diff options
Diffstat (limited to 'Documentation/admin-guide/hw-vuln/spectre.rst')
| -rw-r--r-- | Documentation/admin-guide/hw-vuln/spectre.rst | 47 |
1 files changed, 35 insertions, 12 deletions
diff --git a/Documentation/admin-guide/hw-vuln/spectre.rst b/Documentation/admin-guide/hw-vuln/spectre.rst index 9e9556826450..cce768afec6b 100644 --- a/Documentation/admin-guide/hw-vuln/spectre.rst +++ b/Documentation/admin-guide/hw-vuln/spectre.rst @@ -422,6 +422,14 @@ The possible values in this file are: 'RSB filling' Protection of RSB on context switch enabled ============= =========================================== + - EIBRS Post-barrier Return Stack Buffer (PBRSB) protection status: + + =========================== ======================================================= + 'PBRSB-eIBRS: SW sequence' CPU is affected and protection of RSB on VMEXIT enabled + 'PBRSB-eIBRS: Vulnerable' CPU is vulnerable + 'PBRSB-eIBRS: Not affected' CPU is not affected by PBRSB + =========================== ======================================================= + Full mitigation might require a microcode update from the CPU vendor. When the necessary microcode is not available, the kernel will report vulnerability. @@ -465,14 +473,25 @@ Spectre variant 2 -mindirect-branch=thunk-extern -mindirect-branch-register options. If the kernel is compiled with a Clang compiler, the compiler needs to support -mretpoline-external-thunk option. The kernel config - CONFIG_RETPOLINE needs to be turned on, and the CPU needs to run with - the latest updated microcode. + CONFIG_MITIGATION_RETPOLINE needs to be turned on, and the CPU needs + to run with the latest updated microcode. 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. + + On Intel's enhanced IBRS systems, this includes cross-thread branch target + injections on SMT systems (STIBP). In other words, Intel eIBRS enables + STIBP, too. + + AMD Automatic IBRS does not protect userspace, and Legacy IBRS systems clear + the IBRS bit on exit to userspace, therefore both explicitly enable STIBP. The retpoline mitigation is turned on by default on vulnerable CPUs. It can be forced on or off by the administrator @@ -496,9 +515,12 @@ 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. @@ -587,8 +609,8 @@ kernel command line. Selecting 'on' will, and 'auto' may, choose a mitigation method at run time according to the CPU, the available microcode, the setting of the - CONFIG_RETPOLINE configuration option, and the - compiler with which the kernel was built. + CONFIG_MITIGATION_RETPOLINE configuration option, + and the compiler with which the kernel was built. Selecting 'on' will also enable the mitigation against user space to user space task attacks. @@ -602,9 +624,10 @@ kernel command line. retpoline,generic Retpolines retpoline,lfence LFENCE; indirect branch retpoline,amd alias for retpoline,lfence - eibrs enhanced IBRS - eibrs,retpoline enhanced IBRS + Retpolines - eibrs,lfence enhanced IBRS + LFENCE + eibrs Enhanced/Auto IBRS + eibrs,retpoline Enhanced/Auto IBRS + Retpolines + eibrs,lfence Enhanced/Auto IBRS + LFENCE + ibrs use IBRS to protect kernel Not specifying this option is equivalent to spectre_v2=auto. |