Hi Kees, On Tue, Sep 09, 2014 at 03:33:11PM +0100, Kees Cook wrote: > On Tue, Sep 9, 2014 at 5:38 AM, Will Deacon <will.dea...@arm.com> wrote: > > On Mon, Sep 08, 2014 at 11:40:43PM +0100, Kees Cook wrote: > >> Ah, so it was, yes! Will, which version of this logic would you prefer? > > > > I still don't think we're solving the general problem here -- we're actually > > just making the ftrace case work. It is perfectly possible for another CPU > > to undergo a TLB miss and refill whilst the page table is being modified by > > the CPU with preemption disabled. In this case, a local tlb flush won't > > invalidate that entry on the other core, and we have no way of knowing when > > the original permissions are actually observed across the system. > > The fixmap is used by anything doing patching _except_ ftrace, > actually. It's used by jump labels, kprobes, and kgdb. This code is > the general case. Access to set_fixmap is done via the kernel patching > interface: patch_text(). > > Right now, the patch_text interface checks cache_ops_need_broadcast(), > and conditionally runs under stop_machine(). We could make this > unconditional, and we'll avoid any problem with TLB misses on another > CPU.
Yes, it we always use stop_machine, that solves the TLB broadcast problem and we could do that if CONFIG_ARM_ERRATA_798181 is set. > > So I think we need to figure out a way to invalidate the TLB properly. What > > do architectures that use IPIs for TLB broadcasting do (x86, some powerpc, > > mips, ...)? They must have exactly the same problem. > > I don't think this should be done at the set_fixmap level, as it is > more a primitive. I think making sure patch_text() always works would > be best. What do you think of using an unconditional stop_machine() > instead? Why not move the TLB invalidation into patch_text, then we can do stop_machine if IS_ENABLED(CONFIG_ARM_ERRATA_798181) || tlb_ops_need_broadcast()? Then that just leaves ftrace. Will -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
