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Add additional object id sanity checks

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Ensure objects with impossibly large ids are ignored.
This commit is contained in:
m-holger 2024-07-01 18:11:51 +01:00
parent 42c511198b
commit 6d640c569a
5 changed files with 37 additions and 10 deletions
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3
include/qpdf/QPDF.hh
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@ -1502,6 +1502,9 @@ class QPDF
std::shared_ptr<EncryptionParameters> encp;
std::string pdf_version;
std::map<QPDFObjGen, QPDFXRefEntry> xref_table;
// Various tables are indexed by object id, with potential size id + 1
int xref_table_max_id{std::numeric_limits<int>::max() - 1};
qpdf_offset_t xref_table_max_offset{0};
std::set<int> deleted_objects;
std::map<QPDFObjGen, ObjCache> obj_cache;
std::set<QPDFObjGen> resolving;

39
libqpdf/QPDF.cc
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@ -441,6 +441,12 @@ QPDF::parse(char const* password)
// 30 characters to leave room for the startxref stuff.
m->file->seek(0, SEEK_END);
qpdf_offset_t end_offset = m->file->tell();
m->xref_table_max_offset = end_offset;
// Sanity check on object ids. All objects must appear in xref table / stream. In all realistic
// scenarios at least 3 bytes are required.
if (m->xref_table_max_id > m->xref_table_max_offset / 3) {
m->xref_table_max_id = static_cast<int>(m->xref_table_max_offset / 3);
}
qpdf_offset_t start_offset = (end_offset > 1054 ? end_offset - 1054 : 0);
PatternFinder sf(*this, &QPDF::findStartxref);
qpdf_offset_t xref_offset = 0;
@ -554,9 +560,6 @@ QPDF::reconstruct_xref(QPDFExc& e)
m->file->seek(0, SEEK_END);
qpdf_offset_t eof = m->file->tell();
// Sanity check on object ids. All objects must appear in xref table / stream. In all realistic
// scenarios at leat 3 bytes are required.
auto max_obj_id = eof / 3;
m->file->seek(0, SEEK_SET);
qpdf_offset_t line_start = 0;
// Don't allow very long tokens here during recovery.
@ -574,7 +577,7 @@ QPDF::reconstruct_xref(QPDFExc& e)
if ((t2.isInteger()) && (readToken(m->file, MAX_LEN).isWord("obj"))) {
int obj = QUtil::string_to_int(t1.getValue().c_str());
int gen = QUtil::string_to_int(t2.getValue().c_str());
if (obj <= max_obj_id) {
if (obj <= m->xref_table_max_id) {
insertReconstructedXrefEntry(obj, token_start, gen);
} else {
warn(damagedPDF(
@ -709,7 +712,7 @@ QPDF::read_xref(qpdf_offset_t xref_offset)
int size = m->trailer.getKey("/Size").getIntValueAsInt();
int max_obj = 0;
if (!m->xref_table.empty()) {
max_obj = (*(m->xref_table.rbegin())).first.getObj();
max_obj = m->xref_table.rbegin()->first.getObj();
}
if (!m->deleted_objects.empty()) {
max_obj = std::max(max_obj, *(m->deleted_objects.rbegin()));
@ -1262,11 +1265,21 @@ QPDF::insertXrefEntry(int obj, int f0, qpdf_offset_t f1, int f2)
// If there is already an entry for this object and generation in the table, it means that a
// later xref table has registered this object. Disregard this one.
if (obj > m->xref_table_max_id) {
// ignore impossibly large object ids or object ids > Size.
return;
}
if (m->deleted_objects.count(obj)) {
QTC::TC("qpdf", "QPDF xref deleted object");
return;
}
if (f0 == 2 && static_cast<int>(f1) == obj) {
warn(damagedPDF("xref stream", "self-referential object stream " + std::to_string(obj)));
return;
}
auto [iter, created] = m->xref_table.try_emplace(QPDFObjGen(obj, (f0 == 2 ? 0 : f2)));
if (!created) {
QTC::TC("qpdf", "QPDF xref reused object");
@ -1303,12 +1316,11 @@ QPDF::insertFreeXrefEntry(QPDFObjGen og)
void
QPDF::insertReconstructedXrefEntry(int obj, qpdf_offset_t f1, int f2)
{
// Various tables are indexed by object id, with potential size id + 1
constexpr static int max_id = std::numeric_limits<int>::max() - 1;
if (!(obj > 0 && obj <= max_id && 0 <= f2 && f2 < 65535)) {
if (!(obj > 0 && obj <= m->xref_table_max_id && 0 <= f2 && f2 < 65535)) {
QTC::TC("qpdf", "QPDF xref overwrite invalid objgen");
return;
}
QPDFObjGen og(obj, f2);
if (!m->deleted_objects.count(obj)) {
// deleted_objects stores the uncompressed objects removed from the xref table at the start
@ -1918,6 +1930,17 @@ QPDF::resolveObjectsInStream(int obj_stream_number)
int num = QUtil::string_to_int(tnum.getValue().c_str());
long long offset = QUtil::string_to_int(toffset.getValue().c_str());
if (num > m->xref_table_max_id) {
continue;
}
if (num == obj_stream_number) {
warn(damagedPDF(
input,
m->last_object_description,
input->getLastOffset(),
"object stream claims to contain itself"));
continue;
}
offsets[num] = toI(offset + first);
}

3
qpdf/qtest/qpdf/issue-118.out
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@ -1,4 +1,3 @@
WARNING: issue-118.pdf: can't find PDF header
WARNING: issue-118.pdf (offset 732): loop detected resolving object 2 0
WARNING: issue-118.pdf (xref stream: object 8 0, offset 732): supposed object stream 2 is not a stream
WARNING: issue-118.pdf (xref stream, offset 732): self-referential object stream 2
issue-118.pdf: unable to find /Root dictionary

1
qpdf/qtest/qpdf/issue-120.out
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@ -1 +1,2 @@
WARNING: issue-120.pdf (xref stream, offset 712): self-referential object stream 3
qpdf: issue-120.pdf: unable to find page tree

1
qpdf/qtest/qpdf/issue-143.out
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@ -3,6 +3,7 @@ WARNING: issue-143.pdf (xref stream: object 3 0, offset 654): stream keyword not
WARNING: issue-143.pdf (xref stream: object 3 0, offset 607): stream dictionary lacks /Length key
WARNING: issue-143.pdf (xref stream: object 3 0, offset 654): attempting to recover stream length
WARNING: issue-143.pdf (xref stream: object 3 0, offset 654): recovered stream length: 36
WARNING: issue-143.pdf (xref stream, offset 654): self-referential object stream 3
WARNING: issue-143.pdf: file is damaged
WARNING: issue-143.pdf (object 1 0, offset 48): expected n n obj
WARNING: issue-143.pdf: Attempting to reconstruct cross-reference table