Hi Peter, 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 ;-)
We could also guard it with a mutex. First task to set the TID wins, the other thread just reuses the cookie of the TID that won. But I think we cannot just use the TID value as the cookie, due to TID wrap-around and reuse. Otherwise we could accidentally group 2 tasks. Instead, I suggest let us keep TID as the interface per your suggestion and do the needed ptrace checks, but convert the TID to the task_struct pointer value and use that as the cookie for the group of tasks sharing a core. Thoughts? thanks, - Joel > > > 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. > > 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. > > Also, fundamentally, we cannot have more threads than TID space, it's a > natural identifier.
