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.
In terms of security, the problematic case is guest PTEs mapping the
shared alias GFNs, since the VMM has control of the shared mapping in the
EPT/NPT.

Such conversion errors may herald future system instability, but are
temporarily survivable with proper handling in the caller. The kernel
traditionally makes every effort to keep running, but it is expected that
some coco guests may prefer to play it safe security-wise, and panic in
this case. To accommodate both cases, warn when the arch breakouts for
converting memory at the VMM layer return an error to CPA. Security focused
users can rely on panic_on_warn to defend against bugs in the callers. Some
VMMs are not known to behave in the troublesome way, so users that would
like to terminate on any unusual behavior by the VMM around this will be
covered as well.

Since the arch breakouts host the logic for handling coco implementation
specific errors, an error returned from them means that the set_memory()
call is out of options for handling the error internally. Make this the
condition to warn about.

It is possible that very rarely these functions could fail due to guest
memory pressure (in the case of failing to allocate a huge page when
splitting a page table). Don't warn in this case because it is a lot less
likely to indicate an attack by the host and it is not clear which
set_memory() calls should get the same treatment. That corner should be
addressed by future work that considers the more general problem and not
just papers over a single set_memory() variant.
Suggested-by: Michael Kelley (LINUX) <mikelley@microsoft.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Reviewed-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/all/20240122184003.129104-1-rick.p.edgecombe%40intel.com