From fd96b61389f8650d880df4b7f8f55f880c19bff9 Mon Sep 17 00:00:00 2001 From: Kim Phillips Date: Mon, 8 Aug 2022 09:32:33 -0500 Subject: x86/bugs: Enable STIBP for IBPB mitigated RETBleed commit e6cfcdda8cbe81eaf821c897369a65fec987b404 upstream. AMD's "Technical Guidance for Mitigating Branch Type Confusion, Rev. 1.0 2022-07-12" whitepaper, under section 6.1.2 "IBPB On Privileged Mode Entry / SMT Safety" says: Similar to the Jmp2Ret mitigation, if the code on the sibling thread cannot be trusted, software should set STIBP to 1 or disable SMT to ensure SMT safety when using this mitigation. So, like already being done for retbleed=unret, and now also for retbleed=ibpb, force STIBP on machines that have it, and report its SMT vulnerability status accordingly. [ bp: Remove the "we" and remove "[AMD]" applicability parameter which doesn't work here. ] Fixes: 3ebc17006888 ("x86/bugs: Add retbleed=ibpb") Signed-off-by: Kim Phillips Signed-off-by: Borislav Petkov Cc: stable@vger.kernel.org # 5.10, 5.15, 5.19 Link: https://bugzilla.kernel.org/show_bug.cgi?id=206537 Link: https://lore.kernel.org/r/20220804192201.439596-1-kim.phillips@amd.com Signed-off-by: Greg Kroah-Hartman --- Documentation/admin-guide/kernel-parameters.txt | 29 ++++++++++++++++++------- 1 file changed, 21 insertions(+), 8 deletions(-) (limited to 'Documentation') diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt index 8b7c26d09045..f577c29f2093 100644 --- a/Documentation/admin-guide/kernel-parameters.txt +++ b/Documentation/admin-guide/kernel-parameters.txt @@ -4662,20 +4662,33 @@ Speculative Code Execution with Return Instructions) vulnerability. + AMD-based UNRET and IBPB mitigations alone do not stop + sibling threads from influencing the predictions of other + sibling threads. For that reason, STIBP is used on pro- + cessors that support it, and mitigate SMT on processors + that don't. + off - no mitigation auto - automatically select a migitation auto,nosmt - automatically select a mitigation, disabling SMT if necessary for the full mitigation (only on Zen1 and older without STIBP). - ibpb - mitigate short speculation windows on - basic block boundaries too. Safe, highest - perf impact. - unret - force enable untrained return thunks, - only effective on AMD f15h-f17h - based systems. - unret,nosmt - like unret, will disable SMT when STIBP - is not available. + ibpb - On AMD, mitigate short speculation + windows on basic block boundaries too. + Safe, highest perf impact. It also + enables STIBP if present. Not suitable + on Intel. + ibpb,nosmt - Like "ibpb" above but will disable SMT + when STIBP is not available. This is + the alternative for systems which do not + have STIBP. + unret - Force enable untrained return thunks, + only effective on AMD f15h-f17h based + systems. + unret,nosmt - Like unret, but will disable SMT when STIBP + is not available. This is the alternative for + systems which do not have STIBP. Selecting 'auto' will choose a mitigation method at run time according to the CPU. -- cgit v1.2.3