Messages in this thread | | | Date | Wed, 10 Jan 2018 08:13:32 +0100 | From | Ingo Molnar <> | Subject | Re: [RFC PATCH v2 2/6] x86/arch_prctl: add ARCH_GET_NOPTI and ARCH_SET_NOPTI to enable/disable PTI |
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* Andy Lutomirski <luto@kernel.org> wrote:
> On Tue, Jan 9, 2018 at 6:54 AM, Willy Tarreau <w@1wt.eu> wrote: > > On Tue, Jan 09, 2018 at 03:51:57PM +0100, Borislav Petkov wrote: > >> On Tue, Jan 09, 2018 at 03:36:53PM +0100, Willy Tarreau wrote: > >> > I see and am not particularly against this, but what use case do you > >> > have in mind precisely ? I doubt it's just saving a few tens of bytes, > >> > so probably you're more concerned about the potential risks this opens ? > >> > But given we only allow this for CAP_SYS_RAWIO and these ones already > >> > have access to /dev/mem and many other things, don't you think there > >> > are much easier ways to dump kernel memory in this case than trying to > >> > inject some meltdown code into the victim process ? Or maybe you have > >> > other cases in mind that I'm not seeing. > >> > >> I'd like this to be config-controllable so that distros can make the > >> decision whether/if they want to support the whole per-mm thing. > > > > OK. > > > >> Also, if CAP_SYS_RAWIO is going to protect, please make the > >> ARCH_GET_NOPTI variant check it too. > > > > Interestingly I removed the check consecutive to the discussions. But > > I think I'll simply remove the whole ARCH_GET_NOPTI as it has no real > > value beyond initial development. > > > > I've thought about this a bit more. Here are my thoughts: > > 1. I don't like it being per-mm. I think it should be a per-thread > control so that a program can have a thread with PTI that runs > less-trusted JavaScript and other network threads with PTI off. > Obviously we lose NX protection mm-wide if any threads have PTI off. > I think the way to implement this is:
Btw., the "NX protection", the NX bit set in the PTI kernel pagetables for the user range really just matters for non-SMEP hardware, right? On SMEP a CPU in kernel privilege mode cannot execute user pages, i.e. the fact that it's user pages is already NX, guaranteed by the CPU.
And note how there's a happy circumstance for users, regarding SMEP and PTI NX:
- All Intel desktop/server CPUs currently sold and those built in the last ~3 years have SMEP enabled already, so are not affected.
- AMD CPUs don't have PTI enabled, so they already don't have NX for their user pages - no change in behavior.
I.e.: non-issue and not a real constraint on the flexibility of this ABI, AFAICS - it's "only" an implementational matter.
Thanks,
Ingo
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