On Jul 11, 2016 8:48 AM, "Dave Hansen" <dave.hansen@xxxxxxxxx> wrote: > > On 07/11/2016 07:45 AM, Andy Lutomirski wrote: > > On Mon, Jul 11, 2016 at 7:34 AM, Dave Hansen <dave@xxxxxxxx> wrote: > >> Should we instead just recommend to userspace that they lock down access > >> to keys by default in all threads as a best practice? > > > > Is that really better than doing it in-kernel? My concern is that > > we'll find library code that creates a thread, and that code could run > > before the pkey-aware part of the program even starts running. > > Yeah, so let's assume we have some pkey-unaware thread. The upside of a > scheme where the kernel preemptively (and transparently to the thread) > locks down PKRU is that the thread can't go corrupting any non-zero-pkey > structures that came from other threads. > > But, the downside is that the thread can not access any non-zero-pkey > structures without taking some kind of action with PKRU. That obviously > won't happen since the thread is pkeys-unaware to begin with. Would > that break these libraries unless everything using pkeys knows to only > share pkey=0 data with those threads? > Yes, but at least for the cases I can think of, that's probably a good thing. OTOH, I can see cases where you want everyone to be able to read but only specific code paths to be able to write. I think it's more or less impossible to get sensible behavior passing pkey != 0 data to legacy functions. If you call: void frob(struct foo *p); If frob in turn passes p to a thread, what PKRU is it supposed to use? > > So how is user code supposed lock down all of its threads? > > > > seccomp has TSYNC for this, but I don't think that PKRU allows > > something like that. > > I'm not sure this is possible for PKRU. Think of a simple PKRU > manipulation in userspace: > > pkru = rdpkru(); > pkru |= PKEY_DENY_ACCESS<<key*2; > wrpkru(pkru); > > If we push a PKRU value into a thread between the rdpkru() and wrpkru(), > we'll lose the content of that "push". I'm not sure there's any way to > guarantee this with a user-controlled register. We could try to insist that user code uses some vsyscall helper that tracks which bits are as-yet-unassigned. That's quite messy, though. We could also arbitrarily partition the key space into initially-wide-open, initially-read-only, and initially-no-access and let pkey_alloc say which kind it wants. --Andy -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>