Fix pkcs7 parsing in some cases (#12519)

* Fix pkcs7 parsing in some cases

brings in mozilla-services/pkcs7#61 from upstream

In some cases but not all, aws includes a certificate in the pkcs7 response,
and currently vault fails to parse those certificates:
```
URL: PUT https://vault.example.com/v1/auth/aws/login
Code: 500. Errors
* failed to parse the BER encoded PKCS#7 signature: ber2der: Invalid BER format
```

This fixes logins on those instances.  Note we could not readily ascertain why
some instances have those certificates and others don't.

* Add changelog entry

* Correct missed line

builtin/credential/aws/pkcs7/ber.go

@@ -175,7 +175,7 @@ func readObject(ber []byte, offset int) (asn1Object, int, error) {
 	if offset > berLen {
 		return nil, 0, errors.New("ber2der: cannot move offset forward, end of ber data reached")
 	}
-	hack := 0
+	indefinite := false
 	if l > 0x80 {
 		numberOfBytes := (int)(l & 0x7F)
 		if numberOfBytes > 4 { // int is only guaranteed to be 32bit
@@ -197,14 +197,7 @@ func readObject(ber []byte, offset int) (asn1Object, int, error) {
 			}
 		}
 	} else if l == 0x80 {
-		// find length by searching content
-		markerIndex := bytes.LastIndex(ber[offset:], []byte{0x0, 0x0})
-		if markerIndex == -1 {
-			return nil, 0, errors.New("ber2der: Invalid BER format")
-		}
-		length = markerIndex
-		hack = 2
-		debugprint("--> (compute length) marker found at offset: %d\n", markerIndex+offset)
+		indefinite = true
 	} else {
 		length = (int)(l)
 	}
@@ -220,6 +213,9 @@ func readObject(ber []byte, offset int) (asn1Object, int, error) {
 	debugprint("--> content end   : %d\n", contentEnd)
 	debugprint("--> content       : % X\n", ber[offset:contentEnd])
 	var obj asn1Object
+	if indefinite && kind == 0 {
+		return nil, 0, errors.New("ber2der: Indefinite form tag must have constructed encoding")
+	}
 	if kind == 0 {
 		obj = asn1Primitive{
 			tagBytes: ber[tagStart:tagEnd],
@@ -228,24 +224,49 @@ func readObject(ber []byte, offset int) (asn1Object, int, error) {
 		}
 	} else {
 		var subObjects []asn1Object
-		for offset < contentEnd {
+		for (offset < contentEnd) || indefinite {
 			var subObj asn1Object
 			var err error
-			subObj, offset, err = readObject(ber[:contentEnd], offset)
+			subObj, offset, err = readObject(ber, offset)
 			if err != nil {
 				return nil, 0, err
 			}
 			subObjects = append(subObjects, subObj)
+
+			if indefinite {
+				terminated, err := isIndefiniteTermination(ber, offset)
+				if err != nil {
+					return nil, 0, err
+				}
+
+				if terminated {
+					break
+				}
+			}
 		}
 		obj = asn1Structured{
 			tagBytes: ber[tagStart:tagEnd],
 			content:  subObjects,
 		}
 	}
 
-	return obj, contentEnd + hack, nil
+	// Apply indefinite form length with 0x0000 terminator.
+	if indefinite {
+		contentEnd = offset + 2
+	}
+
+	return obj, contentEnd, nil
+}
+
+func isIndefiniteTermination(ber []byte, offset int) (bool, error) {
+	if len(ber) - offset < 2 {
+		return false, errors.New("ber2der: Invalid BER format")
+	}
+
+	return bytes.Index(ber[offset:], []byte{0x0, 0x0}) == 0, nil
 }
 
 func debugprint(format string, a ...interface{}) {
 	//fmt.Printf(format, a)
 }
+

builtin/credential/aws/pkcs7/ber_test.go

@@ -3,6 +3,8 @@ package pkcs7
 import (
 	"bytes"
 	"encoding/asn1"
+	"encoding/pem"
+	"fmt"
 	"strings"
 	"testing"
 )
@@ -45,7 +47,8 @@ func TestBer2Der_Negatives(t *testing.T) {
 		{[]byte{0x30, 0x85}, "tag length too long"},
 		{[]byte{0x30, 0x84, 0x80, 0x0, 0x0, 0x0}, "length is negative"},
 		{[]byte{0x30, 0x82, 0x0, 0x1}, "length has leading zero"},
-		{[]byte{0x30, 0x80, 0x1, 0x2}, "Invalid BER format"},
+		{[]byte{0x30, 0x80, 0x1, 0x2, 0x1, 0x2}, "Invalid BER format"},
+		{[]byte{0x30, 0x80, 0x1, 0x2}, "BER tag length is more than available data"},
 		{[]byte{0x30, 0x03, 0x01, 0x02}, "length is more than available data"},
 		{[]byte{0x30}, "end of ber data reached"},
 	}
@@ -60,3 +63,88 @@ func TestBer2Der_Negatives(t *testing.T) {
 		}
 	}
 }
+
+func TestVerifyIndefiniteLengthBer(t *testing.T) {
+	decoded := mustDecodePEM([]byte(testPKCS7))
+
+	_, err := ber2der(decoded)
+	if err != nil {
+		t.Errorf("cannot parse indefinite length ber: %v", err)
+	}
+}
+
+func mustDecodePEM(data []byte) []byte {
+	var block *pem.Block
+	block, rest := pem.Decode(data)
+	if len(rest) != 0 {
+		panic(fmt.Errorf("unexpected remaining PEM block during decode"))
+	}
+	return block.Bytes
+}
+
+const testPKCS7 = `
+-----BEGIN PKCS7-----
+MIAGCSqGSIb3DQEHAqCAMIACAQExDzANBglghkgBZQMEAgEFADCABgkqhkiG9w0B
+BwGggCSABIIDfXsiQWdlbnRBY3Rpb25PdmVycmlkZXMiOnsiQWdlbnRPdmVycmlk
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+Tb7e7EjgDGJsjOTMdTrMfv8ho8kAAAAAAAA=
+-----END PKCS7-----
+`

changelog/12519.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+auth/aws: Fix ec2 auth on instances that have a cert in their PKCS7 signature
+```