diff options
Diffstat (limited to 'Documentation/admin-guide/hw-vuln')
| -rw-r--r-- | Documentation/admin-guide/hw-vuln/gather_data_sampling.rst | 109 | ||||
| -rw-r--r-- | Documentation/admin-guide/hw-vuln/index.rst | 1 | ||||
| -rw-r--r-- | Documentation/admin-guide/hw-vuln/spectre.rst | 21 |
3 files changed, 126 insertions, 5 deletions
diff --git a/Documentation/admin-guide/hw-vuln/gather_data_sampling.rst b/Documentation/admin-guide/hw-vuln/gather_data_sampling.rst new file mode 100644 index 000000000000..264bfa937f7d --- /dev/null +++ b/Documentation/admin-guide/hw-vuln/gather_data_sampling.rst @@ -0,0 +1,109 @@ +.. 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 gather_data_sampling="force" or +"clearcpuid=avx" on the kernel command-line. + +If used, these options will disable AVX use by turning off XSAVE YMM support. +However, the processor will still enumerate AVX support. Userspace that +does not follow proper AVX enumeration to check both AVX *and* XSAVE YMM +support will break. + +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. Specifying "gather_data_sampling=force" will +use the microcode mitigation when available or disable AVX on affected systems +where the microcode hasn't been updated to include the mitigation. + +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: AVX disabled, + no microcode Processor is vulnerable and microcode is missing + mitigation. AVX disabled as 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. diff --git a/Documentation/admin-guide/hw-vuln/index.rst b/Documentation/admin-guide/hw-vuln/index.rst index 2adec1e6520a..245468b0f2be 100644 --- a/Documentation/admin-guide/hw-vuln/index.rst +++ b/Documentation/admin-guide/hw-vuln/index.rst @@ -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 diff --git a/Documentation/admin-guide/hw-vuln/spectre.rst b/Documentation/admin-guide/hw-vuln/spectre.rst index 7e061ed449aa..0fba3758d0da 100644 --- a/Documentation/admin-guide/hw-vuln/spectre.rst +++ b/Documentation/admin-guide/hw-vuln/spectre.rst @@ -479,8 +479,16 @@ Spectre variant 2 On Intel Skylake-era systems the mitigation covers most, but not all, cases. See :ref:`[3] <spec_ref3>` for more details. - On CPUs with hardware mitigation for Spectre variant 2 (e.g. Enhanced - IBRS on x86), retpoline is automatically disabled at run time. + On CPUs with hardware mitigation for Spectre variant 2 (e.g. IBRS + or enhanced IBRS on x86), retpoline is automatically disabled at run time. + + Systems which support enhanced IBRS (eIBRS) enable IBRS protection once at + boot, by setting the IBRS bit, and they're automatically protected against + Spectre v2 variant attacks, including cross-thread branch target injections + on SMT systems (STIBP). In other words, eIBRS enables STIBP too. + + Legacy IBRS systems clear the IBRS bit on exit to userspace and + therefore explicitly enable STIBP for that The retpoline mitigation is turned on by default on vulnerable CPUs. It can be forced on or off by the administrator @@ -504,9 +512,12 @@ Spectre variant 2 For Spectre variant 2 mitigation, individual user programs can be compiled with return trampolines for indirect branches. This protects them from consuming poisoned entries in the branch - target buffer left by malicious software. Alternatively, the - programs can disable their indirect branch speculation via prctl() - (See :ref:`Documentation/userspace-api/spec_ctrl.rst <set_spec_ctrl>`). + target buffer left by malicious software. + + On legacy IBRS systems, at return to userspace, implicit STIBP is disabled + because the kernel clears the IBRS bit. In this case, the userspace programs + can disable indirect branch speculation via prctl() (See + :ref:`Documentation/userspace-api/spec_ctrl.rst <set_spec_ctrl>`). On x86, this will turn on STIBP to guard against attacks from the sibling thread when the user program is running, and use IBPB to flush the branch target buffer when switching to/from the program. |