Re: [RFC PATCH v3 13/20] x86: DMA support for memory encryption

[Michael S. Tsirkin]

On Tue, Nov 22, 2016 at 12:38:59PM +0100, Borislav Petkov wrote:
> On Tue, Nov 15, 2016 at 12:29:35PM -0600, Tom Lendacky wrote:
> > > Makes sense, but I think at least a dmesg warning here
> > > might be a good idea.
> > 
> > Good idea.  Should it be a warning when it is first being set up or
> > a warning the first time the bounce buffers need to be used.  Or maybe
> > both?
> 
> Ok, let me put my user hat on...
> 
> (... puts a felt hat ...)
> 
> so what am I supposed to do about this as a user? Go and physically
> remove those devices because I want to enable SME?!
> 
> IMO, the only thing we should do is issue a *single* warning -
> pr_warn_once - along the lines of:
> 
> "... devices present which due to SME will use bounce buffers and will
> cause their speed to diminish. Boot with sme=debug to see full info".
> 
> And then sme=debug will dump the whole gory details. I don't think
> screaming for each device is going to change anything in many cases.
> 99% of people don't care - they just want shit to work.

The issue is it's a (potential) security hole, not a slowdown.


> > > A boot flag that says "don't enable devices that don't support
> > > encryption" might be a good idea, too, since most people
> > > don't read dmesg output and won't notice the message.
> > 
> > I'll look into this. It might be something that can be checked as
> > part of the device setting its DMA mask or the first time a DMA
> > API is used if the device doesn't explicitly set its mask.
> 
> Still with my user hat on, what would be the purpose of such an option?
> 
> We already use bounce buffers so those devices do support encryption,
> albeit slower.
> 
> felt hat is confused.

To disable unsecure things. If someone enables SEV one might have an
expectation of security.  Might help push vendors to do the right thing
as a side effect.

> -- 
> Regards/Gruss,
>     Boris.
> 
> Good mailing practices for 400: avoid top-posting and trim the reply.