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Commit e81494b7 authored by Daniel Sneddon's avatar Daniel Sneddon Committed by Greg Kroah-Hartman
Browse files

x86/speculation: Add Gather Data Sampling mitigation 



commit 8974eb588283b7d44a7c91fa09fcbaf380339f3a upstream

Gather Data Sampling (GDS) is a hardware vulnerability which allows
unprivileged speculative access to data which was previously stored in
vector registers.

Intel processors that support AVX2 and AVX512 have gather instructions
that fetch non-contiguous data elements from memory. On vulnerable
hardware, when a gather instruction is transiently executed and
encounters a fault, stale data from architectural or internal vector
registers may get transiently stored to the destination vector
register allowing an attacker to infer the stale data using typical
side channel techniques like cache timing attacks.

This mitigation is different from many earlier ones for two reasons.
First, it is enabled by default and a bit must be set to *DISABLE* it.
This is the opposite of normal mitigation polarity. This means GDS can
be mitigated simply by updating microcode and leaving the new control
bit alone.

Second, GDS has a "lock" bit. This lock bit is there because the
mitigation affects the hardware security features KeyLocker and SGX.
It needs to be enabled and *STAY* enabled for these features to be
mitigated against GDS.

The mitigation is enabled in the microcode by default. Disable it by
setting gather_data_sampling=off or by disabling all mitigations with
mitigations=off. The mitigation status can be checked by reading:

    /sys/devices/system/cpu/vulnerabilities/gather_data_sampling

Signed-off-by: default avatarDaniel Sneddon <daniel.sneddon@linux.intel.com>
Signed-off-by: default avatarDave Hansen <dave.hansen@linux.intel.com>
Acked-by: default avatarJosh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: default avatarDaniel Sneddon <daniel.sneddon@linux.intel.com>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent 2323f105

Documentation/ABI/testing/sysfs-devices-system-cpu

+6 −5
Original line number Diff line number Diff line
@@ -472,16 +472,17 @@ Description: information about CPUs heterogeneity. 
		cpu_capacity: capacity of cpu#.



What:		/sys/devices/system/cpu/vulnerabilities

		/sys/devices/system/cpu/vulnerabilities/gather_data_sampling

		/sys/devices/system/cpu/vulnerabilities/itlb_multihit

		/sys/devices/system/cpu/vulnerabilities/l1tf

		/sys/devices/system/cpu/vulnerabilities/mds

		/sys/devices/system/cpu/vulnerabilities/meltdown

		/sys/devices/system/cpu/vulnerabilities/mmio_stale_data

		/sys/devices/system/cpu/vulnerabilities/spec_store_bypass

		/sys/devices/system/cpu/vulnerabilities/spectre_v1

		/sys/devices/system/cpu/vulnerabilities/spectre_v2

		/sys/devices/system/cpu/vulnerabilities/spec_store_bypass

		/sys/devices/system/cpu/vulnerabilities/l1tf

		/sys/devices/system/cpu/vulnerabilities/mds

		/sys/devices/system/cpu/vulnerabilities/srbds

		/sys/devices/system/cpu/vulnerabilities/tsx_async_abort

		/sys/devices/system/cpu/vulnerabilities/itlb_multihit

		/sys/devices/system/cpu/vulnerabilities/mmio_stale_data

Date:		January 2018

Contact:	Linux kernel mailing list <linux-kernel@vger.kernel.org>

Description:	Information about CPU vulnerabilities

Documentation/admin-guide/hw-vuln/gather_data_sampling.rst

0 → 100644
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.. SPDX-License-Identifier: GPL-2.0



GDS - Gather Data Sampling

==========================



Gather Data Sampling is a hardware vulnerability which allows unprivileged

speculative access to data which was previously stored in vector registers.



Problem

-------

When a gather instruction performs loads from memory, different data elements

are merged into the destination vector register. However, when a gather

instruction that is transiently executed encounters a fault, stale data from

architectural or internal vector registers may get transiently forwarded to the

destination vector register instead. This will allow a malicious attacker to

infer stale data using typical side channel techniques like cache timing

attacks. GDS is a purely sampling-based attack.



The attacker uses gather instructions to infer the stale vector register data.

The victim does not need to do anything special other than use the vector

registers. The victim does not need to use gather instructions to be

vulnerable.



Because the buffers are shared between Hyper-Threads cross Hyper-Thread attacks

are possible.



Attack scenarios

----------------

Without mitigation, GDS can infer stale data across virtually all

permission boundaries:



	Non-enclaves can infer SGX enclave data

	Userspace can infer kernel data

	Guests can infer data from hosts

	Guest can infer guest from other guests

	Users can infer data from other users



Because of this, it is important to ensure that the mitigation stays enabled in

lower-privilege contexts like guests and when running outside SGX enclaves.



The hardware enforces the mitigation for SGX. Likewise, VMMs should  ensure

that guests are not allowed to disable the GDS mitigation. If a host erred and

allowed this, a guest could theoretically disable GDS mitigation, mount an

attack, and re-enable it.



Mitigation mechanism

--------------------

This issue is mitigated in microcode. The microcode defines the following new

bits:



 ================================   ===   ============================

 IA32_ARCH_CAPABILITIES[GDS_CTRL]   R/O   Enumerates GDS vulnerability

                                          and mitigation support.

 IA32_ARCH_CAPABILITIES[GDS_NO]     R/O   Processor is not vulnerable.

 IA32_MCU_OPT_CTRL[GDS_MITG_DIS]    R/W   Disables the mitigation

                                          0 by default.

 IA32_MCU_OPT_CTRL[GDS_MITG_LOCK]   R/W   Locks GDS_MITG_DIS=0. Writes

                                          to GDS_MITG_DIS are ignored

                                          Can't be cleared once set.

 ================================   ===   ============================



GDS can also be mitigated on systems that don't have updated microcode by

disabling AVX. This can be done by setting "clearcpuid=avx" on the kernel

command-line.



Mitigation control on the kernel command line

---------------------------------------------

The mitigation can be disabled by setting "gather_data_sampling=off" or

"mitigations=off" on the kernel command line. Not specifying either will

default to the mitigation being enabled.



GDS System Information

------------------------

The kernel provides vulnerability status information through sysfs. For

GDS this can be accessed by the following sysfs file:



/sys/devices/system/cpu/vulnerabilities/gather_data_sampling



The possible values contained in this file are:



 ============================== =============================================

 Not affected                   Processor not vulnerable.

 Vulnerable                     Processor vulnerable and mitigation disabled.

 Vulnerable: No microcode       Processor vulnerable and microcode is missing

                                mitigation.

 Mitigation: Microcode          Processor is vulnerable and mitigation is in

                                effect.

 Mitigation: Microcode (locked) Processor is vulnerable and mitigation is in

                                effect and cannot be disabled.

 Unknown: Dependent on

 hypervisor status              Running on a virtual guest processor that is

                                affected but with no way to know if host

                                processor is mitigated or vulnerable.

 ============================== =============================================



GDS Default mitigation

----------------------

The updated microcode will enable the mitigation by default. The kernel's

default action is to leave the mitigation enabled.

Documentation/admin-guide/hw-vuln/index.rst

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@@ -16,3 +16,4 @@ are configurable at compile, boot or run time. 
   multihit.rst

   special-register-buffer-data-sampling.rst

   processor_mmio_stale_data.rst

   gather_data_sampling.rst

Documentation/admin-guide/kernel-parameters.txt

+24 −9
Original line number Diff line number Diff line
@@ -1290,6 +1290,20 @@ 
			Format: off | on

			default: on



	gather_data_sampling=

			[X86,INTEL] Control the Gather Data Sampling (GDS)

			mitigation.



			Gather Data Sampling is a hardware vulnerability which

			allows unprivileged speculative access to data which was

			previously stored in vector registers.



			This issue is mitigated by default in updated microcode.

			The mitigation may have a performance impact but can be

			disabled.



			off:    Disable GDS mitigation.



	gcov_persist=	[GCOV] When non-zero (default), profiling data for

			kernel modules is saved and remains accessible via

			debugfs, even when the module is unloaded/reloaded.
@@ -2555,22 +2569,23 @@ 
				Disable all optional CPU mitigations.  This

				improves system performance, but it may also

				expose users to several CPU vulnerabilities.

				Equivalent to: nopti [X86,PPC]

				Equivalent to: gather_data_sampling=off [X86]

					       kpti=0 [ARM64]

					       nospectre_v1 [PPC]

					       kvm.nx_huge_pages=off [X86]

					       l1tf=off [X86]

					       mds=off [X86]

					       mmio_stale_data=off [X86]

					       no_entry_flush [PPC]

					       no_uaccess_flush [PPC]

					       nobp=0 [S390]

					       nopti [X86,PPC]

					       nospectre_v1 [PPC]

					       nospectre_v1 [X86]

					       nospectre_v2 [X86,PPC,S390,ARM64]

					       spectre_v2_user=off [X86]

					       spec_store_bypass_disable=off [X86,PPC]

					       spectre_v2_user=off [X86]

					       ssbd=force-off [ARM64]

					       l1tf=off [X86]

					       mds=off [X86]

					       tsx_async_abort=off [X86]

					       kvm.nx_huge_pages=off [X86]

					       no_entry_flush [PPC]

					       no_uaccess_flush [PPC]

					       mmio_stale_data=off [X86]



				Exceptions:

					       This does not have any effect on

arch/x86/include/asm/cpufeatures.h

+1 −0
Original line number Diff line number Diff line
@@ -409,5 +409,6 @@ 
#define X86_BUG_MMIO_UNKNOWN		X86_BUG(26) /* CPU is too old and its MMIO Stale Data status is unknown */

#define X86_BUG_RETBLEED		X86_BUG(27) /* CPU is affected by RETBleed */

#define X86_BUG_EIBRS_PBRSB		X86_BUG(28) /* EIBRS is vulnerable to Post Barrier RSB Predictions */

#define X86_BUG_GDS			X86_BUG(29) /* CPU is affected by Gather Data Sampling */



#endif /* _ASM_X86_CPUFEATURES_H */