Hi,
On Nov 10, 2016 5:10 PM, php-dev@coydogsoftware.net wrote:
>
> 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).
I didn't see the problem in real life, but it's clear, that serving of few chroot environments using the same cache may lead to duplicate keys. FPM with separate pools shouldn't be affected.
>
> 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)?
Serving different users by the same process opens a lot of ways to stole data (sessions, persistent caches, persistent conections, dirty memory, etc).
They should be served by different FPM pools or separate Apache processes.
>
> > 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.
Yeah, inode is not an exelent solution as well.
> I still strongly believe a user identifier is needed in addition to
> dev+inode due to the permissions bypass issue.
Opcache just wasn't designed to solve permission problems in shared hosting evvironment.
"user id prefix", looks for me like a quick hack, that doesn't solve the whole problem anyway.
>
> > 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.
Do you talk about executing "unreadable" PHP scripts of different users?
I think, the proper way to fix this, whould executing access() check on each cached script access (this might be enabled/disabled through php.ini)
>
> > 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.
RINIT
>
> > 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.
Let me check this.
>. 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?
Let methink a bit more.
Thanks. Dmitry.
