From 97b4aad955ea816d7cc2273c1fd85fe139ec6207 Mon Sep 17 00:00:00 2001
From: Guilhem Moulin <guilhem@fripost.org>
Date: Thu, 29 Jun 2017 14:53:40 +0200
Subject: webserver: improve serving logic for ACME challenge responses.

In particular, we now return "403 Forbidden" for
/.well-known/acme-challenge/
---
 webserver | 20 ++++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

diff --git a/webserver b/webserver
index 211f646..90be70c 100755
--- a/webserver
+++ b/webserver
@@ -37,7 +37,7 @@ use warnings;
 # as "www-data:www-data"; bind(2)'ing to a privileged port such as 80 is
 # not a problem since FD can be bound as root prior to the execve(2).
 
-use Errno 'EINTR';
+use Errno qw/EINTR ENOENT/;
 use Fcntl qw/O_NOFOLLOW O_RDONLY/;
 use Socket qw/AF_UNIX AF_INET AF_INET6/;
 
@@ -92,19 +92,23 @@ while (1) {
     while (defined (my $h = $conn->getline())) { last if $h eq "\r\n" };
 
     my ($status_line, $content_type, $content);
-    if ($req =~ /\A\Q$ROOT\E\/([A-Za-z0-9_\-]+)\z/ and -f $1) {
-        # only open files in the cwd, and refuse to follow symlinks
-        if (sysopen(my $fh, $1, O_NOFOLLOW|O_RDONLY)) {
+    if ($req =~ /\A\Q$ROOT\E\/([A-Za-z0-9_\-]+)\z/) {
+        # only serve base64-encoded filenames (tokens) in the cwd
+        # XXX stat(2) followed by open(2) is racy; open(2) would hang if
+        # an attacker manages to replace a regular file with a FIFO
+        # between the syscalls, leading to denial of service; however
+        # there shouldn't be any risk of information disclosure as we're
+        # not following symlinks (and the cwd is not owned by us)
+        if (-f $1 and sysopen(my $fh, $1, O_NOFOLLOW|O_RDONLY)) {
             ($status_line, $content_type) = ('200 OK', 'application/jose+json');
             $content = do { local $/ = undef; $fh->getline() };
             $fh->close() or die "close: $!";
-        }
-        else {
-            $status_line = '403 Forbidden';
+        } elsif ($! == ENOENT) { # errno from either stat(2) or open(2)
+            $status_line = '404 Not Found';
         }
     }
 
-    $conn->print( "HTTP/$proto ", ($status_line // '404 Not Found'), "\r\n" );
+    $conn->print( "HTTP/$proto ", ($status_line // '403 Forbidden'), "\r\n" );
     $conn->print( "Content-Type: $content_type\r\n" ) if defined $content_type;
     $conn->print( "Content-Length: ".length($content)."\r\n" ) if defined $content;
     $conn->print( "Connection: close\r\n" );
-- 
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