On Fri, Jun 14, 2019 at 12:11:23PM -0700, Dave Hansen wrote: > On 6/14/19 11:46 AM, Alison Schofield wrote: > > 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 ? > > Well, it got to be plaintext because they wrote it to memory in > plaintext in the first place, so it's kinda hard to disappoint them. :) > > IMNHO, the *vast* majority of cases, folks will allocate memory and then > put a secret in it. They aren't going to *get* a secret in some > mysterious fashion and then later decide they want to protect it. In > other words, the inability to convert it is pretty academic and not > worth the complexity. I'm not saying it is (required to preserve); but I do think it is somewhat surprising to have an mprotect() call destroy content. It's traditionally specified to not do that.
