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Keep UBX-NAV-SIG frames longer than the receive buffer #11921

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Keep UBX-NAV-SIG frames longer than the receive buffer #11921

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dd087e9
gps: keep UBX-NAV-SIG frames longer than the receive buffer
Sep 10, 2026
ad36673
Reject inconsistent oversized UBX navigation lengths
Raffi1202 Sep 11, 2026
92b479f
gps: reduce the oversized UBX frame change to the parser
Raffi1202 Oct 2, 2026
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30 changes: 25 additions & 5 deletions src/main/io/gps_ublox.c
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Original file line number Diff line number Diff line change
Expand Up @@ -897,11 +897,16 @@ static bool gpsNewFrameUBLOX(uint8_t data)
_step++;
_ck_b += (_ck_a += data); // checksum byte
_payload_length |= (uint16_t)(data << 8);
if (_payload_length > MAX_UBLOX_PAYLOAD_SIZE ) {
// we can't receive the whole packet, just log the error and start searching for the next packet.
gpsStats.errors++;
_step = 0;
break;
if (_payload_length > MAX_UBLOX_PAYLOAD_SIZE) {
// Multi-band receivers send more NAV-SIG/NAV-SAT records than fit: keep the first ones, checksum all
const bool truncatable = (_class == CLASS_NAV) &&
(_msg_id == MSG_NAV_SIG || _msg_id == MSG_NAV_SAT) &&
(_payload_length <= UBLOX_MAX_ACCEPTED_PAYLOAD_SIZE);
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if (!truncatable) {
gpsStats.errors++;
_step = 0;
break;
}
}
// prepare to receive payload
_payload_counter = 0;
Expand All @@ -914,6 +919,16 @@ static bool gpsNewFrameUBLOX(uint8_t data)
if (_payload_counter < MAX_UBLOX_PAYLOAD_SIZE) {
_buffer.bytes[_payload_counter] = data;
}
if (_payload_length > MAX_UBLOX_PAYLOAD_SIZE && _payload_counter == 7) {
// A corrupt length would swallow the following frames: it must match the record count at byte 5
const uint16_t recordSize = (_msg_id == MSG_NAV_SIG) ? sizeof(ubx_nav_sig_info) : sizeof(ubx_nav_svinfo_channel);
const uint16_t expectedLength = 8 + _buffer.bytes[5] * recordSize;
if (_payload_length != expectedLength) {
gpsStats.errors++;
_step = 0;
break;
}
}
// NOTE: check counter BEFORE increasing so that a payload_size of 65535 is correctly handled. This can happen if garbage data is received.
if (_payload_counter == _payload_length - 1) {
_step++;
Expand Down Expand Up @@ -942,6 +957,11 @@ static bool gpsNewFrameUBLOX(uint8_t data)
break;
}

// Handlers must not look past the truncated payload
if (_payload_length > MAX_UBLOX_PAYLOAD_SIZE) {
_payload_length = MAX_UBLOX_PAYLOAD_SIZE;
}

if (gpsParseFrameUBLOX()) {
parsed = true;
}
Expand Down
2 changes: 2 additions & 0 deletions src/main/io/gps_ublox.h
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Original file line number Diff line number Diff line change
Expand Up @@ -36,10 +36,12 @@ extern "C" {
#endif
#define MAX_UBLOX_PAYLOAD_SIZE ((UBLOX_MAX_SIGNALS * 16) + 8) // UBX-NAV-SIG info would be UBLOX_MAX_SIGNALS * 16 + 8
#define UBLOX_BUFFER_SIZE MAX_UBLOX_PAYLOAD_SIZE
#define UBLOX_MAX_ACCEPTED_PAYLOAD_SIZE ((255 * 16) + 8) // numSigs is U1, no valid NAV-SIG/NAV-SAT is longer
#define UBLOX_SBAS_MESSAGE_LENGTH 16
#define GPS_CAPA_INTERVAL 5000

STATIC_ASSERT(MAX_UBLOX_PAYLOAD_SIZE >= 256, ubx_size_too_small);
STATIC_ASSERT(UBLOX_MAX_ACCEPTED_PAYLOAD_SIZE >= MAX_UBLOX_PAYLOAD_SIZE, ubx_accepted_size_too_small);

#define UBX_DYNMODEL_PORTABLE 0
#define UBX_DYNMODEL_STATIONARY 2
Expand Down
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