From: Andy Lutomirski luto@kernel.org

[ Upstream commit 716572b0003ef67a4889bd7d85baf5099c5a0248 ]

Setting GS to 1, 2, or 3 causes a nonsensical part of the IRET microcode to change GS back to zero on a return from kernel mode to user mode. The result is that these tests fail randomly depending on when interrupts happen. Detect when this happens and let the test pass.

Signed-off-by: Andy Lutomirski luto@kernel.org Signed-off-by: Borislav Petkov bp@suse.de Link: https://lkml.kernel.org/r/7567fd44a1d60a9424f25b19a998f12149993b0d.160434659... Signed-off-by: Sasha Levin sashal@kernel.org --- tools/testing/selftests/x86/fsgsbase.c | 12 ++++++++++-- 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/x86/fsgsbase.c b/tools/testing/selftests/x86/fsgsbase.c index f249e042b3b51..026cd644360f6 100644 --- a/tools/testing/selftests/x86/fsgsbase.c +++ b/tools/testing/selftests/x86/fsgsbase.c @@ -318,8 +318,8 @@ static void set_gs_and_switch_to(unsigned long local, local = read_base(GS);

/* - * Signal delivery seems to mess up weird selectors. Put it - * back. + * Signal delivery is quite likely to change a selector + * of 1, 2, or 3 back to 0 due to IRET being defective. */ asm volatile ("mov %0, %%gs" : : "rm" (force_sel)); } else { @@ -337,6 +337,14 @@ static void set_gs_and_switch_to(unsigned long local, if (base == local && sel_pre_sched == sel_post_sched) { printf("[OK]\tGS/BASE remained 0x%hx/0x%lx\n", sel_pre_sched, local); + } else if (base == local && sel_pre_sched >= 1 && sel_pre_sched <= 3 && + sel_post_sched == 0) { + /* + * IRET is misdesigned and will squash selectors 1, 2, or 3 + * to zero. Don't fail the test just because this happened. + */ + printf("[OK]\tGS/BASE changed from 0x%hx/0x%lx to 0x%hx/0x%lx because IRET is defective\n", + sel_pre_sched, local, sel_post_sched, base); } else { nerrs++; printf("[FAIL]\tGS/BASE changed from 0x%hx/0x%lx to 0x%hx/0x%lx\n",