Re: [PATCH v5 3/8] arm: fixmap: implement __set_fixmap()

[Will Deacon <will.deacon@arm.com>]

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.deacon@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