On Thu, Aug 04, 2022 at 02:22:01PM -0500, Kim Phillips wrote:
For retbleed=ibpb, force STIBP on machines that have it,
Because?
and report its SMT vulnerability status accordingly.
Fixes: 3ebc17006888 ("x86/bugs: Add retbleed=ibpb")
Signed-off-by: Kim Phillips <kim.phillips@xxxxxxx>
---
Documentation/admin-guide/kernel-parameters.txt | 4 +++-
arch/x86/kernel/cpu/bugs.c | 10 ++++++----
2 files changed, 9 insertions(+), 5 deletions(-)
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 597ac77b541c..127fa4328360 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5212,10 +5212,12 @@
ibpb - mitigate short speculation windows on
basic block boundaries too. Safe, highest
perf impact.
You should put some blurb here about STIBP and why it is being enabled,
where present.
@@ -2346,10 +2347,11 @@ 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");
Well, you can't lump those together.
> You can't especially say "Vulnerable" and "IBPB" in one line.
To quote from the BTC paper:
"Software may choose to perform an IBPB command on entry into privileged
code in order to avoid any previous branch prediction information from
subsequently being used. This effectively mitigates all forms of BTC for
scenarios like user-to-supervisor or VM-to-hypervisor attacks."
Especially if we disable SMT only on !STIBP parts:
if (mitigate_smt && !boot_cpu_has(X86_FEATURE_STIBP) &&
(retbleed_nosmt || cpu_mitigations_auto_nosmt()))
cpu_smt_disable(false);
If there are AMD parts which have IBPB but DO NOT have STIBP, then you
can say "Vulnerable... IBPB" but then you need to check for !STIBP and
issue that on a separate line.
I'd say...