crypto/x509: don't panic when decrypting invalid PEM data.

If an encrypted PEM block contained ciphertext that was not a multiple
of the block size then the code would panic. This change tests for that
case and returns an error.

Fixes #11215.

Change-Id: I7b700f99e20810c4f545519b1e9d766b4640e8a7
Reviewed-on: https://go-review.googlesource.com/11097Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/crypto/x509/pem_decrypt.go

@@ -144,6 +144,10 @@ func DecryptPEMBlock(b *pem.Block, password []byte) ([]byte, error) {
 		return nil, err
 	}
 
+	if len(b.Bytes)%block.BlockSize() != 0 {
+		return nil, errors.New("x509: encrypted PEM data is not a multiple of the block size")
+	}
+
 	data := make([]byte, len(b.Bytes))
 	dec := cipher.NewCBCDecrypter(block, iv)
 	dec.CryptBlocks(data, b.Bytes)

src/crypto/x509/pem_decrypt_test.go

@@ -9,6 +9,7 @@ import (
 	"crypto/rand"
 	"encoding/base64"
 	"encoding/pem"
+	"strings"
 	"testing"
 )
 
@@ -221,3 +222,26 @@ AgkA8SEfu/2i3g0CCQDGNlXbBHX7kQIIK3Ww5o0cYbECCQDCimPb0dYGsQIIeQ7A
 jryIst8=`,
 	},
 }
+
+const incompleteBlockPEM = `
+-----BEGIN RSA PRIVATE KEY-----
+Proc-Type: 4,ENCRYPTED
+DEK-Info: AES-128-CBC,74611ABC2571AF11B1BF9B69E62C89E7
+
+6L8yXK2MTQUWBk4ZD6OvCiYp+mXyR1594TQ1K38MxGvDw5pwcDME2Lek8RrR5fd40P2XsL2Z4KKt
+ai+OP1BZUetfK6AW4MiqB2FDyIdOAJ8XeWuZy21Wtsh8wPD6yYOFM/w7WZL8weX3Y0TSeG/T
+-----END RSA PRIVATE KEY-----`
+
+func TestIncompleteBlock(t *testing.T) {
+	// incompleteBlockPEM contains ciphertext that is not a multiple of the
+	// block size. This previously panicked. See #11215.
+	block, _ := pem.Decode([]byte(incompleteBlockPEM))
+	_, err := DecryptPEMBlock(block, []byte("foo"))
+	if err == nil {
+		t.Fatal("Bad PEM data decrypted successfully")
+	}
+	const expectedSubstr = "block size"
+	if e := err.Error(); !strings.Contains(e, expectedSubstr) {
+		t.Fatalf("Expected error containing %q but got: %q", expectedSubstr, e)
+	}
+}