Re: [PATCH 04/10] swiotlb: Use free_decrypted_pages()

[Petr Tesařík]

On Tue, 31 Oct 2023 15:54:52 +0000
"Edgecombe, Rick P" <rick.p.edgecombe@xxxxxxxxx> wrote:

> On Tue, 2023-10-31 at 11:43 +0100, Petr Tesařík wrote:
> > 
> > I admit I'm not familiar with the encryption/decryption API, but if a
> > __free_pages() is not sufficient here, then it is quite confusing.
> > The error label is reached only if set_memory_decrypted() returns
> > non-zero. My naive expectation is that the memory is *not* decrypted
> > in
> > that case and does not require special treatment. Is this assumption
> > wrong?  
> 
> Yea, the memory can still be decrypted, or partially decrypted. On x86,
> all the set_memory() calls can fail part way through the work, and they
> don't rollback the changes they had made up to that point.

Thank you for the explanation. So, after set_memory_decrypted() fails,
the pages become Schroedinger-crypted, but since its true state cannot
be observed by the guest kernel, it stays as such forever.

Sweet.

>[...]
> > OTOH I believe there is a bug in the logic. The subsequent
> > __free_pages() in swiotlb_alloc_tlb() would have to be changed to a
> > free_decrypted_pages(). However, I'm proposing a different approach
> > to
> > address the latter issue here:
> > 
> > https://lore.kernel.org/linux-iommu/20231026095123.222-1-petrtesarik@xxxxxxxxxxxxxxx/T/  
> 
> Oh, yes, that makes sense. I was planning to send a patch to just leak
> the pages if set_memory_decrypted() fails, after my v2 linked above is
> accepted. It could have a different label than the phys_limit check
> error path added in your linked patch, so that case would still free
> the perfectly fine encrypted pages.

Hm, should I incorporate this knowledge into a v2 of my patch and
address both issues?

Petr T