From f5c65949e87e54f5b7c493c5acfe70844ec78329 Mon Sep 17 00:00:00 2001
From: Jason Ish <jason.ish@oisf.net>
Date: Tue, 1 Feb 2022 15:44:43 -0600
Subject: [PATCH] dns: better error handling when parsing names

The DNS name parser will error out with an error even if the
error is incomplete. Instead of manually generating errors,
use '?' to let the nom error ripple up the error handling chain.

The reason this wasn't done in the first place is this code
predates the ? operator, or we were not aware of it at the time.

This prevents the case where probing fails when there is enough data to
parse the header, but not enough to complete name parser. In such a case
a parse error is returned (instead of incomplete) resulting in the
payload not being detected as DNS.

Ticket #5034

(cherry picked from commit 0623ada24df1da99c72bb8cd4959b2cb0e64ccc2)
---
 rust/src/dns/parser.rs | 41 +++++++++++++----------------------------
 1 file changed, 13 insertions(+), 28 deletions(-)

diff --git a/rust/src/dns/parser.rs b/rust/src/dns/parser.rs
index e7d7fb32fb..9759b91253 100644
--- a/rust/src/dns/parser.rs
+++ b/rust/src/dns/parser.rs
@@ -67,36 +67,21 @@ pub fn dns_parse_name<'a, 'b>(start: &'b [u8],
             pos = &pos[1..];
             break;
         } else if len & 0b1100_0000 == 0 {
-            match length_bytes!(pos, be_u8) {
-                Ok((rem, label)) => {
-                    if name.len() > 0 {
-                        name.push('.' as u8);
-                    }
-                    name.extend(label);
-                    pos = rem;
-                }
-                _ => {
-                    return Err(nom::Err::Error(
-                        error_position!(pos, nom::ErrorKind::OctDigit)));
-                }
+            let (rem, label) = nom::length_data!(pos, be_u8)?;
+            if name.len() > 0 {
+                name.push('.' as u8);
             }
+            name.extend(label);
+            pos = rem;
         } else if len & 0b1100_0000 == 0b1100_0000 {
-            match be_u16(pos) {
-                Ok((rem, leader)) => {
-                    let offset = leader & 0x3fff;
-                    if offset as usize > message.len() {
-                        return Err(nom::Err::Error(
-                            error_position!(pos, nom::ErrorKind::OctDigit)));
-                    }
-                    pos = &message[offset as usize..];
-                    if pivot == start {
-                        pivot = rem;
-                    }
-                }
-                _ => {
-                    return Err(nom::Err::Error(
-                        error_position!(pos, nom::ErrorKind::OctDigit)));
-                }
+            let (rem, leader) = be_u16(pos)?;
+            let offset = usize::from(leader) & 0x3fff;
+            if offset > message.len() {
+                return Err(nom::Err::Error(error_position!(pos, nom::ErrorKind::OctDigit)));
+            }
+            pos = &message[offset..];
+            if pivot == start {
+                pivot = rem;
             }
         } else {
             return Err(nom::Err::Error(
-- 
2.47.2