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

[Will Deacon]

On Mon, Sep 08, 2014 at 11:40:43PM +0100, Kees Cook wrote:
> On Mon, Sep 8, 2014 at 2:55 PM, Rabin Vincent <rabin@xxxxxx> wrote:
> > On Mon, Sep 08, 2014 at 12:16:34PM -0700, Kees Cook wrote:
> >> On Thu, Sep 04, 2014 at 06:27:48PM +0100, Will Deacon wrote:
> >> > On Thu, Sep 04, 2014 at 06:23:42PM +0100, Kees Cook wrote:
> >> > > Ah! If this is the case, perhaps we can get away with
> >> > > local_flush_tlb_kernel_range() then?
> >> >
> >> > That's a bit tricky, since you need to ensure that preemption is disabled
> >> > until the mapping is put back like it was.
> >>
> >> Okay, under both real hardware with the errata, and under QEMU, things seem
> >> to work with this change to the series. What do you think?
> >
> > Preemption is already disabled until the mapping is put back in this
> > patch.c code because interrupts are disabled from before the time
> > set_fixmap() is called until after clear_fixmap() is called.
> 
> Should I drop the preempt_disable/enable(), and just add a comment to
> set_fixmap()?
> 
> > I'd guess that Will meant other (future) callers of set_fixmap() would
> > have to ensure similar behaviour with set_fixmap() / clear_fixmap().
> >
> > Unless I'm missing something set/clear_fixmap() seem to be quite arch
> > specific and only really used on x86, so we could ensure that future
> > users on ARM perform the correct tlb flush:  the first user on ARM with
> > a non-atomic context (or you) could implement a set_fixmap() which does
> > the global flush and have this patch.c (and any other atomic context
> > callers) call __set_fixmap() directly.
> >
> > The change to local_flush_tlb_kernel_range() in __set_fixmap() would of
> > course be needed in that case, and IIRC that was what my original patch
> > had (via set_top_pte()).
> 
> 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.

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.

Will
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