Re: [PATCH] [v2] x86/sgx: Allow enclaves to use Asynchrounous Exit Notification

[Jarkko Sakkinen <jarkko@kernel.org>]

On Wed, Jul 20, 2022 at 12:13:47PM -0700, Dave Hansen wrote:
> Changes from v1:
>  * Make sure SGX_ATTR_ASYNC_EXIT_NOTIFY is in the masks that are
>    used at bare-metal enclave initialization and that enumerates
>    available attributes to KVM guests.
> 
> --
> 
> 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.  Instead
> of adding to the open-coded lists of individual attributes,
> add named lists of privileged (disallowed by default) and
> unprivileged (allowed by default) attributes.  Add the AEX notify
> attribute as an unprivileged attribute, which will keep the kernel
> from rejecting enclaves with it set.
> 
> I just built this and ran it to make sure there were no obvious
> regressions since I do not have the hardware (and new microcde)
> to test it.
> 
> Testing on bare-metal and in VMs accompanied by Tested-by's
> would be much appreciated.  (This means you, Intel folks who
> actually have systems with the microcode that can do this.)
> 
> 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: "H. Peter Anvin" <hpa@zytor.com>
> Cc: Sean Christopherson <seanjc@google.com>
> Cc: Kai Huang <kai.huang@intel.com>
> Cc: Haitao Huang <haitao.huang@linux.intel.com>
> Cc: x86@kernel.org
> Cc: linux-sgx@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org

I think it would make sense to have a co-maintainer with better
access to unreleased ucode patches, if anyone is willing to
consider. Perhaps someone from Intel given the constraints.

This would help with features such as AEX Notify. I can work
out issues with existing hardware and also make sure that the
whole stack is usable (as I'm also consumer for SGX).

BR, Jarkko