Re: [PATCH 4/4] x86/tdx: Disable NOTIFY_ENABLES

[Dave Hansen]

On 12/9/22 05:25, Kirill A. Shutemov wrote:
> == Background ==
> 
> There is a class of side-channel attacks against SGX enclaves called
> "SGX Step"[1]. These attacks create lots of exceptions inside of
> enclaves. Basically, run an in-enclave instruction, cause an exception.
> Over and over.
> 
> There is a concern that a VMM could attack a TDX guest in the same way
> by causing lots of #VE's. The TDX architecture includes new
> countermeasures for these attacks. It basically counts the number of
> exceptions and can send another *special* exception once the number of
> VMM-induced #VE's hits a critical threshold[2].
> 
> == Problem ==
> 
> But, these special exceptions are independent of any action that the
> guest takes. They can occur anywhere that the guest executes. This
> includes sensitive areas like the entry code. The (non-paranoid) #VE
> handler is incapable of handling exceptions in these areas.
> 
> == Solution ==
> 
> Fortunately, the special exceptions can be disabled by the guest via
> write to NOTIFY_ENABLES TDCS field. NOTIFY_ENABLES is disabled by
> default, but might be enabled by a bootloader, firmware or an earlier
> kernel before the current kernel runs.
> 
> Disable NOTIFY_ENABLES feature explicitly and unconditionally. Any
> NOTIFY_ENABLES-based #VE's that occur before this point will end up
> in the early #VE exception handler and die due to unexpected exit
> reason.
> 
> [1] https://github.com/jovanbulck/sgx-step
> [2] https://intel.github.io/ccc-linux-guest-hardening-docs/security-spec.html#safety-against-ve-in-kernel-code
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>

Reviewed-by: Dave Hansen <dave.hansen@xxxxxxxxx>