dispatch: protect against malicious 'hg serve --stdio' invocations (sec)

Some shared-ssh installations assume that 'hg serve --stdio' is a safe
command to run for minimally trusted users. Unfortunately, the messy
implementation of argument parsing here meant that trying to access a
repo named '--debugger' would give the user a pdb prompt, thereby
sidestepping any hoped-for sandboxing. Serving repositories over HTTP(S)
is unaffected.

We're not currently hardening any subcommands other than 'serve'. If
your service exposes other commands to users with arbitrary repository
names, it is imperative that you defend against repository names of
'--debugger' and anything starting with '--config'.

The read-only mode of hg-ssh stopped working because it provided its hook
configuration to "hg serve --stdio" via --config parameter. This is banned for
security reasons now. This patch switches it to directly call ui.setconfig().
If your custom hosting infrastructure relies on passing --config to
"hg serve --stdio", you'll need to find a different way to get that configuration
into Mercurial, either by using ui.setconfig() as hg-ssh does in this patch,
or by placing an hgrc file someplace where Mercurial will read it.

[e-mail] provided some extra fixes for the dispatch code and
for hg-ssh in places that I overlooked.

contrib/hg-ssh

@@ -32,7 +32,7 b' command="hg-ssh --read-only repos/*"'
 # enable importing on demand to reduce startup time
 from mercurial import demandimport; demandimport.enable()
 
-from mercurial import dispatch
+from mercurial import dispatch, ui as uimod
 
 import sys, os, shlex
 
@@ -61,14 +61,15 b' def main():'
         repo = os.path.normpath(os.path.join(cwd, os.path.expanduser(path)))
         if repo in allowed_paths:
             cmd = ['-R', repo, 'serve', '--stdio']
+            req = dispatch.request(cmd)
             if readonly:
-                cmd += [
-                    '--config',
-                    'hooks.pretxnopen.hg-ssh=python:__main__.rejectpush',
-                    '--config',
-                    'hooks.prepushkey.hg-ssh=python:__main__.rejectpush'
-                    ]
-            dispatch.dispatch(dispatch.request(cmd))
+                if not req.ui:
+                    req.ui = uimod.ui.load()
+                req.ui.setconfig('hooks', 'pretxnopen.hg-ssh',
+                                 'python:__main__.rejectpush', 'hg-ssh')
+                req.ui.setconfig('hooks', 'prepushkey.hg-ssh',
+                                 'python:__main__.rejectpush', 'hg-ssh')
+            dispatch.dispatch(req)
         else:
             sys.stderr.write('Illegal repository "%s"\n' % repo)
             sys.exit(255)

mercurial/dispatch.py

@@ -155,6 +155,37 b' def _runcatch(req):'
         pass # happens if called in a thread
 
     def _runcatchfunc():
+        realcmd = None
+        try:
+            cmdargs = fancyopts.fancyopts(req.args[:], commands.globalopts, {})
+            cmd = cmdargs[0]
+            aliases, entry = cmdutil.findcmd(cmd, commands.table, False)
+            realcmd = aliases[0]
+        except (error.UnknownCommand, error.AmbiguousCommand,
+                IndexError, getopt.GetoptError):
+            # Don't handle this here. We know the command is
+            # invalid, but all we're worried about for now is that
+            # it's not a command that server operators expect to
+            # be safe to offer to users in a sandbox.
+            pass
+        if realcmd == 'serve' and '--stdio' in cmdargs:
+            # We want to constrain 'hg serve --stdio' instances pretty
+            # closely, as many shared-ssh access tools want to grant
+            # access to run *only* 'hg -R $repo serve --stdio'. We
+            # restrict to exactly that set of arguments, and prohibit
+            # any repo name that starts with '--' to prevent
+            # shenanigans wherein a user does something like pass
+            # --debugger or --config=ui.debugger=1 as a repo
+            # name. This used to actually run the debugger.
+            if (len(req.args) != 4 or
+                req.args[0] != '-R' or
+                req.args[1].startswith('--') or
+                req.args[2] != 'serve' or
+                req.args[3] != '--stdio'):
+                raise error.Abort(
+                    _('potentially unsafe serve --stdio invocation: %r') %
+                    (req.args,))
+
         try:
             debugger = 'pdb'
             debugtrace = {

tests/test-ssh.t

@@ -357,6 +357,19 b' Test (non-)escaping of remote paths with'
   abort: destination 'a repo' is not empty
   [255]
 
+Make sure hg is really paranoid in serve --stdio mode. It used to be
+possible to get a debugger REPL by specifying a repo named --debugger.
+  $ hg -R --debugger serve --stdio
+  abort: potentially unsafe serve --stdio invocation: ['-R', '--debugger', 'serve', '--stdio']
+  [255]
+  $ hg -R --config=ui.debugger=yes serve --stdio
+  abort: potentially unsafe serve --stdio invocation: ['-R', '--config=ui.debugger=yes', 'serve', '--stdio']
+  [255]
+Abbreviations of 'serve' also don't work, to avoid shenanigans.
+  $ hg -R narf serv --stdio
+  abort: potentially unsafe serve --stdio invocation: ['-R', 'narf', 'serv', '--stdio']
+  [255]
+
 Test hg-ssh using a helper script that will restore PYTHONPATH (which might
 have been cleared by a hg.exe wrapper) and invoke hg-ssh with the right
 parameters: