Re: [PATCH 06/10] dma: Use free_decrypted_pages()

[Edgecombe, Rick P]

On Wed, 2023-10-18 at 18:42 +0100, Robin Murphy wrote:
> On 2023-10-17 21:25, Rick Edgecombe wrote:
> > On TDX it is possible for the untrusted host to cause
> > set_memory_encrypted() or set_memory_decrypted() to fail such that
> > an
> > error is returned and the resulting memory is shared. Callers need
> > to take
> > care to handle these errors to avoid returning decrypted (shared)
> > memory to
> > the page allocator, which could lead to functional or security
> > issues.
> > 
> > DMA could free decrypted/shared pages if set_memory_decrypted()
> > fails.
> > Use the recently added free_decrypted_pages() to avoid this.
> > 
> > Several paths also result in proper encrypted pages being freed
> > through
> > the same freeing function. Rely on free_decrypted_pages() to not
> > leak the
> > memory in these cases.
> 
> If something's needed in the fallback path here, what about the 
> cma_release() paths?

You mean inside cma_release(). If so, unfortunately I think it won't
fit great because there are callers that are never dealing with shared
memory (huge tlb). The reset-to-private operation does extra work that
would be nice to avoid when possible.

The cases I thought exhibited the issue were the two calls sites of
dma_set_decrypted(). Playing around with it, I was thinking it might be
easier to just fix those to open code leaking the pages on
dma_set_decrypted() error. In which case it won't have the re-encrypt
problem.

It make's it less fool proof, but more efficient. And
free_decrypted_pages() doesn't fit great anyway, as pointed out by
Christoph.