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. 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@xxxxxxxxxxxxxxx> Cc: Jarkko Sakkinen <jarkko@xxxxxxxxxx> Cc: Andy Lutomirski <luto@xxxxxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Cc: Ingo Molnar <mingo@xxxxxxxxxx> Cc: Borislav Petkov <bp@xxxxxxxxx> Cc: x86@xxxxxxxxxx Cc: "H. Peter Anvin" <hpa@xxxxxxxxx> Cc: linux-sgx@xxxxxxxxxxxxxxx Cc: linux-kernel@xxxxxxxxxxxxxxx --- 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
