xmlsecurity: detect if PDF signature doesn't sign the whole file

For ODF signatures we require that all streams of the storage are
signed.  The PDF equivalent of this is to ensure that the byte range is
the entire file, including the signature dictionary but excluding the
signature value itself.

Change-Id: Ie47f42913e2aa960f35079eb981768cd47fb9f92
Reviewed-on: https://gerrit.libreoffice.org/29890Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>

xmlsecurity/source/pdfio/pdfdocument.cxx

@@ -777,6 +777,54 @@ bool PDFDocument::ValidateSignature(SvStream& rStream, PDFObjectElement* pSignat
             rInformation.ouDescription = aBuffer.makeStringAndClear();
     }
 
+    // Build a list of offset-length pairs, representing the signed bytes.
+    std::vector<std::pair<size_t, size_t>> aByteRanges;
+    size_t nByteRangeOffset = 0;
+    const std::vector<PDFElement*>& rByteRangeElements = pByteRange->GetElements();
+    for (size_t i = 0; i < rByteRangeElements.size(); ++i)
+    {
+        auto pNumber = dynamic_cast<PDFNumberElement*>(rByteRangeElements[i]);
+        if (!pNumber)
+        {
+            SAL_WARN("xmlsecurity.pdfio", "PDFDocument::ValidateSignature: signature offset and length has to be a number");
+            return false;
+        }
+
+        if (i % 2 == 0)
+        {
+            nByteRangeOffset = pNumber->GetValue();
+            continue;
+        }
+        size_t nByteRangeLength = pNumber->GetValue();
+        aByteRanges.push_back(std::make_pair(nByteRangeOffset, nByteRangeLength));
+    }
+
+    // Detect if the byte ranges don't cover everything, but the signature itself.
+    if (aByteRanges.size() < 2)
+    {
+        SAL_WARN("xmlsecurity.pdfio", "PDFDocument::ValidateSignature: expected 2 byte ranges");
+        return false;
+    }
+    if (aByteRanges[0].first != 0)
+    {
+        SAL_WARN("xmlsecurity.pdfio", "PDFDocument::ValidateSignature: first range start is not 0");
+        return false;
+    }
+    // 2 is the leading "<" and the trailing ">" around the hex string.
+    size_t nSignatureLength = pContents->GetValue().getLength() + 2;
+    if (aByteRanges[1].first != (aByteRanges[0].second + nSignatureLength))
+    {
+        SAL_WARN("xmlsecurity.pdfio", "PDFDocument::ValidateSignature: second range start is not the end of the signature");
+        return false;
+    }
+    rStream.Seek(STREAM_SEEK_TO_END);
+    size_t nFileEnd = rStream.Tell();
+    if ((aByteRanges[1].first + aByteRanges[1].second) != nFileEnd)
+    {
+        SAL_WARN("xmlsecurity.pdfio", "PDFDocument::ValidateSignature: second range end is not the end of the file");
+        return false;
+    }
+
     // At this point there is no obviously missing info to validate the
     // signature.
     std::vector<unsigned char> aSignature = PDFDocument::DecodeHexString(pContents);
@@ -837,37 +885,21 @@ bool PDFDocument::ValidateSignature(SvStream& rStream, PDFObjectElement* pSignat
     }
 
     // We have a hash, update it with the byte ranges.
-    size_t nByteRangeOffset = 0;
-    const std::vector<PDFElement*>& rByteRangeElements = pByteRange->GetElements();
-    for (size_t i = 0; i < rByteRangeElements.size(); ++i)
+    for (const auto& rByteRange : aByteRanges)
     {
-        auto pNumber = dynamic_cast<PDFNumberElement*>(rByteRangeElements[i]);
-        if (!pNumber)
-        {
-            SAL_WARN("xmlsecurity.pdfio", "PDFDocument::ValidateSignature: signature offset and length has to be a number");
-            return false;
-        }
-
-        if (i % 2 == 0)
-        {
-            nByteRangeOffset = pNumber->GetValue();
-            continue;
-        }
-
-        rStream.Seek(nByteRangeOffset);
-        size_t nByteRangeLength = pNumber->GetValue();
+        rStream.Seek(rByteRange.first);
 
         // And now hash this byte range.
         const int nChunkLen = 4096;
         std::vector<unsigned char> aBuffer(nChunkLen);
-        for (size_t nByte = 0; nByte < nByteRangeLength;)
+        for (size_t nByte = 0; nByte < rByteRange.second;)
         {
-            size_t nRemainingSize = nByteRangeLength - nByte;
+            size_t nRemainingSize = rByteRange.second - nByte;
             if (nRemainingSize < nChunkLen)
             {
                 rStream.ReadBytes(aBuffer.data(), nRemainingSize);
                 HASH_Update(pHASHContext, aBuffer.data(), nRemainingSize);
-                nByte = nByteRangeLength;
+                nByte = rByteRange.second;
             }
             else
             {