[PATCH 5.18 0971/1095] x86/bugs: Enable STIBP for IBPB mitigated RETBleed

Greg Kroah-Hartman
2022-08-15T23:10:24Z

From: Kim Phillips <kim.phillips@amd.com>

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 <kim.phillips@amd.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
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 <gregkh@linuxfoundation.org>
---
Documentation/admin-guide/kernel-parameters.txt | 29 +++++++++++++++++-------
arch/x86/kernel/cpu/bugs.c | 10 ++++----
2 files changed, 27 insertions(+), 12 deletions(-)

--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5130,20 +5130,33 @@
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.
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -152,7 +152,7 @@ void __init check_bugs(void)
/*
* spectre_v2_user_select_mitigation() relies on the state set by
* retbleed_select_mitigation(); specifically the STIBP selection is
- * forced for UNRET.
+ * forced for UNRET or IBPB.
*/
spectre_v2_user_select_mitigation();
ssb_select_mitigation();
@@ -1172,7 +1172,8 @@ spectre_v2_user_select_mitigation(void)
boot_cpu_has(X86_FEATURE_AMD_STIBP_ALWAYS_ON))
mode = SPECTRE_V2_USER_STRICT_PREFERRED;

- if (retbleed_mitigation == RETBLEED_MITIGATION_UNRET) {
+ if (retbleed_mitigation == RETBLEED_MITIGATION_UNRET ||
+ retbleed_mitigation == RETBLEED_MITIGATION_IBPB) {
if (mode != SPECTRE_V2_USER_STRICT &&
mode != SPECTRE_V2_USER_STRICT_PREFERRED)
pr_info("Selecting STIBP always-on mode to complement retbleed mitigation\n");
@@ -2353,10 +2354,11 @@ static ssize_t srbds_show_state(char *bu

static ssize_t retbleed_show_state(char *buf)
{
- if (retbleed_mitigation == RETBLEED_MITIGATION_UNRET) {
+ if (retbleed_mitigation == RETBLEED_MITIGATION_UNRET ||
+ retbleed_mitigation == RETBLEED_MITIGATION_IBPB) {
if (boot_cpu_data.x86_vendor != X86_VENDOR_AMD &&
boot_cpu_data.x86_vendor != X86_VENDOR_HYGON)
- return sprintf(buf, "Vulnerable: untrained return thunk on non-Zen uarch\n");
+ return sprintf(buf, "Vulnerable: untrained return thunk / IBPB on non-AMD based uarch\n");

return sprintf(buf, "%s; SMT %s\n",
retbleed_strings[retbleed_mitigation],