Add size limits on responses (#351)

This prevents a malicious server from running the client out of memory
by returning a very large body.

Limit into_string() to 1 megabyte.
Limit individual header fields to 100 kilobytes.
Limit number of header fields to 100.

Part of #267

src/response.rs

@@ -20,6 +20,11 @@ use encoding_rs::Encoding;
 
 pub const DEFAULT_CONTENT_TYPE: &str = "text/plain";
 pub const DEFAULT_CHARACTER_SET: &str = "utf-8";
+const INTO_STRING_LIMIT: usize = 10 * 1_024 * 1_024;
+// Follow the example of curl and limit a single header to 100kB:
+// https://curl.se/libcurl/c/CURLOPT_HEADERFUNCTION.html
+const MAX_HEADER_SIZE: usize = 100 * 1_024;
+const MAX_HEADER_COUNT: usize = 100;
 
 /// Response instances are created as results of firing off requests.
 ///
@@ -211,6 +216,11 @@ impl Response {
     ///    length regardless of how many bytes the server sends.
     /// 3. If no length header, the reader is until server stream end.
     ///
+    /// Note: If you use `read_to_end()` on the resulting reader, a malicious
+    /// server might return enough bytes to exhaust available memroy. If you're
+    /// making requests to untrusted servers, you should use `.take()` to
+    /// limit the response bytes read.
+    ///
     /// Example:
     ///
     /// ```
@@ -226,6 +236,7 @@ impl Response {
     ///
     /// let mut bytes: Vec<u8> = Vec::with_capacity(len);
     /// resp.into_reader()
+    ///     .take(10_000_000)
     ///     .read_to_end(&mut bytes)?;
     ///
     /// assert_eq!(bytes.len(), len);
@@ -291,6 +302,8 @@ impl Response {
     /// implementation first reads the reader to end into a `Vec<u8>` and then
     /// attempts to decode it using the charset.
     ///
+    /// If the response is larger than 10 megabytes, this will return an error.
+    ///
     /// Example:
     ///
     /// ```
@@ -321,14 +334,30 @@ impl Response {
                 .or_else(|| Encoding::for_label(DEFAULT_CHARACTER_SET.as_bytes()))
                 .unwrap();
             let mut buf: Vec<u8> = vec![];
-            self.into_reader().read_to_end(&mut buf)?;
+            self.into_reader()
+                .take((INTO_STRING_LIMIT + 1) as u64)
+                .read_to_end(&mut buf)?;
+            if buf.len() > INTO_STRING_LIMIT {
+                return Err(io::Error::new(
+                    io::ErrorKind::Other,
+                    "response too big for into_string",
+                ));
+            }
             let (text, _, _) = encoding.decode(&buf);
             Ok(text.into_owned())
         }
         #[cfg(not(feature = "charset"))]
         {
             let mut buf: Vec<u8> = vec![];
-            self.into_reader().read_to_end(&mut buf)?;
+            self.into_reader()
+                .take((INTO_STRING_LIMIT + 1) as u64)
+                .read_to_end(&mut buf)?;
+            if buf.len() > INTO_STRING_LIMIT {
+                return Err(io::Error::new(
+                    io::ErrorKind::Other,
+                    "response too big for into_string",
+                ));
+            }
             Ok(String::from_utf8_lossy(&buf).to_string())
         }
     }
@@ -427,7 +456,7 @@ impl Response {
         let (index, status) = parse_status_line(status_line.as_str())?;
 
         let mut headers: Vec<Header> = Vec::new();
-        loop {
+        while headers.len() <= MAX_HEADER_COUNT {
             let line = read_next_line(&mut stream, "a header")?;
             if line.is_empty() {
                 break;
@@ -437,6 +466,12 @@ impl Response {
             }
         }
 
+        if headers.len() > MAX_HEADER_COUNT {
+            return Err(ErrorKind::BadHeader.msg(
+                format!("more than {} header fields in response", MAX_HEADER_COUNT).as_str(),
+            ));
+        }
+
         Ok(Response {
             url: None,
             status_line,
@@ -549,27 +584,33 @@ impl FromStr for Response {
 
 fn read_next_line(reader: &mut impl BufRead, context: &str) -> io::Result<HeaderLine> {
     let mut buf = Vec::new();
-    let result = reader.read_until(b'\n', &mut buf);
+    let result = reader
+        .take((MAX_HEADER_SIZE + 1) as u64)
+        .read_until(b'\n', &mut buf);
 
-    if let Err(e) = result {
-        // Provide context to errors encountered while reading the line.
-        let reason = format!("Error encountered in {}", context);
-
-        let kind = e.kind();
-
-        // Use an intermediate wrapper type which carries the error message
-        // as well as a .source() reference to the original error.
-        let wrapper = Error::new(ErrorKind::Io, Some(reason)).src(e);
-
-        return Err(io::Error::new(kind, wrapper));
-    }
-
-    if result? == 0 {
-        return Err(io::Error::new(
+    match result {
+        Ok(0) => Err(io::Error::new(
             io::ErrorKind::ConnectionAborted,
             "Unexpected EOF",
-        ));
-    }
+        )),
+        Ok(n) if n > MAX_HEADER_SIZE => Err(io::Error::new(
+            io::ErrorKind::Other,
+            format!("header field longer than {} bytes", MAX_HEADER_SIZE),
+        )),
+        Ok(_) => Ok(()),
+        Err(e) => {
+            // Provide context to errors encountered while reading the line.
+            let reason = format!("Error encountered in {}", context);
+
+            let kind = e.kind();
+
+            // Use an intermediate wrapper type which carries the error message
+            // as well as a .source() reference to the original error.
+            let wrapper = Error::new(ErrorKind::Io, Some(reason)).src(e);
+
+            Err(io::Error::new(kind, wrapper))
+        }
+    }?;
 
     if !buf.ends_with(b"\n") {
         return Err(io::Error::new(
@@ -672,6 +713,8 @@ impl Read for ErrorReader {
 
 #[cfg(test)]
 mod tests {
+    use std::io::Cursor;
+
     use super::*;
 
     #[test]
@@ -755,6 +798,25 @@ mod tests {
         assert_eq!("hello world!!!", resp.into_string().unwrap());
     }
 
+    #[test]
+    fn into_string_large() {
+        const LEN: usize = INTO_STRING_LIMIT + 1;
+        let s = format!(
+            "HTTP/1.1 200 OK\r\n\
+                 Content-Length: {}\r\n
+                 \r\n
+                 {}",
+            LEN,
+            "A".repeat(LEN),
+        );
+        let result = s.parse::<Response>().unwrap();
+        let err = result
+            .into_string()
+            .expect_err("didn't error with too-long body");
+        assert_eq!(err.to_string(), "response too big for into_string");
+        assert_eq!(err.kind(), io::ErrorKind::Other);
+    }
+
     #[test]
     #[cfg(feature = "json")]
     fn parse_simple_json() {
@@ -801,6 +863,43 @@ mod tests {
         assert_eq!(resp.status_text(), "");
     }
 
+    #[test]
+    fn read_next_line_large() {
+        const LEN: usize = MAX_HEADER_SIZE + 1;
+        let s = format!("Long-Header: {}\r\n", "A".repeat(LEN),);
+        let mut cursor = Cursor::new(s);
+        let result = read_next_line(&mut cursor, "some context");
+        let err = result.expect_err("did not error on too-large header");
+        assert_eq!(err.kind(), io::ErrorKind::Other);
+        assert_eq!(
+            err.to_string(),
+            format!("header field longer than {} bytes", MAX_HEADER_SIZE)
+        );
+    }
+
+    #[test]
+    fn too_many_headers() {
+        const LEN: usize = MAX_HEADER_COUNT + 1;
+        let s = format!(
+            "HTTP/1.1 200 OK\r\n\
+                 {}
+                 \r\n
+                 hi",
+            "Header: value\r\n".repeat(LEN),
+        );
+        let err = s
+            .parse::<Response>()
+            .expect_err("did not error on too many headers");
+        assert_eq!(err.kind(), ErrorKind::BadHeader);
+        assert_eq!(
+            err.to_string(),
+            format!(
+                "Bad Header: more than {} header fields in response",
+                MAX_HEADER_COUNT
+            )
+        );
+    }
+
     #[test]
     #[cfg(feature = "charset")]
     fn read_next_line_non_ascii_reason() {