On Thu, May 28, 2020 at 02:17:19PM -0400 Phil Auld wrote: > On Thu, May 28, 2020 at 07:01:28PM +0200 Peter Zijlstra wrote: > > On Sun, May 24, 2020 at 10:00:46AM -0400, Phil Auld wrote: > > > On Fri, May 22, 2020 at 05:35:24PM -0400 Joel Fernandes wrote: > > > > On Fri, May 22, 2020 at 02:59:05PM +0200, Peter Zijlstra wrote: > > > > [..] > > > > > > > It doens't allow tasks for form their own groups (by for example > > > > > > > setting > > > > > > > the key to that of another task). > > > > > > > > > > > > So for this, I was thinking of making the prctl pass in an integer. > > > > > > And 0 > > > > > > would mean untagged. Does that sound good to you? > > > > > > > > > > A TID, I think. If you pass your own TID, you tag yourself as > > > > > not-sharing. If you tag yourself with another tasks's TID, you can do > > > > > ptrace tests to see if you're allowed to observe their junk. > > > > > > > > But that would require a bunch of tasks agreeing on which TID to tag > > > > with. > > > > For example, if 2 tasks tag with each other's TID, then they would have > > > > different tags and not share. > > > > Well, don't do that then ;-) > > > > That was a poorly worded example :) >
Heh, sorry, I thought that was my statement. I do not mean to belittle Joel's example... That's a fine example of a totally different problem than I was thinking of :) Cheers, Phil > The point I was trying to make was more that one TID of a group (not cgroup!) > of tasks is just an arbitrary value. > > At a single process (or pair rather) level, sure, you can see it as an > identifier of whom you want to share with, but even then you have to tag > both processes with this. And it has less meaning when the whom you want to > share with is mutltiple tasks. > > > > > What's wrong with passing in an integer instead? In any case, we would > > > > do the > > > > CAP_SYS_ADMIN check to limit who can do it. > > > > So the actual permission model can be different depending on how broken > > the hardware is. > > > > > > Also, one thing CGroup interface allows is an external process to set > > > > the > > > > cookie, so I am wondering if we should use sched_setattr(2) instead of, > > > > or in > > > > addition to, the prctl(2). That way, we can drop the CGroup interface > > > > completely. How do you feel about that? > > > > > > > > > > I think it should be an arbitrary 64bit value, in both interfaces to avoid > > > any potential reuse security issues. > > > > > > I think the cgroup interface could be extended not to be a boolean but > > > take > > > the value. With 0 being untagged as now. > > > > How do you avoid reuse in such a huge space? That just creates yet > > another problem for the kernel to keep track of who is who. > > > > The kernel doesn't care or have to track anything. The admin does. > At the kernel level it's just matching cookies. > > Tasks A,B,C all can share core so you give them each A's TID as a cookie. > Task A then exits. Now B and C are using essentially a random value. > Task D comes along and want to share with B and C. You have to tag it > with A's old TID, which has no meaning at this point. > > And if A's TID ever gets reused. The new A` gets to share too. At some > level aren't those still 32bits? > > > With random u64 numbers, it even becomes hard to determine if you're > > sharing at all or not. > > > > Now, with the current SMT+MDS trainwreck, any sharing is bad because it > > allows leaking kernel privates. But under a less severe thread scenario, > > say where only user data would be at risk, the ptrace() tests make > > sense, but those become really hard with random u64 numbers too. > > > > What would the purpose of random u64 values be for cgroups? That only > > replicates the problem of determining uniqueness there. Then you can get > > two cgroups unintentionally sharing because you got lucky. > > > > Seems that would be more flexible for the admin. > > What if you had two cgroups you wanted to allow to run together? Or a > cgroup and a few processes from a different one (say with different > quotas or something). > > I don't have such use cases so I don't feel that strongly but it seemed > more flexible and followed the mechanism-in-kernel/policy-in-userspace > dictum rather than basing the functionality on the implementation details. > > > Cheers, > Phil > > > > Also, fundamentally, we cannot have more threads than TID space, it's a > > natural identifier. > > > > -- --
