Messages in this thread |  | | Date | Mon, 17 Feb 2014 12:34:12 +0400 | From | Pavel Emelyanov <> | Subject | Re: [CRIU] [PATCH 1/3] prctl: reduce permissions to change boundaries of data, brk and stack |
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On 02/15/2014 12:09 AM, Eric W. Biederman wrote: > Pavel Emelyanov <xemul@parallels.com> writes: > >> On 02/14/2014 11:16 PM, Eric W. Biederman wrote: >>> Cyrill Gorcunov <gorcunov@gmail.com> writes: >>> >>>> On Fri, Feb 14, 2014 at 09:43:14PM +0400, Andrew Vagin wrote: >>>>>> My brain hurts just looking at this patch and how you are justifying it. >>>>>> >>>>>> For the resources you are mucking with below all you have to do is to >>>>>> verify that you are below the appropriate rlimit at all times and no >>>>>> CAP_SYS_RESOURCE check is needed. You only need CAP_SYS_RESOURCE >>>>>> to exceed your per process limits. >>>>>> >>>>>> All you have to do is to fix the current code to properly enforce the >>>>>> limits. >>>>> >>>>> I'm afraid what you are suggesting doesn't work. >>>>> >>>>> The first reason is that we can not change both boundaries in one call. >>>>> But when we are restoring these attributes, we may need to move their >>>>> too far. >>>> >>>> When this code was introduced, there were no user-namespace implementation, >>>> if I remember correctly, so CAP_SYS_RESOURCE was enough barrier point >>>> to prevent modifying this values by anyone. Now user-ns brings a limit -- >>>> we need somehow to provide a way to modify these mm fields having no >>>> CAP_SYS_RESOURCE set. "Verifying rlimit" not an option here because >>>> we're modifying members one by one (looking back I think this was not >>>> a good idea to modify the fields in this manner). >>>> >>>> Maybe we could improve this api and provide argument as a pointer >>>> to a structure, which would have all the fields we're going to >>>> modify, which in turn would allow us to verify that all new values >>>> are sane and fit rlimits, then we could (probably) deprecate old >>>> api if noone except c/r camp is using it (I actually can't imagine >>>> who else might need this api). Then CAP_SYS_RESOURCE requirement >>>> could be ripped off. Hm? (sure touching api is always "no-no" >>>> case, but maybe...) >>> >>> Hmm. Let me rewind this a little bit. >>> >>> I want to be very stupid and ask the following. >>> >>> Why can't you have the process of interest do: >>> ptrace(PTRACE_ATTACHME); >>> execve(executable, args, ...); >>> >>> /* Have the ptracer inject the recovery/fixup code */ >>> /* Fix up the mostly correct process to look like it has been >>> * executing for a while. >>> */ >> >> Let's imagine we do that. >> >> This means, that the whole memory contents should be restored _after_ >> the execve() call, since the execve() flushes old mappings. In >> that case we lose the ability to preserve any shared memory regions >> between any two processes. This "shared" can be either regular >> MAP_SHARED mappings or MAP_ANONYMOUS but still not COW-ed ones. > > If we have MAP_ANONYMOUS but not COW-ed mappings we have the correct > executable, which implies we have everything else correct except for the > brk and the stack addresses, because the process was started with fork. > > So while that sounds like an interesting case to handle it does not seem > to invalidate the idea of using exec to set all of the other fields when > we need to set them.
Well, yes, what you propose we call "inheritable resources". These are, e.g. SIDs or shared FD-table/MM-s. That's OK to restore them at fork(), but I'd like to draw your attention to two concerns I have with this approach.
1. Inheritable resources can be potentially restored more than one time. Consider you have tasks tree look like this:
task-A -[exe]-> A `- task-C1 -[exe]-> C `- task-C2 -[exe]-> C
IOW -- task A has executable A and two kids C1 and C2 that share executable C.
In that case the restore sequence should look like this
* Task A calls execve() on C * Task A forks C1 * Task A forks C2 * Task A calls execve() on A
This does work, I agree, but task A has to call execve() two times. And even more, if we had e.g. D1 and D2 kids with different exe D. Now, why I think that's a problem? Please, see concern #2 :)
2. What you propose means we have to effectively strace and execve-ing task. As compared with plain prlctl this is up to ~600 times slower. I've made such an experiment.
* Idle node with plenty of free RAM * Simple proggie doing execve() on self for 1000 times, compiled statically to avoid ld.so spoiling the times, run under strace * Another proggie doing open() + prlctl() 1000 times.
The first task took ~12 sec to complete. The second -- ~0.02 seconds.
If we take an average container of 100 tasks, even with all different exe links, your approach would give us ~1 sec more to restore, while existing one would be almost no op. And this hits us even without the inheritance scenario I demonstrated above.
Please, keep in mind, that checkpoint-restore in not only live-migration, we have use cases where restore cannot be pre-restored for better down-time. It _must_ be as fast as possible.
That said, Eric, I do agree with your concern about security, I _am_ ready to rework this stuff and kill the whole bunch of prctls we have. But please! Very please! Can we come up with mm->foo-s and ->exe_link restoration API that is at most ... 5 times slower than existing prlctl? It's really-really important for us!
Maybe we can make prlctl() do lite-execve()? It will open the executable, read the required amount of headers and just put data red from there onto mm-struct? This should be MUCH better, that full execve() with loading all binary data plus strace and flushing old mm-s.
> Eric
Thanks, Pavel
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