From 48482f4dd3432e5e62873bf0f2e254cfb8ce2ac2 Mon Sep 17 00:00:00 2001 From: Russell Currey Date: Tue, 8 Jun 2021 16:48:09 +1000 Subject: [PATCH] selftests/powerpc: Better reporting in spectre_v2 In commit f3054ffd71b5 ("selftests/powerpc: Return skip code for spectre_v2"), the spectre_v2 selftest is updated to be aware of cases where the vulnerability status reported in sysfs is incorrect, skipping the test instead. This happens because qemu can misrepresent the mitigation status of the host to the guest. If the count cache is disabled in the host, and this is correctly reported to the guest, then the guest won't apply mitigations. If the guest is then migrated to a new host where mitigations are necessary, it is now vulnerable because it has not applied mitigations. Update the selftest to report when we see excessive misses, indicative of the count cache being disabled. If software flushing is enabled, also warn that these flushes are just wasting performance. Signed-off-by: Russell Currey [mpe: Rebase and update change log appropriately] Signed-off-by: Michael Ellerman Link: https://lore.kernel.org/r/20210608064809.199116-1-ruscur@russell.cc --- .../selftests/powerpc/security/spectre_v2.c | 24 ++++++++++++------- 1 file changed, 15 insertions(+), 9 deletions(-) diff --git a/tools/testing/selftests/powerpc/security/spectre_v2.c b/tools/testing/selftests/powerpc/security/spectre_v2.c index 80dc97e3ec57..0e231e89bfa4 100644 --- a/tools/testing/selftests/powerpc/security/spectre_v2.c +++ b/tools/testing/selftests/powerpc/security/spectre_v2.c @@ -182,17 +182,23 @@ int spectre_v2_test(void) case COUNT_CACHE_FLUSH_HW: // These should all not affect userspace branch prediction if (miss_percent > 15) { + if (miss_percent > 95) { + /* + * Such a mismatch may be caused by a system being unaware + * the count cache is disabled. This may be to enable + * guest migration between hosts with different settings. + * Return skip code to avoid detecting this as an error. + * We are not vulnerable and reporting otherwise, so + * missing such a mismatch is safe. + */ + printf("Branch misses > 95%% unexpected in this configuration.\n"); + printf("Count cache likely disabled without Linux knowing.\n"); + if (state == COUNT_CACHE_FLUSH_SW) + printf("WARNING: Kernel performing unnecessary flushes.\n"); + return 4; + } printf("Branch misses > 15%% unexpected in this configuration!\n"); printf("Possible mis-match between reported & actual mitigation\n"); - /* - * Such a mismatch may be caused by a guest system - * reporting as vulnerable when the host is mitigated. - * Return skip code to avoid detecting this as an error. - * We are not vulnerable and reporting otherwise, so - * missing such a mismatch is safe. - */ - if (miss_percent > 95) - return 4; return 1; }