110 lines
5.0 KiB
ReStructuredText
110 lines
5.0 KiB
ReStructuredText
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.. SPDX-License-Identifier: GPL-2.0
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GDS - Gather Data Sampling
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==========================
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Gather Data Sampling is a hardware vulnerability which allows unprivileged
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speculative access to data which was previously stored in vector registers.
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Problem
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-------
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When a gather instruction performs loads from memory, different data elements
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are merged into the destination vector register. However, when a gather
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instruction that is transiently executed encounters a fault, stale data from
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architectural or internal vector registers may get transiently forwarded to the
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destination vector register instead. This will allow a malicious attacker to
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infer stale data using typical side channel techniques like cache timing
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attacks. GDS is a purely sampling-based attack.
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The attacker uses gather instructions to infer the stale vector register data.
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The victim does not need to do anything special other than use the vector
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registers. The victim does not need to use gather instructions to be
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vulnerable.
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Because the buffers are shared between Hyper-Threads cross Hyper-Thread attacks
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are possible.
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Attack scenarios
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----------------
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Without mitigation, GDS can infer stale data across virtually all
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permission boundaries:
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Non-enclaves can infer SGX enclave data
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Userspace can infer kernel data
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Guests can infer data from hosts
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Guest can infer guest from other guests
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Users can infer data from other users
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Because of this, it is important to ensure that the mitigation stays enabled in
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lower-privilege contexts like guests and when running outside SGX enclaves.
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The hardware enforces the mitigation for SGX. Likewise, VMMs should ensure
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that guests are not allowed to disable the GDS mitigation. If a host erred and
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allowed this, a guest could theoretically disable GDS mitigation, mount an
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attack, and re-enable it.
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Mitigation mechanism
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--------------------
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This issue is mitigated in microcode. The microcode defines the following new
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bits:
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================================ === ============================
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IA32_ARCH_CAPABILITIES[GDS_CTRL] R/O Enumerates GDS vulnerability
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and mitigation support.
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IA32_ARCH_CAPABILITIES[GDS_NO] R/O Processor is not vulnerable.
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IA32_MCU_OPT_CTRL[GDS_MITG_DIS] R/W Disables the mitigation
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0 by default.
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IA32_MCU_OPT_CTRL[GDS_MITG_LOCK] R/W Locks GDS_MITG_DIS=0. Writes
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to GDS_MITG_DIS are ignored
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Can't be cleared once set.
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================================ === ============================
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GDS can also be mitigated on systems that don't have updated microcode by
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disabling AVX. This can be done by setting gather_data_sampling="force" or
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"clearcpuid=avx" on the kernel command-line.
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If used, these options will disable AVX use by turning off XSAVE YMM support.
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However, the processor will still enumerate AVX support. Userspace that
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does not follow proper AVX enumeration to check both AVX *and* XSAVE YMM
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support will break.
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Mitigation control on the kernel command line
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---------------------------------------------
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The mitigation can be disabled by setting "gather_data_sampling=off" or
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"mitigations=off" on the kernel command line. Not specifying either will default
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to the mitigation being enabled. Specifying "gather_data_sampling=force" will
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use the microcode mitigation when available or disable AVX on affected systems
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where the microcode hasn't been updated to include the mitigation.
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GDS System Information
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------------------------
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The kernel provides vulnerability status information through sysfs. For
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GDS this can be accessed by the following sysfs file:
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/sys/devices/system/cpu/vulnerabilities/gather_data_sampling
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The possible values contained in this file are:
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============================== =============================================
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Not affected Processor not vulnerable.
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Vulnerable Processor vulnerable and mitigation disabled.
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Vulnerable: No microcode Processor vulnerable and microcode is missing
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mitigation.
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Mitigation: AVX disabled,
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no microcode Processor is vulnerable and microcode is missing
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mitigation. AVX disabled as mitigation.
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Mitigation: Microcode Processor is vulnerable and mitigation is in
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effect.
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Mitigation: Microcode (locked) Processor is vulnerable and mitigation is in
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effect and cannot be disabled.
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Unknown: Dependent on
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hypervisor status Running on a virtual guest processor that is
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affected but with no way to know if host
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processor is mitigated or vulnerable.
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============================== =============================================
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GDS Default mitigation
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----------------------
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The updated microcode will enable the mitigation by default. The kernel's
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default action is to leave the mitigation enabled.
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