From 318e8c339c9a0891c389298bb328ed0762a9935e Mon Sep 17 00:00:00 2001 From: Patrick Bellasi Date: Wed, 5 Feb 2025 14:04:41 +0000 Subject: [PATCH] x86/cpu/kvm: SRSO: Fix possible missing IBPB on VM-Exit 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. Fixes: 864bcaa38ee4 ("x86/cpu/kvm: Provide UNTRAIN_RET_VM") [1] Fixes: d893832d0e1e ("x86/srso: Add IBPB on VMEXIT") [2] Reported-by: Yosry Ahmed Signed-off-by: Patrick Bellasi Reviewed-by: Borislav Petkov (AMD) Cc: stable@kernel.org Signed-off-by: Linus Torvalds --- arch/x86/Kconfig | 3 ++- arch/x86/kernel/cpu/bugs.c | 21 ++++++++++++++------- 2 files changed, 16 insertions(+), 8 deletions(-) diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig index 87198d957e2f..be2c311f5118 100644 --- a/arch/x86/Kconfig +++ b/arch/x86/Kconfig @@ -2599,7 +2599,8 @@ config MITIGATION_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 MITIGATION_IBRS_ENTRY bool "Enable IBRS on kernel entry" diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c index 5a505aa65489..a5d0998d7604 100644 --- a/arch/x86/kernel/cpu/bugs.c +++ b/arch/x86/kernel/cpu/bugs.c @@ -1115,6 +1115,8 @@ 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 @@ -1124,9 +1126,6 @@ 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, @@ -2646,6 +2645,7 @@ static void __init srso_select_mitigation(void) if (IS_ENABLED(CONFIG_MITIGATION_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; /* @@ -2655,6 +2655,13 @@ 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 MITIGATION_IBPB_ENTRY.\n"); @@ -2663,8 +2670,8 @@ static void __init srso_select_mitigation(void) ibpb_on_vmexit: case SRSO_CMD_IBPB_ON_VMEXIT: - if (IS_ENABLED(CONFIG_MITIGATION_SRSO)) { - if (!boot_cpu_has(X86_FEATURE_ENTRY_IBPB) && has_microcode) { + if (IS_ENABLED(CONFIG_MITIGATION_IBPB_ENTRY)) { + if (has_microcode) { setup_force_cpu_cap(X86_FEATURE_IBPB_ON_VMEXIT); srso_mitigation = SRSO_MITIGATION_IBPB_ON_VMEXIT; @@ -2676,8 +2683,8 @@ 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 MITIGATION_IBPB_ENTRY.\n"); + } break; default: break;