net/http: check RemoteAddr isn't nil before dereferencing

RemoteAddr can return nil in some cases, this fix prevents a panic.

I chatted with @neild about this beforehand, but what's happening in our
case is that a connection comes in to the HTTP server which is then
immediately closed (we discovered this issue by accident using nmap).
The network implementation that we're using (it happens to be gVisor
via its gonet adaptor) is returning nil from RemoteAddr(), presumably
as there is no remote at that point.

But, ultimately, since RemoteAddr returns an interface it is always
possible for it to return nil, and indeed conn.RemoteAddr in this file
does exactly that if the conn is not ok.

Change-Id: Ibe67ae6e30b68e2776df5ee2911bf5f1dc539641
GitHub-Last-Rev: ff3505d
GitHub-Pull-Request: #60823
Reviewed-on: https://go-review.googlesource.com/c/go/+/503656
Auto-Submit: Dmitri Shuralyov <[email protected]>
Reviewed-by: Damien Neil <[email protected]>
Run-TryBot: Damien Neil <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: AlCutter

src/net/http/server.go

@@ -1856,7 +1856,9 @@ func isCommonNetReadError(err error) bool {
 
 // Serve a new connection.
 func (c *conn) serve(ctx context.Context) {
-	c.remoteAddr = c.rwc.RemoteAddr().String()
+	if ra := c.rwc.RemoteAddr(); ra != nil {
+		c.remoteAddr = ra.String()
+	}
 	ctx = context.WithValue(ctx, LocalAddrContextKey, c.rwc.LocalAddr())
 	var inFlightResponse *response
 	defer func() {