[SDC-1140] - fix(vex): allow VEX matching when no subcomponents (#124)

* fix(vex): allow VEX matching when no subcomponents

Signed-off-by: felipecruz91 <[email protected]>

* Apply suggestions from code review

* Apply suggestions from code review

---------

Signed-off-by: felipecruz91 <[email protected]>

policy/data/vulnerabilities.go

@@ -3,6 +3,7 @@ package data
 import (
 	"context"
 
+	"github.com/openvex/go-vex/pkg/vex"
 	govex "github.com/openvex/go-vex/pkg/vex"
 
 	"github.com/atomist-skills/go-skill/policy/data/query"
@@ -43,23 +44,7 @@ func (ds *DataSource) GetImageVulnerabilities(ctx context.Context, evalCtx goals
 		normalization.DenormalizeSBOM(&vulnsResponse, purlMapping)
 
 		for _, vulnsByPurl := range vulnsResponse.VulnerabilitiesByPackage {
-			affected := true
-			for _, v := range imageSbom.VexDocuments {
-				for _, stmt := range v.Statements {
-					purl, upstreamPurl := normalization.NormalizePURL(vulnsByPurl.Purl, nil)
-					for _, p := range stmt.Products {
-						if normalization.ContainsPurl(p.Subcomponents, purl) || normalization.ContainsPurl(p.Subcomponents, upstreamPurl) {
-							if stmt.Status == govex.StatusNotAffected || stmt.Status == govex.StatusFixed {
-								affected = false
-							}
-						}
-					}
-				}
-			}
-
-			if affected {
-				vulns[vulnsByPurl.Purl] = vulnsByPurl.Vulnerabilities
-			}
+			vulns[vulnsByPurl.Purl] = applyVEX(vulnsByPurl, imageSbom.VexDocuments)
 		}
 	} else {
 		var response jynx.ImagePackagesByDigestResponse
@@ -80,3 +65,72 @@ func (ds *DataSource) GetImageVulnerabilities(ctx context.Context, evalCtx goals
 
 	return &query.QueryResponse{}, packages, vulns, nil
 }
+
+// applyVEX returns the CVEs that remain relevant after cross-referencing them with VEX documents.
+func applyVEX(vulnsByPurl types.VulnerabilitiesByPurl, vexDocs []vex.VEX) []types.Vulnerability {
+	filteredOutCVEs := []types.Vulnerability{}
+
+	for _, cve := range vulnsByPurl.Vulnerabilities {
+		for _, v := range vexDocs {
+			for _, stmt := range v.Statements {
+				if cveMatch(cve.SourceId, stmt) {
+					if purlMatch(vulnsByPurl.Purl, stmt) {
+						if notAffectedOrFixed(stmt) {
+							filteredOutCVEs = append(filteredOutCVEs, cve)
+						}
+					}
+				}
+			}
+		}
+	}
+
+	vexedCVEsMap := make(map[string]bool, len(filteredOutCVEs))
+	for _, cve := range filteredOutCVEs {
+		vexedCVEsMap[cve.SourceId] = true
+	}
+
+	// Filter out the VEXed CVEs
+	cves := make([]types.Vulnerability, 0, len(vulnsByPurl.Vulnerabilities))
+	for _, cve := range vulnsByPurl.Vulnerabilities {
+		if !vexedCVEsMap[cve.SourceId] {
+			cves = append(cves, cve)
+		}
+	}
+	return cves
+}
+
+// cveMatch checks whether a CVE is present in a VEX statement
+func cveMatch(cveID string, stmt govex.Statement) bool {
+	return stmt.Vulnerability.ID == cveID || string(stmt.Vulnerability.Name) == cveID
+}
+
+// purlMatch checks whether a purl is present in at least one of the following locations:
+// - Component
+// - Subcomponent(s)
+// - Special case for image-scoped exceptions.
+func purlMatch(purl string, stmt govex.Statement) bool {
+	purl, upstreamPurl := normalization.NormalizePURL(purl, nil)
+
+	for _, p := range stmt.Products {
+		// Check if purl is defined as the top-level component
+		if purl == p.Component.ID {
+			return true
+		}
+		// Check if purl is defined as one of the subcomponents
+		if normalization.ContainsPurl(p.Subcomponents, purl) || normalization.ContainsPurl(p.Subcomponents, upstreamPurl) {
+			return true
+		}
+		// If none of the previous conditions matched, we add this special case to support image-scoped exceptions.
+		// The purpose of this is to align with how VEX works in the platform side.
+		if len(p.Subcomponents) == 0 {
+			return true
+		}
+	}
+
+	return false
+}
+
+// notAffectedOrFixed checks whether the statement status is not affected or fixed.
+func notAffectedOrFixed(stmt govex.Statement) bool {
+	return stmt.Status == govex.StatusNotAffected || stmt.Status == govex.StatusFixed
+}