[PATCH 6.6 111/122] x86/bugs: Fix BHI handling of RRSBA

Greg Kroah-Hartman
2024-04-15T14:37:44Z

From: Josh Poimboeuf <jpoimboe@kernel.org>

commit 1cea8a280dfd1016148a3820676f2f03e3f5b898 upstream.

The ARCH_CAP_RRSBA check isn't correct: RRSBA may have already been
disabled by the Spectre v2 mitigation (or can otherwise be disabled by
the BHI mitigation itself if needed). In that case retpolines are fine.

Fixes: ec9404e40e8f ("x86/bhi: Add BHI mitigation knob")
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/6f56f13da34a0834b69163467449be7f58f253dc.1712813475.git.jpoimboe@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/x86/kernel/cpu/bugs.c | 30 ++++++++++++++++++------------
1 file changed, 18 insertions(+), 12 deletions(-)

--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1537,20 +1537,25 @@ static enum spectre_v2_mitigation __init
return SPECTRE_V2_RETPOLINE;
}

+static bool __ro_after_init rrsba_disabled;
+
/* Disable in-kernel use of non-RSB RET predictors */
static void __init spec_ctrl_disable_kernel_rrsba(void)
{
- u64 x86_arch_cap_msr;
+ if (rrsba_disabled)
+ return;

- if (!boot_cpu_has(X86_FEATURE_RRSBA_CTRL))
+ if (!(x86_arch_cap_msr & ARCH_CAP_RRSBA)) {
+ rrsba_disabled = true;
return;
+ }

- x86_arch_cap_msr = x86_read_arch_cap_msr();
+ if (!boot_cpu_has(X86_FEATURE_RRSBA_CTRL))
+ return;

- if (x86_arch_cap_msr & ARCH_CAP_RRSBA) {
- x86_spec_ctrl_base |= SPEC_CTRL_RRSBA_DIS_S;
- update_spec_ctrl(x86_spec_ctrl_base);
- }
+ x86_spec_ctrl_base |= SPEC_CTRL_RRSBA_DIS_S;
+ update_spec_ctrl(x86_spec_ctrl_base);
+ rrsba_disabled = true;
}

static void __init spectre_v2_determine_rsb_fill_type_at_vmexit(enum spectre_v2_mitigation mode)
@@ -1651,9 +1656,11 @@ static void __init bhi_select_mitigation
return;

/* Retpoline mitigates against BHI unless the CPU has RRSBA behavior */
- if (cpu_feature_enabled(X86_FEATURE_RETPOLINE) &&
- !(x86_read_arch_cap_msr() & ARCH_CAP_RRSBA))
- return;
+ if (cpu_feature_enabled(X86_FEATURE_RETPOLINE)) {
+ spec_ctrl_disable_kernel_rrsba();
+ if (rrsba_disabled)
+ return;
+ }

if (spec_ctrl_bhi_dis())
return;
@@ -2808,8 +2815,7 @@ static const char *spectre_bhi_state(voi
return "; BHI: BHI_DIS_S";
else if (boot_cpu_has(X86_FEATURE_CLEAR_BHB_LOOP))
return "; BHI: SW loop, KVM: SW loop";
- else if (boot_cpu_has(X86_FEATURE_RETPOLINE) &&
- !(x86_arch_cap_msr & ARCH_CAP_RRSBA))
+ else if (boot_cpu_has(X86_FEATURE_RETPOLINE) && rrsba_disabled)
return "; BHI: Retpoline";
else if (boot_cpu_has(X86_FEATURE_CLEAR_BHB_LOOP_ON_VMEXIT))
return "; BHI: Syscall hardening, KVM: SW loop";