[release-branch.go1.21] archive/zip: treat truncated EOCDR comment as an error

When scanning for an end of central directory record,
treat an EOCDR signature with a record containing a truncated
comment as an error. Previously, we would skip over the invalid
record and look for another one. Other implementations do not
do this (they either consider this a hard error, or just ignore
the truncated comment). This parser misalignment allowed
presenting entirely different archive contents to Go programs
and other zip decoders.

For #66869
Fixes #67553

Change-Id: I94e5cb028534bb5704588b8af27f1e22ea49c7c6
Reviewed-on: https://go-review.googlesource.com/c/go/+/585397
Reviewed-by: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
(cherry picked from commit 33d725e5758bf1fea62e6c77fc70b57a828a49f5)
Reviewed-on: https://go-review.googlesource.com/c/go/+/588795
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/archive/zip/reader.go

@@ -699,9 +699,13 @@ func findSignatureInBlock(b []byte) int {
 		if b[i] == 'P' && b[i+1] == 'K' && b[i+2] == 0x05 && b[i+3] == 0x06 {
 			// n is length of comment
 			n := int(b[i+directoryEndLen-2]) | int(b[i+directoryEndLen-1])<<8
-			if n+directoryEndLen+i <= len(b) {
+			if n+directoryEndLen+i > len(b) {
-				return i
+				// Truncated comment.
+				// Some parsers (such as Info-ZIP) ignore the truncated comment
+				// rather than treating it as a hard error.
+				return -1
 			}
+			return i
 		}
 	}
 	return -1

src/archive/zip/reader_test.go

@@ -570,6 +570,14 @@ var tests = []ZipTest{
 			},
 		},
 	},
+	// Issue 66869: Don't skip over an EOCDR with a truncated comment.
+	// The test file sneakily hides a second EOCDR before the first one;
+	// previously we would extract one file ("file") from this archive,
+	// while most other tools would reject the file or extract a different one ("FILE").
+	{
+		Name:  "comment-truncated.zip",
+		Error: ErrFormat,
+	},
 }
 
 func TestReader(t *testing.T) {