Re: [PATCH, RFC 44/62] x86/mm: Set KeyIDs in encrypted VMAs for MKTME

[Alison Schofield]

On Fri, Jun 14, 2019 at 11:26:10AM -0700, Dave Hansen wrote:
> On 6/14/19 10:33 AM, Alison Schofield wrote:
> > Preserving the data across encryption key changes has not
> > been a requirement. I'm not clear if it was ever considered
> > and rejected. I believe that copying in order to preserve
> > the data was never considered.
> 
> We could preserve the data pretty easily.  It's just annoying, though.
> Right now, our only KeyID conversions happen in the page allocator.  If
> we were to convert in-place, we'd need something along the lines of:
> 
> 	1. Allocate a scratch page
> 	2. Unmap target page, or at least make it entirely read-only
> 	3. Copy plaintext into scratch page
> 	4. Do cache KeyID conversion of page being converted:
> 	   Flush caches, change page_ext metadata
> 	5. Copy plaintext back into target page from scratch area
> 	6. Re-establish PTEs with new KeyID

Seems like the 'Copy plaintext' steps might disappoint the user, as
much as the 'we don't preserve your data' design. Would users be happy
w the plain text steps ?
Alison

> 
> #2 is *really* hard.  It's similar to the problems that the poor
> filesystem guys are having with RDMA these days when RDMA is doing writes.
> 
> What we have here (destroying existing data) is certainly the _simplest_
> semantic.  We can certainly give it a different name, or even non-PROT_*
> semantics where it shares none of mprotect()'s functionality.
> 
> Doesn't really matter to me at all.