x86/cpu/kvm: SRSO: Fix possible missing IBPB on VM-Exit
PATCH 1/1 x86/cpu/kvm: SRSO: Fix possible missing IBPB on VM-Exit
commit 318e8c339c9a0891c389298bb328ed0762a9935e upstream.
In [1] the meaning of the synthetic IBPB flags has been redefined for a better separation of concerns:
- ENTRY_IBPB -- issue IBPB on entry only
- IBPB_ON_VMEXIT -- issue IBPB on VM-Exit only
and the Retbleed mitigations have been updated to match this new semantics.
Commit [2] was merged shortly before [1], and their interaction was not handled properly. This resulted in IBPB not being triggered on VM-Exit in all SRSO mitigation configs requesting an IBPB there.
Specifically, an IBPB on VM-Exit is triggered only when X86_FEATURE_IBPB_ON_VMEXIT is set. However:
- X86_FEATURE_IBPB_ON_VMEXIT is not set for "spec_rstack_overflow=ibpb", because before [1] having X86_FEATURE_ENTRY_IBPB was enough. Hence, an IBPB is triggered on entry but the expected IBPB on VM-exit is not.
- X86_FEATURE_IBPB_ON_VMEXIT is not set also when "spec_rstack_overflow=ibpb-vmexit" if X86_FEATURE_ENTRY_IBPB is already set.
That's because before [1] this was effectively redundant. Hence, e.g. a "retbleed=ibpb spec_rstack_overflow=bpb-vmexit" config mistakenly reports the machine still vulnerable to SRSO, despite an IBPB being triggered both on entry and VM-Exit, because of the Retbleed selected mitigation config.
- UNTRAIN_RET_VM won't still actually do anything unless CONFIG_MITIGATION_IBPB_ENTRY is set.
For "spec_rstack_overflow=ibpb", enable IBPB on both entry and VM-Exit and clear X86_FEATURE_RSB_VMEXIT which is made superfluous by X86_FEATURE_IBPB_ON_VMEXIT. This effectively makes this mitigation option similar to the one for 'retbleed=ibpb', thus re-order the code for the RETBLEED_MITIGATION_IBPB option to be less confusing by having all features enabling before the disabling of the not needed ones.
For "spec_rstack_overflow=ibpb-vmexit", guard this mitigation setting with CONFIG_MITIGATION_IBPB_ENTRY to ensure UNTRAIN_RET_VM sequence is effectively compiled in. Drop instead the CONFIG_MITIGATION_SRSO guard, since none of the SRSO compile cruft is required in this configuration. Also, check only that the required microcode is present to effectively enabled the IBPB on VM-Exit.
Finally, update the KConfig description for CONFIG_MITIGATION_IBPB_ENTRY to list also all SRSO config settings enabled by this guard.
--- arch/x86/Kconfig | 3 ++- arch/x86/kernel/cpu/bugs.c | 20 ++++++++++++++------ 2 files changed, 16 insertions(+), 7 deletions(-)
arch/x86/Kconfig +2 -1
@@ -2506,7 +2506,8 @@ config CPU_IBPB_ENTRY depends on CPU_SUP_AMD && X86_64 default y help - Compile the kernel with support for the retbleed=ibpb mitigation. + Compile the kernel with support for the retbleed=ibpb and + spec_rstack_overflow={ibpb,ibpb-vmexit} mitigations. config CPU_IBRS_ENTRY bool "Enable IBRS on kernel entry"
arch/x86/kernel/cpu/bugs.c +14 -6
@@ -1092,6 +1092,8 @@ static void __init retbleed_select_mitigation(void)case RETBLEED_MITIGATION_IBPB :setup_force_cpu_cap (X86_FEATURE_ENTRY_IBPB ); +setup_force_cpu_cap (X86_FEATURE_IBPB_ON_VMEXIT ); + mitigate_smt = true;/* * IBPB on entry already obviates the need for
@@ -1101,8 +1103,6 @@ static void __init retbleed_select_mitigation(void)setup_clear_cpu_cap (X86_FEATURE_UNRET );setup_clear_cpu_cap (X86_FEATURE_RETHUNK ); - mitigate_smt = true; -/* * There is no need for RSB filling: entry_ibpb() ensures * all predictions, including the RSB, are invalidated,
@@ -2607,6 +2607,7 @@ static void __init srso_select_mitigation(void)if (IS_ENABLED (CONFIG_CPU_IBPB_ENTRY )) {if (has_microcode) {setup_force_cpu_cap (X86_FEATURE_ENTRY_IBPB ); +setup_force_cpu_cap (X86_FEATURE_IBPB_ON_VMEXIT ); srso_mitigation =SRSO_MITIGATION_IBPB ;/*
@@ -2616,6 +2617,13 @@ static void __init srso_select_mitigation(void)*/ setup_clear_cpu_cap (X86_FEATURE_UNRET );setup_clear_cpu_cap (X86_FEATURE_RETHUNK ); + +/* +* There is no need for RSB filling: entry_ibpb() ensures +* all predictions, including the RSB, are invalidated, +* regardless of IBPB implementation. +*/ +setup_clear_cpu_cap (X86_FEATURE_RSB_VMEXIT ); } }else {pr_err ("WARNING: kernel not compiled with CPU_IBPB_ENTRY.\n" );
@@ -2624,8 +2632,8 @@ static void __init srso_select_mitigation(void)break ;case SRSO_CMD_IBPB_ON_VMEXIT : -if (IS_ENABLED (CONFIG_CPU_SRSO )) { -if (!boot_cpu_has (X86_FEATURE_ENTRY_IBPB ) && has_microcode) { +if (IS_ENABLED (CONFIG_CPU_IBPB_ENTRY )) { +if (has_microcode) {setup_force_cpu_cap (X86_FEATURE_IBPB_ON_VMEXIT ); srso_mitigation =SRSO_MITIGATION_IBPB_ON_VMEXIT ;
@@ -2637,9 +2645,9 @@ static void __init srso_select_mitigation(void)setup_clear_cpu_cap (X86_FEATURE_RSB_VMEXIT ); } }else { -pr_err ("WARNING: kernel not compiled with CPU_SRSO.\n" ); +pr_err ("WARNING: kernel not compiled with CPU_IBPB_ENTRY.\n" ); goto pred_cmd; - } + }break ;default :
Signature · 1 line
2.48.1.601.g30ceb7b040-goog
Sasha Levin
maintainer[ Sasha's backport helper bot ]
Hi,
Summary of potential issues:
ℹ️ Patch is missing in 6.6.y (ignore if backport was sent)
The upstream commit SHA1 provided is correct: 318e8c339c9a0891c389298bb328ed0762a9935e
Status in newer kernel trees:
6.13.y | Present (different SHA1: 0bdda736ef7f)
6.12.y | Present (different SHA1: eea6d16f56e9)
6.6.y | Not found
Note: The patch differs from the upstream commit:
---
1: 318e8c339c9a0 ! 1: e509c2d77c608 x86/cpu/kvm: SRSO: Fix possible missing IBPB on VM-Exit
@@ Metadata
## Commit message ##
x86/cpu/kvm: SRSO: Fix possible missing IBPB on VM-Exit
+ commit 318e8c339c9a0891c389298bb328ed0762a9935e upstream.
+
In [1] the meaning of the synthetic IBPB flags has been redefined for a
better separation of concerns:
- ENTRY_IBPB -- issue IBPB on entry only
@@ Commit message
Signed-off-by: Linus Torvalds <[email protected]>
## arch/x86/Kconfig ##
-@@ arch/x86/Kconfig: config MITIGATION_IBPB_ENTRY
+@@ arch/x86/Kconfig: config CPU_IBPB_ENTRY
depends on CPU_SUP_AMD && X86_64
default y
help
@@ arch/x86/Kconfig: config MITIGATION_IBPB_ENTRY
+ Compile the kernel with support for the retbleed=ibpb and
+ spec_rstack_overflow={ibpb,ibpb-vmexit} mitigations.
- config MITIGATION_IBRS_ENTRY
+ config CPU_IBRS_ENTRY
bool "Enable IBRS on kernel entry"
## arch/x86/kernel/cpu/bugs.c ##
@@ arch/x86/kernel/cpu/bugs.c: static void __init retbleed_select_mitigation(void)
setup_clear_cpu_cap(X86_FEATURE_UNRET);
setup_clear_cpu_cap(X86_FEATURE_RETHUNK);
-- setup_force_cpu_cap(X86_FEATURE_IBPB_ON_VMEXIT);
- mitigate_smt = true;
-
/*
* There is no need for RSB filling: entry_ibpb() ensures
* all predictions, including the RSB, are invalidated,
@@ arch/x86/kernel/cpu/bugs.c: static void __init srso_select_mitigation(void)
- if (IS_ENABLED(CONFIG_MITIGATION_IBPB_ENTRY)) {
+ if (IS_ENABLED(CONFIG_CPU_IBPB_ENTRY)) {
if (has_microcode) {
setup_force_cpu_cap(X86_FEATURE_ENTRY_IBPB);
+ setup_force_cpu_cap(X86_FEATURE_IBPB_ON_VMEXIT);
@@ arch/x86/kernel/cpu/bugs.c: static void __init srso_select_mitigation(void)
+ setup_clear_cpu_cap(X86_FEATURE_RSB_VMEXIT);
}
} else {
- pr_err("WARNING: kernel not compiled with MITIGATION_IBPB_ENTRY.\n");
+ pr_err("WARNING: kernel not compiled with CPU_IBPB_ENTRY.\n");
@@ arch/x86/kernel/cpu/bugs.c: static void __init srso_select_mitigation(void)
+ break;
- ibpb_on_vmexit:
case SRSO_CMD_IBPB_ON_VMEXIT:
-- if (IS_ENABLED(CONFIG_MITIGATION_SRSO)) {
+- if (IS_ENABLED(CONFIG_CPU_SRSO)) {
- if (!boot_cpu_has(X86_FEATURE_ENTRY_IBPB) && has_microcode) {
-+ if (IS_ENABLED(CONFIG_MITIGATION_IBPB_ENTRY)) {
++ if (IS_ENABLED(CONFIG_CPU_IBPB_ENTRY)) {
+ if (has_microcode) {
setup_force_cpu_cap(X86_FEATURE_IBPB_ON_VMEXIT);
srso_mitigation = SRSO_MITIGATION_IBPB_ON_VMEXIT;
@@ arch/x86/kernel/cpu/bugs.c: static void __init srso_select_mitigation(void)
setup_clear_cpu_cap(X86_FEATURE_RSB_VMEXIT);
}
} else {
-- pr_err("WARNING: kernel not compiled with MITIGATION_SRSO.\n");
+- pr_err("WARNING: kernel not compiled with CPU_SRSO.\n");
++ pr_err("WARNING: kernel not compiled with CPU_IBPB_ENTRY.\n");
+ goto pred_cmd;
- }
-+ pr_err("WARNING: kernel not compiled with MITIGATION_IBPB_ENTRY.\n");
+ }
break;
+
default:
- break;---
Results of testing on various branches:
| Branch | Patch Apply | Build Test | |---------------------------|-------------|------------| | stable/linux-6.1.y | Success | Success |