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Subject[GIT PULL] x86 fix
Linus,

Please pull the latest x86/urgent git tree from:

git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git x86-urgent-2022-08-13

# HEAD: e6cfcdda8cbe81eaf821c897369a65fec987b404 x86/bugs: Enable STIBP for IBPB mitigated RETBleed

Fix the "IBPB mitigated RETBleed" mode of operation on AMD CPUs
(not turned on by default), which also need STIBP enabled (if
available) to be '100% safe' on even the shortest speculation
windows.

Thanks,

Ingo

------------------>
Kim Phillips (1):
x86/bugs: Enable STIBP for IBPB mitigated RETBleed


Documentation/admin-guide/kernel-parameters.txt | 29 ++++++++++++++++++-------
arch/x86/kernel/cpu/bugs.c | 10 +++++----
2 files changed, 27 insertions(+), 12 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 5e9147fe8968..523b19624026 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5209,20 +5209,33 @@
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.
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 6761668100b9..d50686ca5870 100644
--- 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();
@@ -1179,7 +1179,8 @@ 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");
@@ -2320,10 +2321,11 @@ static ssize_t srbds_show_state(char *buf)

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],
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