In qpdf_fuzzer and dct_fuzzer add a scan limit for Pl_DCT

fuzz/dct_fuzzer.cc

@@ -31,6 +31,7 @@ FuzzHelper::doChecks()
     // jpeg_start_decompress is called. During normal use of qpdf very large JPEGs can occasionally
     // occur legitimately and therefore must be allowed during normal operations.
     Pl_DCT::setMemoryLimit(200'000'000);
+    Pl_DCT::setScanLimit(50);
 
     // Do not decompress corrupt data. This may cause extended runtime within jpeglib without
     // exercising additional code paths in qpdf.

fuzz/qpdf_fuzzer.cc

@@ -181,6 +181,7 @@ FuzzHelper::doChecks()
     // jpeg_start_decompress is called. During normal use of qpdf very large JPEGs can occasionally
     // occur legitimately and therefore must be allowed during normal operations.
     Pl_DCT::setMemoryLimit(100'000'000);
+    Pl_DCT::setScanLimit(50);
 
     Pl_PNGFilter::setMemoryLimit(1'000'000);
     Pl_TIFFPredictor::setMemoryLimit(1'000'000);

include/qpdf/Pl_DCT.hh

@@ -39,6 +39,11 @@ class QPDF_DLL_CLASS Pl_DCT: public Pipeline
     QPDF_DLL
     static void setMemoryLimit(long limit);
 
+    // Limit the number of scans used by jpeglib when decompressing progressive jpegs.
+    // NB This is a static option affecting all Pl_DCT instances.
+    QPDF_DLL
+    static void setScanLimit(int limit);
+
     // Treat corrupt data as a runtime error rather than attempting to decompress regardless. This
     // is the qpdf default behaviour. To attempt to decompress corrupt data set 'treat_as_error' to
     // false.

libqpdf/Pl_DCT.cc

@@ -22,6 +22,7 @@ namespace
     };
 
     long memory_limit{0};
+    int scan_limit{0};
     bool throw_on_corrupt_data{true};
 } // namespace
 
@@ -45,6 +46,17 @@ emit_message(j_common_ptr cinfo, int msg_level)
     }
 }
 
+static void
+progress_monitor(j_common_ptr cinfo)
+{
+    if (cinfo->is_decompressor &&
+        reinterpret_cast<jpeg_decompress_struct*>(cinfo)->input_scan_number > scan_limit) {
+        auto* jerr = reinterpret_cast<qpdf_jpeg_error_mgr*>(cinfo->err);
+        jerr->msg = "Pl_DCT::decompress: JPEG data has too many scans";
+        longjmp(jerr->jmpbuf, 1);
+    }
+}
+
 Pl_DCT::Members::Members() :
     action(a_decompress),
     buf("DCT compressed image")
@@ -74,6 +86,12 @@ Pl_DCT::setMemoryLimit(long limit)
     memory_limit = limit;
 }
 
+void
+Pl_DCT::setScanLimit(int limit)
+{
+    scan_limit = limit;
+}
+
 void
 Pl_DCT::setThrowOnCorruptData(bool treat_as_error)
 {
@@ -341,6 +359,11 @@ Pl_DCT::decompress(void* cinfo_p, Buffer* b)
         // first warning is encountered causing a timeout in oss-fuzz.
         throw std::runtime_error("Pl_DCT::decompress: JPEG data large - may be too slow");
     }
+    jpeg_progress_mgr progress_mgr;
+    if (scan_limit > 0) {
+        progress_mgr.progress_monitor = &progress_monitor;
+        cinfo->progress = &progress_mgr;
+    }
     JSAMPARRAY buffer =
         (*cinfo->mem->alloc_sarray)(reinterpret_cast<j_common_ptr>(cinfo), JPOOL_IMAGE, width, 1);