Explain why STIBP is needed with legacy IBRS as currently implemented (KERNEL_IBRS) and why STIBP is not needed when enhanced IBRS is enabled.
Fixes: 7c693f54c873 ("x86/speculation: Add spectre_v2=ibrs option to support Kernel IBRS") Cc: stable@vger.kernel.org Signed-off-by: KP Singh kpsingh@kernel.org --- Documentation/admin-guide/hw-vuln/spectre.rst | 21 ++++++++++++++----- 1 file changed, 16 insertions(+), 5 deletions(-)
diff --git a/Documentation/admin-guide/hw-vuln/spectre.rst b/Documentation/admin-guide/hw-vuln/spectre.rst index 3fe6511c5405..4d186f599d90 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.