Messages in this thread | | | Date | Thu, 11 Sep 2014 08:27:06 -0700 | Subject | Re: [PATCH v5 3/8] arm: fixmap: implement __set_fixmap() | From | Kees Cook <> |
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On Wed, Sep 10, 2014 at 10:51 AM, Will Deacon <will.deacon@arm.com> wrote: > 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.
Okay, sounds good.
> >> > 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()?
The (local) TLB flush needs to happen for patch_text to do its work, so I'd rather it stay in set_fixmap, otherwise the flush calls will have to follow each call of set_fixmap.
Is there a reason tlb_ops_need_broadcast() doesn't check IS_ENABLED(CONFIG_ARM_ERRATA_798181) itself?
> Then that just leaves ftrace.
ftrace is already covered by stop_machine. Is there something I missing there?
-Kees
-- Kees Cook Chrome OS Security
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