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

Sathyanarayanan Kuppuswamy
2022-12-09T15:51:32Z

On 12/9/22 5:25 AM, 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@linux.intel.com>
> ---

I don't think you need to explicitly use section names (Background,
problem or solution) in the commit log. But it is up to you.

Rest looks good.

> arch/x86/coco/tdx/tdx.c | 7 +++++++
> 1 file changed, 7 insertions(+)
>
> diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
> index 0e47846ff8ff..c93c2fd2e113 100644
> --- a/arch/x86/coco/tdx/tdx.c
> +++ b/arch/x86/coco/tdx/tdx.c
> @@ -19,6 +19,10 @@
> #define TDX_GET_VEINFO 3
> #define TDX_GET_REPORT 4
> #define TDX_ACCEPT_PAGE 6
> +#define TDX_WR 8
> +
> +/* TDCS fields. To be used by TDG.VM.WR and TDG.VM.RD module calls */
> +#define TDCS_NOTIFY_ENABLES 0x9100000000000010
>
> /* TDX hypercall Leaf IDs */
> #define TDVMCALL_MAP_GPA 0x10001
> @@ -858,6 +862,9 @@ void __init tdx_early_init(void)
> tdx_parse_tdinfo(&cc_mask);
> cc_set_mask(cc_mask);
>
> + /* Kernel does not use NOTIFY_ENABLES and does not need random #VEs */
> + tdx_module_call(TDX_WR, 0, TDCS_NOTIFY_ENABLES, 0, -1ULL, NULL);
> +
> /*
> * All bits above GPA width are reserved and kernel treats shared bit
> * as flag, not as part of physical address.

--
Sathyanarayanan Kuppuswamy
Linux Kernel Developer