encoding/base32: decoder output depends on chunking of underlying reader

After an analysis, I figured that a way to do it could be to check, after
the call to readEncodedData whether the decoder already saw the end or not.

Fixes #38657

Change-Id: I06fd718ea4ee6ded2cb26c2866b28581ad86e271
GitHub-Last-Rev: d0b7bb38e4301a2ae9b8e588944488dbd88b39c4
GitHub-Pull-Request: golang/go#52631
Reviewed-on: https://go-review.googlesource.com/c/go/+/403315
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

2 files changed, 55 insertions, 0 deletions

diff --git a/src/encoding/base32/base32.go b/src/encoding/base32/base32.go
index 5f3af4c8bb..fa6e42e26c 100644
--- a/src/encoding/base32/base32.go
+++ b/src/encoding/base32/base32.go
@@ -445,6 +445,9 @@ func (d *decoder) Read(p []byte) (n int, err error) {
 	if d.nbuf < min {
 		return 0, d.err
 	}
+	if nn > 0 && d.end {
+		return 0, CorruptInputError(0)
+	}
 
 	// Decode chunk into p, or d.out and then p if p is too small.
 	var nr int
diff --git a/src/encoding/base32/base32_test.go b/src/encoding/base32/base32_test.go
index dbd2b613b4..323d04e68b 100644
--- a/src/encoding/base32/base32_test.go
+++ b/src/encoding/base32/base32_test.go
@@ -627,6 +627,58 @@ func TestBufferedDecodingSameError(t *testing.T) {
 	}
 }
 
+func TestBufferedDecodingPadding(t *testing.T) {
+	testcases := []struct {
+		chunks        []string
+		expectedError string
+	}{
+		{[]string{
+			"I4======",
+			"==",
+		}, "unexpected EOF"},
+
+		{[]string{
+			"I4======N4======",
+		}, "illegal base32 data at input byte 2"},
+
+		{[]string{
+			"I4======",
+			"N4======",
+		}, "illegal base32 data at input byte 0"},
+
+		{[]string{
+			"I4======",
+			"========",
+		}, "illegal base32 data at input byte 0"},
+
+		{[]string{
+			"I4I4I4I4",
+			"I4======",
+			"I4======",
+		}, "illegal base32 data at input byte 0"},
+	}
+
+	for _, testcase := range testcases {
+		testcase := testcase
+		pr, pw := io.Pipe()
+		go func() {
+			for _, chunk := range testcase.chunks {
+				_, _ = pw.Write([]byte(chunk))
+			}
+			_ = pw.Close()
+		}()
+
+		decoder := NewDecoder(StdEncoding, pr)
+		_, err := io.ReadAll(decoder)
+
+		if err == nil && len(testcase.expectedError) != 0 {
+			t.Errorf("case %q: got nil error, want %v", testcase.chunks, testcase.expectedError)
+		} else if err.Error() != testcase.expectedError {
+			t.Errorf("case %q: got %v, want %v", testcase.chunks, err, testcase.expectedError)
+		}
+	}
+}
+
 func TestEncodedDecodedLen(t *testing.T) {
 	type test struct {
 		in      int