -7,7 +7,7 @@ uint16_t calc_checksum(uint16_t start, const uint8_t *buffer, size_t length);

int compare_u32(uint32_t seq1, uint32_t seq2);

int sdk_int(JNIEnv *env);
//int sdk_int(JNIEnv *env);

void hex2bytes(const char *hex, uint8_t *buffer);

Expand Down
50 changes: 27 additions & 23 deletions src/netguard/tls_parser.c
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Original file line number Diff line number Diff line change
Expand Up @@ -10,6 +10,7 @@
#include <netinet/ip6.h>
#include "platform.h"
#include "tls.h"
#include "util.h"

static int parse_tls_server_name(const uint8_t *data, const size_t data_len, char *server_name);
static int parse_extensions(const uint8_t*, size_t, char *);
Expand Down Expand Up @@ -69,19 +70,18 @@ static int parse_tls_server_name(const uint8_t *data, const size_t data_len, cha
}

/* TLS record length */
// uint16_t len = ((size_t)data[3] << 8) + (size_t)data[4] + TLS_HEADER_LEN;
size_t len = ntohs(*((uint16_t *) (data + 3))) + TLS_HEADER_LEN;
// data_len = MIN(len, data_len);
log_print(PLATFORM_LOG_PRIORITY_INFO, "data len %d, record len %d\n", data_len, len);
if (data_len < len) {
// purposely don't return as we have checks later on
log_print(PLATFORM_LOG_PRIORITY_WARN, "TLS data length smaller than expected, proceed anyways");
}

/* handshake */
size_t pos = TLS_HEADER_LEN;
// if (pos + 1 > data_len) {
// return -5;
// }
if (pos + 1 > data_len) {
return -5;
}

if (data[pos] != 0x1) {
// not a client hello
Expand All @@ -98,18 +98,17 @@ static int parse_tls_server_name(const uint8_t *data, const size_t data_len, cha
pos += 38;

// Session ID
// if (pos + 1 > data_len) return -7;
if (pos + 1 > data_len) return -7;
len = (size_t)data[pos];
pos += 1 + len;

/* Cipher Suites */
// if (pos + 2 > data_len) return -8;
// len = ((size_t)data[pos] << 8) + (size_t)data[pos + 1];
if (pos + 2 > data_len) return -8;
len = ntohs(*((uint16_t *) (data + pos)));
pos += 2 + len;

/* Compression Methods */
// if (pos + 1 > data_len) return -9;
if (pos + 1 > data_len) return -9;
len = (size_t)data[pos];
pos += 1 + len;

Expand All @@ -119,16 +118,17 @@ static int parse_tls_server_name(const uint8_t *data, const size_t data_len, cha
}

/* Extensions */
// if (pos + 2 > data_len) {
// return -11;
// }
// len = ((size_t)data[pos] << 8) + (size_t)data[pos + 1];
if (pos + 2 > data_len) {
return -11;
}
len = ntohs(*((uint16_t *) (data + pos)));
pos += 2;

// if (pos + len > data_len) {
if (pos + len > data_len) {
// Possibly a TLS fragmented record, continue anyways to see if we find SNI in the fragment
log_print(PLATFORM_LOG_PRIORITY_WARN, "Out of bounds at extensions length, pos(%d) + len(%d) > data_len(%d)", pos, len, data_len);
// return -12;
// }
}
return parse_extensions(data + pos, len, server_name);
}

Expand All @@ -139,15 +139,14 @@ static int parse_extensions(const uint8_t *data, size_t data_len, char *hostname
/* Parse each 4 bytes for the extension header */
while (pos + 4 <= data_len) {
/* Extension Length */
// len = ((size_t)data[pos + 2] << 8) +(size_t)data[pos + 3];
len = ntohs(*((uint16_t *) (data + pos + 2)));

/* Check if it's a server name extension */
if (data[pos] == 0x00 && data[pos + 1] == 0x00) {
/* There can be only one extension of each type, so we break
our state and move p to beinnging of the extension here */
// if (pos + 4 + len > data_len)
// return -20;
if (pos + 4 + len > data_len)
return -20;
return parse_server_name_extension(data + pos + 4, len, hostname);
}
pos += 4 + len; /* Advance to the next extension header */
Expand All @@ -164,12 +163,11 @@ static int parse_server_name_extension(const uint8_t *data, size_t data_len, cha
size_t len;

while (pos + 3 < data_len) {
// len = ((size_t)data[pos + 1] << 8) + (size_t)data[pos + 2];
len = ntohs(*((uint16_t *) (data + pos + 1)));

// if (pos + 3 + len > data_len) {
// return -30;
// }
if (pos + 3 + len > data_len) {
return -30;
}

switch (data[pos]) { /* name type */
case 0x00: /* host_name */
Expand All @@ -180,7 +178,13 @@ static int parse_server_name_extension(const uint8_t *data, size_t data_len, cha
}
strncpy(hostname, (const char *)(data + pos + 3), len);
(hostname)[len] = '\0';
return len;
if (is_valid_utf8(hostname)) {
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This is a check we had before that got lost when we re-wrote the TLS parsing.
We've seen some instances where the hostname is not UTF-8. The JNI interface (is_domain_blocked) uses NewStringUTF to pass the domain name to JVM, this is just to ensure we never get there with non UTF string.

return len;
} else {
log_print(PLATFORM_LOG_PRIORITY_WARN, "invalid UTF-8");
*hostname = 0;
return -34;
}
default:
log_print(PLATFORM_LOG_PRIORITY_DEBUG, "Unknown server name extension name type: %d", data[pos]);
}
Expand Down
2 changes: 1 addition & 1 deletion src/test/Makefile
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Original file line number Diff line number Diff line change
@@ -1,7 +1,7 @@
CC = gcc
CFLAGS = -Wall -Wimplicit-function-declaration -I../netguard/include

SRC = test_tls.c ../netguard/tls_parser.c
SRC = test_tls.c stubs.c ../netguard/tls_parser.c
OBJ = $(SRC:.c=.o)
EXECUTABLE = test_tls

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3 changes: 3 additions & 0 deletions src/test/stubs.c
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@@ -0,0 +1,3 @@
int is_valid_utf8(const char *str) {
return 1;
}
42 changes: 40 additions & 2 deletions src/test/test_tls.c
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Original file line number Diff line number Diff line change
Expand Up @@ -478,6 +478,28 @@ const unsigned char wrong_sni_length[] = {
0x6c, 0x6f, 0x63, 0x61, 0x6c, 0x68, 0x6f, 0x73, 0x74
};

// Fragmented SNI
const unsigned char fragmentedSNI1[] = {
0x16, 0x3, 0x1, 0x0, 0x6e, 0x1, 0x0, 0x0,
0x6d, 0x3, 0x3, 0x0, 0x0, 0x0, 0x0, 0x0,
0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,
0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,
0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,
0x0, 0x0, 0x0, 0x0, 0x0, 0x4, 0x0, 0x2f,
0x0, 0xff, 0x1, 0x0, 0x0, 0x40, 0x0, 0x0,
0x0, 0xe, 0x0, 0xc, 0x0, 0x0, 0x9, 0x6c,
0x6f, 0x63, 0x61, 0x6c, 0x68, 0x6f, 0x73, 0x74,
0x0, 0xd, 0x0, 0xc, 0x0, 0xa, 0x6, 0x1,
0x5, 0x1, 0x4, 0x1, 0x3, 0x1, 0x2, 0x1,
0x0, 0x32, 0x0, 0x1a, 0x0, 0x18, 0x6, 0x1,
0x5, 0x1, 0x4, 0x1, 0x3, 0x1, 0x2, 0x1,
0x1, 0x1, 0x8, 0x4, 0x8, 0x5, 0x8, 0x6,
0x8, 0x9, 0x8
};
const unsigned char fragmentedSNI2[] = {
0x16, 0x3, 0x1, 0x0, 0x3, 0xa, 0x8, 0xb
};


int main() {
uint8_t *pkt = (uint8_t *)good_data_1;
Expand Down Expand Up @@ -547,7 +569,7 @@ int main() {
error = get_server_name(pkt, sizeof(bad_data_2), pkt, sn);
assert(strcmp("localhost", sn) != 0);
assert(strlen(sn) == 0);
assert(error == -31);
assert(error == -30);

pkt = (uint8_t *)bad_data_3;
memset(sn, 0, FQDN_LENGTH);
Expand All @@ -563,7 +585,23 @@ int main() {
error = get_server_name(pkt, sizeof(wrong_sni_length), pkt, sn);
assert(strcmp("localhost", sn) != 0);
assert(strlen(sn) == 0);
assert(error == -33);
assert(error == -30);

pkt = (uint8_t *)fragmentedSNI2;
memset(sn, 0, FQDN_LENGTH);
*sn = 0;
error = get_server_name(pkt, sizeof(fragmentedSNI2), pkt, sn);
assert(strcmp("localhost", sn) != 0);
assert(strlen(sn) == 0);
assert(error == -6);

pkt = (uint8_t *)fragmentedSNI1;
memset(sn, 0, FQDN_LENGTH);
*sn = 0;
error = get_server_name(pkt, sizeof(fragmentedSNI1), pkt, sn);
assert(strcmp("localhost", sn) == 0);
assert(strlen(sn) == 9);
assert(error == 9);

return 0;
}
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