Messages in this thread | | | Date | Fri, 5 May 2017 08:59:20 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH V2] x86/tboot: add an option to disable iommu force on |
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* Joerg Roedel <jroedel@suse.de> wrote:
> On Thu, Apr 27, 2017 at 08:51:42AM +0200, Ingo Molnar wrote: > > > + tboot_noforce [Default Off] > > > + Do not force the Intel IOMMU enabled under tboot. > > > + By default, tboot will force Intel IOMMU on, which > > > + could harm performance of some high-throughput > > > + devices like 40GBit network cards, even if identity > > > + mapping is enabled. > > > + Note that using this option lowers the security > > > + provided by tboot because it makes the system > > > + vulnerable to DMA attacks. > > > > So what's the purpose of this kernel option? > > > > It sure isn't the proper solution for correctly architectured hardware/firmware > > (which can just choose not to expose the IOMMU!), and for one-time hacks for > > special embedded systems or for debugging why not just add an iommu=off option to > > force it off? > > I guess that tboot requires an IOMMU to be present in order to work. It > will do initial IOMMU setup and hands the hardware over to Linux later > on. > > The problem solved here is that someone wants tboot for security > reasons, but doesn't want the performance penalty of having the IOMMU > enabled and can live with the risk of an DMA attack.
Yes, that makes sense - but in this case it would be far more user friendly to make it a sysctl, not a boot option. This is also much more manageable for distributions and also allows it to be more easily turned into a security policy feature.
New boot options should be for debugging hacks in essence - any serious hardware configuration should be done via more user-friendly methods.
Thanks,
Ingo
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