Re: [PATCH] x86/sgx: Allow enclaves to use Asynchrounous Exit Notification

[Jarkko Sakkinen <jarkko@kernel.org>]

On Tue, Jul 05, 2022 at 11:36:48AM -0700, Dave Hansen wrote:
> Short Version:
> 
> Allow enclaves to use the new Asynchronous EXit (AEX)
> notification mechanism.  This mechanism lets enclaves run a
> handler after an AEX event.  These handlers can run mitigations
> for things like SGX-Step[1].
> 
> AEX Notify will be made available both on upcoming processors and
> on some older processors through microcode updates.
> 
> Long Version:
> 
> == SGX Attribute Background ==
> 
> The SGX architecture includes a list of SGX "attributes".  These
> attributes ensure consistency and transparency around specific
> enclave features.
> 
> As a simple example, the "DEBUG" attribute allows an enclave to
> be debugged, but also destroys virtually all of SGX security.
> Using attributes, enclaves can know that they are being debugged.
> Attributes also affect enclave attestation so an enclave can, for
> instance, be denied access to secrets while it is being debugged.
> 
> The kernel keeps a list of known attributes and will only
> initialize enclaves that use a known set of attributes.  This
> kernel policy eliminates the chance that a new SGX attribute
> could cause undesired effects.
> 
> For example, imagine a new attribute was added called
> "PROVISIONKEY2" that provided similar functionality to
> "PROVISIIONKEY".  A kernel policy that allowed indiscriminate use
> of unknown attributes and thus PROVISIONKEY2 would undermine the
> existing kernel policy which limits use of PROVISIONKEY enclaves.
> 
> == AEX Notify Background ==
> 
> "Intel Architecture Instruction Set Extensions and Future
> Features - Version 45" is out[2].  There is a new chapter:
> 
> 	Asynchronous Enclave Exit Notify and the EDECCSSA User Leaf Function.
> 
> Enclaves exit can be either synchronous and consensual (EEXIT for
> instance) or asynchronous (on an interrupt or fault).  The
> asynchronous ones can evidently be exploited to single step
> enclaves[1], on top of which other naughty things can be built.
> 
> AEX Notify will be made available both on upcoming processors and
> on some older processors through microcode updates.
> 
> == The Problem ==
> 
> These attacks are currently entirely opaque to the enclave since
> the hardware does the save/restore under the covers. The
> Asynchronous Enclave Exit Notify (AEX Notify) mechanism provides
> enclaves an ability to detect and mitigate potential exposure to
> these kinds of attacks.
> 
> == The Solution ==
> 
> Define the new attribute value for AEX Notification.  Ensure the
> attribute is cleared from the list reserved attributes which
> allows it to be used in enclaves.
> 
> I just built this and ran it to make sure there were no obvious
> regressions since I do not have the hardware to test it handy.
> Tested-by's would be much appreciated.

Is this available on recent ucode updates e.g. for Icelake
or Geminilake?

> 
> 1. https://github.com/jovanbulck/sgx-step
> 2. https://cdrdv2.intel.com/v1/dl/getContent/671368?explicitVersion=true
> 
> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: Jarkko Sakkinen <jarkko@kernel.org>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: x86@kernel.org
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: linux-sgx@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> ---
>  arch/x86/include/asm/sgx.h | 25 ++++++++++++++++++-------
>  1 file changed, 18 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/include/asm/sgx.h b/arch/x86/include/asm/sgx.h
> index 3f9334ef67cd..f7328d8efd83 100644
> --- a/arch/x86/include/asm/sgx.h
> +++ b/arch/x86/include/asm/sgx.h
> @@ -110,17 +110,28 @@ enum sgx_miscselect {
>   * %SGX_ATTR_EINITTOKENKEY:	Allow to use token signing key that is used to
>   *				sign cryptographic tokens that can be passed to
>   *				EINIT as an authorization to run an enclave.
> + * %SGX_ATTR_ASYNC_EXIT_NOTIFY:	Allow enclaves to be notified after an
> + *				asynchronous exit has occurred.
>   */
>  enum sgx_attribute {
> -	SGX_ATTR_INIT		= BIT(0),
> -	SGX_ATTR_DEBUG		= BIT(1),
> -	SGX_ATTR_MODE64BIT	= BIT(2),
> -	SGX_ATTR_PROVISIONKEY	= BIT(4),
> -	SGX_ATTR_EINITTOKENKEY	= BIT(5),
> -	SGX_ATTR_KSS		= BIT(7),
> +	SGX_ATTR_INIT		   = BIT(0),
> +	SGX_ATTR_DEBUG		   = BIT(1),
> +	SGX_ATTR_MODE64BIT	   = BIT(2),
> +				  /* BIT(3) is reserved */
> +	SGX_ATTR_PROVISIONKEY	   = BIT(4),
> +	SGX_ATTR_EINITTOKENKEY	   = BIT(5),
> +				  /* BIT(6) is for CET */
> +	SGX_ATTR_KSS		   = BIT(7),
> +				  /* BIT(8) is reserved */
> +				  /* BIT(9) is reserved */
> +	SGX_ATTR_ASYNC_EXIT_NOTIFY = BIT(10),
>  };
>  
> -#define SGX_ATTR_RESERVED_MASK	(BIT_ULL(3) | BIT_ULL(6) | GENMASK_ULL(63, 8))
> +#define SGX_ATTR_RESERVED_MASK	(BIT_ULL(3) | \
> +				 BIT_ULL(6) | \
> +				 BIT_ULL(8) | \
> +				 BIT_ULL(9) | \
> +				 GENMASK_ULL(63, 11))
>  
>  /**
>   * struct sgx_secs - SGX Enclave Control Structure (SECS)
> -- 
> 2.34.1
> 

BR, Jarkko