tighten up security for rsync

Nick Cleaton (nick@cleaton.net) found and reported a security issue
caused by trusting the remote rsync too much.  It appears that rsync
cannot -- without special precautions -- be used in any "restricted"
environment.

Gitolite ships with a "bundle helper" called "rsync" (disabled
by default; more details below).  This fix tightens up this
helper to close this hole.

TLDR for administrators and packagers:

1.  Am I affected?

    Look in ~/.gitolite.rc for "rsync"; if it is there, you are
    affected.

    This is NOT an essential program, and it is not enabled by
    default.  You (or a previous administrator of your site)
    would have to have explicitly enabled it for you to be
    affected.

2.  What's the quick fix?

    Comment out the "rsync" line in ~/.gitolite.rc IMMEDIATELY.

    DO NOT LEAVE IT ENABLED IF YOU ARE UNABLE TO UPGRADE IMMEDIATELY!
    Uncomment it only after you have upgraded or patched.

3.  That bad, huh?

    Sadly, yes :(

DETAILS:

This program is not a core program.  Despite the name, it will not
function as a generic "rsync".

This is *only* meant to help out people who are on flaky connections,
trying to clone a large repo.

Because git clone is not resumable, one common technique is to have
someone create a "bundle", then download the bundle to seed the local
repo, then "git fetch" to finish off.  Since the bundle is a single
file, you can use resumable mechanisms (like rsync) to download it.

What this command does is allow that kind of bundling to happen
automatically, if an administrator enables it.

The user simply rsyncs a bundle file using his gitolite
credentials.  As a result, the rsync helper command that ships
with gitolite is executed.  This program manages the creation
and expiry of bundle files, then passes control to the *real*
rsync program to perform the actual data transfer.

It is this last step that requires special care when used in a
restricted environment, resulting in the need for this patch.

src/commands/rsync

@@ -28,11 +28,6 @@ BUNDLE SUPPORT
 
     (2) Add 'rsync' to the ENABLE list in the rc file
 
-
-GENERIC RSYNC SUPPORT
-
-    TBD
-
 =cut
 
 =for usage
@@ -43,17 +38,16 @@ BUNDLE SUPPORT
     Admins: see src/commands/rsync for setup instructions
 
     Users:
-        rsync -P git@host:repo.bundle .
+        rsync git@host:repo.bundle .
             # downloads a file called "<basename of repo>.bundle"; repeat as
             # needed till the whole thing is downloaded
         git clone repo.bundle repo
         cd repo
         git remote set-url origin git@host:repo
         git fetch origin    # and maybe git pull, etc. to freshen the clone
 
-GENERIC RSYNC SUPPORT
-
-    TBD
+    NOTE on options to the rsync command: you are only allowed to use the
+    "-v", "-n", "-q", and "-P" options.
 
 =cut
 
@@ -62,9 +56,9 @@ usage() if not @ARGV or $ARGV[0] eq '-h';
 # rsync driver program.  Several things can be done later, but for now it
 # drives just the 'bundle' transfer.
 
-if ( $ENV{SSH_ORIGINAL_COMMAND} =~ /^rsync --server --sender (-[-\w=.]+ )+\. (\S+)\.bundle$/ ) {
+if ( $ENV{SSH_ORIGINAL_COMMAND} =~ /^rsync --server --sender (?:-[vn]*(?:e\d*\.\w*)? )?\. (\S+)\.bundle$/ ) {
 
-    my $repo = $2;
+    my $repo = $1;
     $repo =~ s/\.git$//;
 
     # all errors have the same message to avoid leaking info
@@ -81,7 +75,7 @@ if ( $ENV{SSH_ORIGINAL_COMMAND} =~ /^rsync --server --sender (-[-\w=.]+ )+\. (\S
     exit 0;
 }
 
-_warn "invalid rsync command '$ENV{SSH_ORIGINAL_COMMAND}'";
+_warn "Sorry, you are only allowed to use the '-v', '-n', '-q', and '-P' options.";
 usage();
 
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