This is very useful for shared hosting, my current workaround is using:

owner /home/ rw,
owner /home/** mrwlkix,

But this would be much nicer. Anything I can do to help get this patch merged?

Well I also wrote a proposal to the mailing list, but no one answered. So as far as I read the next step would be to create an RFC, let people ask questions/suggest stuff and then put it up for a vote; so I'll try to get RFC karma next;
I'll let you know as soon as I got it up

@notti the no reaction part could be simply due to the fact, that nobody is using it. And RFC is the way to go.

-> https://wiki.php.net/rfc/fpm_change_hat

@lstrojny what happens next, will it help if I try it out?

voting just started and ends on 1.7.2013

So having change_hat functionality has been really useful for apache, and I think it would be great to have it in fpm. I know we have received requests for this functionality in the past.

Hrmm looking at the source, it looks like the change is one direction. That is a return call is never used. In this case it might be better to use aa_change_profile. The change_hat call has the deficiency that if an attacker can obtain the magic_token and do a the return call, then it can escape the confinement. This was necessary as it was designed for task that get reused like in apache_prefork. The magic token keeps the attacker from just being able to directly call the return, because if a wrong token is supplied the task is killed. Of course this assumes that an attacker can get/know the token, and also make the return call, if this is not the case the change_hat is as secure as change_profile.

The aa_change_profile call is designed for a one way transition, where the task does not need to return out of the child confinement. However it does have a deficiency in its current implementation that to transition to a child profile the parent needs to construct a fully qualified path. This basically means the parent needs to know its own confinement (have permission to query its confinement) and the child profile name. So to transition to a child profile like change_hat it needs to do a query like (in C sorry)
if (aa_getcon(&parent_name, NULL) == -1)
goto error;
if (sprintf(&buffer, "%s//%s", parent_name, hat_name) == -1)
goto error;
if (aa_change_profile(buffer) == -1)
goto error;

This of course isn't as convenient as the current change_hat api and will be fixed in the next release of apparmor, but for current versions if you are using hats/child profiles it is less than ideal and change_hat might be the better choice.

I passed 0 as magic token, which according to the manpage for change_hat should make it impossible to ever change back. So if the implementation of change_hat hasn't changed it should work as expected.
Reasoning behind all this is in the proposal at https://wiki.php.net/rfc/fpm_change_hat. Sorry if i haven't written that part clear enough.

Sadly the man page is wrong. Long ago this was indeed the case but it was changed years ago, I am not sure of exactly when. The man page was not correctly updated and the bug was not caught for years (finally fixed in feb 2011). I have searched back in the kernel code as far as I have direct history and the behavior is that 0 is not a permanent change.

I am sorry for the confusion and the man page being incorrect.

Woa that is bad news. I'll have a look at it at the weekend.

On Wed, Jun 26, 2013 at 12:11 PM, jjohansen notifications@github.comwrote:

Sadly the man page is wrong. Long ago this was indeed the case but it was
changed years ago, I am not sure of exactly when. The man page was not
correctly updated and the bug was not caught for years (finally fixed in
feb 2011). I have searched back in the kernel code as far as I have direct
history and the behavior is that 0 is not a permanent change.

I am sorry for the confusion and the man page being incorrect.

—
Reply to this email directly or view it on GitHubhttps://github.com//pull/343#issuecomment-20037622
.

@jjohansen: Yeah I had a look at it and I think your proposal for a different way looks best - I'll be implementing and testing it tomorrow. As soon as I have it working, I'll create a new pull request, add a reference to the new one and close this one.
Thanks for having a look at the code and the tip with the hats/profiles.

Fixed in #373