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Subject[tip: x86/misc] selftests/x86/fsgsbase: Fix GS == 1, 2, and 3 tests
The following commit has been merged into the x86/misc branch of tip:

Commit-ID: 716572b0003ef67a4889bd7d85baf5099c5a0248
Gitweb: https://git.kernel.org/tip/716572b0003ef67a4889bd7d85baf5099c5a0248
Author: Andy Lutomirski <luto@kernel.org>
AuthorDate: Mon, 02 Nov 2020 11:51:10 -08:00
Committer: Borislav Petkov <bp@suse.de>
CommitterDate: Tue, 24 Nov 2020 13:46:16 +01:00

selftests/x86/fsgsbase: Fix GS == 1, 2, and 3 tests

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.1604346596.git.luto@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 7161cfc..8c780cc 100644
--- a/tools/testing/selftests/x86/fsgsbase.c
+++ b/tools/testing/selftests/x86/fsgsbase.c
@@ -392,8 +392,8 @@ 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 {
@@ -411,6 +411,14 @@ 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",
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