On Tue, 14 Nov 2017, Andy Lutomirski wrote: > On Tue, Nov 14, 2017 at 8:05 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > > On Tue, Nov 14, 2017 at 03:17:12PM +0000, Mathieu Desnoyers wrote: > >> I've tried to create a small single-threaded self-modifying loop in > >> user-space to trigger a trace cache or speculative execution quirk, > >> but I have not succeeded yet. I suspect that I would need to know > >> more about the internals of the processor architecture to create the > >> right stalls that would allow speculative execution to move further > >> ahead, and trigger an incoherent execution flow. Ideas on how to > >> trigger this would be welcome. > > > > I thought the whole problem was per definition multi-threaded. > > > > Single-threaded stuff can't get out of sync with itself; you'll always > > observe your own stores. > > > > And ISTR the JIT scenario being something like the JIT overwriting > > previously executed but supposedly no longer used code. And in this > > scenario you'd want to guarantee all CPUs observe the new code before > > jumping into it. > > > > The current approach is using mprotect(), except that on a number of > > platforms the TLB invalidate from that is not guaranteed to be strong > > enough to sync for code changes. > > > > On x86 the mprotect() should work just fine, since we broadcast IPIs for > > the TLB invalidate and the IRET from those will get the things synced up > > again (if nothing else; very likely we'll have done a MOV-CR3 which will > > of course also have sufficient syncness on it). > > > > But PowerPC, s390, ARM et al that do TLB invalidates without interrupts > > and don't guarantee their TLB invalidate sync against execution units > > are left broken by this scheme. > > > > On x86 single-thread, you can still get in trouble, I think. Do a > store, get migrated, execute the stored code. There's no actual > guarantee that the new CPU does a CR3 load due to laziness. The migration IPI will probably prevent that. Thanks, tglx -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
