Hi,
> -----Original Message-----
> From: php-dev@coydogsoftware.net [mailto:php-dev@coydogsoftware.net]
> Sent: Thursday, November 10, 2016 3:10 PM
> To: Dmitry Stogov <dmitry@zend.com>
> Cc: internals@lists.php.net; rasmus@lerdorf.com; Zeev Suraski
> <zeev@zend.com>; Anatol Belski (ab@php.net) <ab@php.net>
> Subject: Re: [PATCH] opcache bug #69090, prepend user identifier to keys
>
> Hello,
>
> Thank you for the response. Replies inline:
>
> On Thu, Nov 10, 2016 at 08:51:58AM +0000, Dmitry Stogov wrote:
> >
> > I see the problem(s) and I took a look into the patch.
>
> Can you confirm that you see the permissions bypass problem? I've seen the
> chroot filename collision problem acknowledged in the bugtracker and in
old
> php-internals posts, but I've seen nobody from the PHP Project explicitly
> acknowledge the permissions bypass vulnerability. If my meaning isn't
clear I can
> provide proof of concept off-list. The permissions bypass affects both
> apache2handler (even with mod_ruid2) and FPM (even with user pools).
>
> They're separate problems both stemming from the simple filename design of
> the cache keys.
>
> > From the first look, I don't like the proposed solution.
> >
> > It makes things a bit better, but can't solve shared-hosting
> > configuration problems.
>
> I'll be the first to agree it's not perfect; it's a band-aid from someone
with no
> prior familiarity with the codebase. I was just surprised a trivially
exploitable
> security hole like this was unpatched for 2+ years and thought I would
take a
> stab at a quick solution. Can you elaborate on what shared-hosting
problems it
> doesn't solve (aside from chroot name collisions)?
>
> > It doesn't solve even the simple chroot file resolution problem in
> > general (one user ma have few chroot environments with conflicting
> > file names).
>
> Agreed. Putting device+inode in the key would properly fix the chroot
scenario,
> but wouldn't the inode be readable if the parent directory is readable?
This could
> result in unexpected behaviour; ie a script belonging to user alice,
readable only
> by alice, can be run by user bob if the parent directory is readable.
>
> Plus there's the performance considerations of stat(); I know better than
to put
> the stat() call in the hot function. Apparently APC used a stat struct
passed from
> the SAPI? But how did this work with included/required scripts which the
SAPI
> wasn't aware of; were they all cached under the parent script's key? I've
> skimmed that code in APC but didn't have time for proper analysis. I admit
> ignorance here and thus I preferred to leave dev+inode keying to the
experts.
>
> I still strongly believe a user identifier is needed in addition to
> dev+inode due to the permissions bypass issue.
>
> > I'm not sure, if it's possible to make chroot on Windows, so why we
> > need to add windows user names?
>
> Frankly I don't know if any Windows configurations are vulnerable. I have
no
> experience with the Windows SAPI's. I didn't want to break the Windows
build,
> and wanted to keep the functionality analagous between Windows and other
> platforms rather than leaving a possibly exploitable design on Windows.
>
> But again I should stress that *chroot filename collisions are not the
only bad
> behavior here.* They're not the bug I'm most concerned with.
>
> > The patch introduces syscall in the hot function (this may be
> > optimized).
>
> Agreed. That isn't ideal. But the geteuid() call shouldn't be done during
opcache
> initialization when the SHM object is initialized, because EUID might
change
> afterwards. I didn't want to get EUID too early so I erred on the side of
caution,
> getting it at the last possible moment. This is slower but safer because
it
> prevents trivial cross-user cache access from PHP userland. I'm open to
> suggestion if there's a more "local" initialization function outside of
key
> generation, which is guaranteed to run after EUID changes in both FPM user
> pool, and mod_ruid2/mod_php.
>
> > I'm open for discussion and may change my mind. I'll also try to find
> > a better solution. Any suggestions are welcome.
>
> Frankly I think the better solution for FPM, would be to avoid doing
OPCache
> SHM object initialization in the FPM master before user pools have forked
and
> set EUID. That would fix both the permissions bypass and probably key
collisions
> in chroots. mod_fcgid didn't have these problems because PHP parent
processes
> were started independently for each vhost.
>
> I'm not familiar enough with FPM code (yet) to be more specific, and I
don't like
> the fact that this doesn't doesn't address permissions bypass with
> mod_php/mod_ruid2 which is a popular configuration which people
> *think* is safe for shared hosting.
>
> Ideally I'd like to see OPCache keys fixed with a user identifier and
> dev+inode, *and* FPM fixed as described above :).
>
> Thank you again for taking time to comment. I look forward to your
thoughts.
> Shall I send proof of concept for the permissions bypass, off-list?
>
While inodes would indeed separate the cached physical entries, the data
would be saved into same shared memory segment, currently at least with MMAP
and SHM. I saw some hostings there, offering the file cache only part,
probably for this reason. So chroot or suphp are unlikely enforce the real
jail for Opcache.
Talking about the Windows side, I can link to the ticket I've investigated
some time ago https://bugs.php.net/bug.php?id=72645#1469569537 . The model
there is in many points different. Under IIS, the current implementation
allows separating caches using impersonation, different app pools, ACLs and
user accounts. This approach is certainly complicated, but gives also more
configuration flexibility. For Apache, running as a service, the config
possibilities would be limited, but still it should be possible to have a
separation per site/user for FCGI.
I was thinking about combiding these approaches. Maybe it might make sense
to use separate memory per site/user/etc. This could be achieved by
extending the SHM model to respect a configurable key. The key could be
generated by ftok() automatically or preconfigured by a shared hoster, or
both, whatsoever.
Pro for separating SHM
- SHM key can be made unique per INI, even if done automatically with ftok -
it's a onetime effort
- no clashes can happen per se
- works for any SAPI
A downside I could think about - higher memory usage caused to hosting. But
that would happen, when inodes are used, too. For multisite approaches, like
it is possible fe with Drupal and others, another smarter configuration
would be needed anyway, to exclude the config files, etc. But having a
separation of memory pools looks to me like a clean and much simpler way to
exclude any thinkable implications to cache mistakes.
Regards
Anatol
