Both the Spectre-v2 and Spectre-BHB mitigations involve running a sequence immediately after exiting a guest, before any branches. In the stable kernels these sequences are built by copying templates into an empty vector slot.
For Spectre-BHB, Cortex-A57 and A72 require the branchy loop with k=8. If Spectre-v2 needs mitigating at the same time, a firmware call to EL3 is needed. The work EL3 does at this point is also enough to mitigate Spectre-BHB.
When enabling the Spectre-BHB mitigation, spectre_bhb_enable_mitigation() should check if a slot has already been allocated for Spectre-v2, meaning no work is needed for Spectre-BHB.
This check was missed in the earlier backport, add it.
Fixes: 3e3904125fcc ("arm64: Mitigate spectre style branch history side channels") Signed-off-by: James Morse james.morse@arm.com --- arch/arm64/kernel/cpu_errata.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-)
diff --git a/arch/arm64/kernel/cpu_errata.c b/arch/arm64/kernel/cpu_errata.c index d783d08bd031..c1d1b37bc118 100644 --- a/arch/arm64/kernel/cpu_errata.c +++ b/arch/arm64/kernel/cpu_errata.c @@ -1104,7 +1104,13 @@ void spectre_bhb_enable_mitigation(const struct arm64_cpu_capabilities *entry) } else if (spectre_bhb_loop_affected(SCOPE_LOCAL_CPU)) { switch (spectre_bhb_loop_affected(SCOPE_SYSTEM)) { case 8: - kvm_setup_bhb_slot(__spectre_bhb_loop_k8_start); + /* + * A57/A72-r0 will already have selected the + * spectre-indirect vector, which is sufficient + * for BHB too. + */ + if (!__this_cpu_read(bp_hardening_data.fn)) + kvm_setup_bhb_slot(__spectre_bhb_loop_k8_start); break; case 24: kvm_setup_bhb_slot(__spectre_bhb_loop_k24_start);