Fix possible out of bounds read in PDF parser

The `find_length()` function in the PDF parser incorrectly assumes that
objects found are located in the main PDF file map, and fails to take
into account whether the objects were in fact found in extracted PDF
object streams. The resulting pointer is then invalid and may be an out
of bounds read.

This issue was found by OSS-Fuzz.

This fix checks if the object is from an object stream, and then
calculates the pointer based on the start of the object stream instead
of based on the start of the PDF.

I've also added extra checks to verify the calculated pointer and object
size are within the stream (or PDF file map). I'm not entirely sure this
is necessary, but better safe than sorry.

Fixes: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=69617

libclamav/pdf.c

@@ -1009,8 +1009,26 @@ static size_t find_length(struct pdf_struct *pdf, struct pdf_obj *obj, const cha
                 return 0;
             }
 
-            indirect_obj_start = pdf->map + obj->start;
-            bytes_remaining    = pdf->size - obj->start;
+            if (NULL == obj->objstm) {
+                indirect_obj_start = (const char *)(obj->start + pdf->map);
+
+                if (!CLI_ISCONTAINED(pdf->map, pdf->size, indirect_obj_start, obj->size)) {
+                    cli_dbgmsg("find_length: indirect object found, but not contained in PDF\n");
+                    return 0;
+                }
+
+                bytes_remaining = pdf->size - obj->start;
+
+            } else {
+                indirect_obj_start = (const char *)(obj->start + obj->objstm->streambuf);
+
+                if (!CLI_ISCONTAINED(obj->objstm->streambuf, obj->objstm->streambuf_len, indirect_obj_start, obj->size)) {
+                    cli_dbgmsg("find_length: indirect object found, but not contained in PDF streambuf\n");
+                    return 0;
+                }
+
+                bytes_remaining = obj->objstm->streambuf_len - obj->start;
+            }
 
             /* Ok so we found the indirect object, lets read the value. */
             index = pdf_nextobject(indirect_obj_start, bytes_remaining);
@@ -3104,15 +3122,30 @@ void pdf_handle_enc(struct pdf_struct *pdf)
 
     obj = find_obj(pdf, pdf->objs[0], pdf->enc_objid);
     if (!obj) {
-        cli_dbgmsg("pdf_handle_enc: can't find encrypted object %d %d\n", pdf->enc_objid >> 8, pdf->enc_objid & 0xff);
-        noisy_warnmsg("pdf_handle_enc: can't find encrypted object %d %d\n", pdf->enc_objid >> 8, pdf->enc_objid & 0xff);
+        cli_dbgmsg("pdf_handle_enc: can't find encryption object %d %d\n", pdf->enc_objid >> 8, pdf->enc_objid & 0xff);
+        noisy_warnmsg("pdf_handle_enc: can't find encryption object %d %d\n", pdf->enc_objid >> 8, pdf->enc_objid & 0xff);
         return;
     }
 
     len = obj->size;
 
-    q = (obj->objstm) ? (const char *)(obj->start + obj->objstm->streambuf)
-                      : (const char *)(obj->start + pdf->map);
+    if (NULL == obj->objstm) {
+        q = (const char *)(obj->start + pdf->map);
+
+        if (!CLI_ISCONTAINED(pdf->map, pdf->size, q, len)) {
+            cli_dbgmsg("pdf_handle_enc: encryption object found, but not contained in PDF\n");
+            noisy_warnmsg("pdf_handle_enc: encryption object found, but not contained in PDF\n");
+            return;
+        }
+    } else {
+        q = (const char *)(obj->start + obj->objstm->streambuf);
+
+        if (!CLI_ISCONTAINED(obj->objstm->streambuf, obj->objstm->streambuf_len, q, len)) {
+            cli_dbgmsg("pdf_handle_enc: encryption object found, but not contained in PDF streambuf\n");
+            noisy_warnmsg("pdf_handle_enc: encryption object found, but not contained in PDF streambuf\n");
+            return;
+        }
+    }
 
     O = U = UE = StmF = StrF = EFF = NULL;
     do {

libclamav/pdfng.c

@@ -456,6 +456,11 @@ char *pdf_parse_string(struct pdf_struct *pdf, struct pdf_obj *obj, const char *
         if (!(newobj))
             return NULL;
 
+        if (!CLI_ISCONTAINED(pdf->map, pdf->size, newobj->start, newobj->size)) {
+            cli_dbgmsg("pdf_parse_string: object not contained in PDF\n");
+            return NULL;
+        }
+
         if (newobj == obj)
             return NULL;