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Subject[tip: x86/fsgsbase] selftests/x86/fsgsbase: Test GS selector on ptracer-induced GS base write
The following commit has been merged into the x86/fsgsbase branch of tip:

Commit-ID: 291fd83569e10f3d305cd8adb62f6ec00f759dc6
Gitweb: https://git.kernel.org/tip/291fd83569e10f3d305cd8adb62f6ec00f759dc6
Author: Chang S. Bae <chang.seok.bae@intel.com>
AuthorDate: Thu, 28 May 2020 16:14:01 -04:00
Committer: Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Thu, 18 Jun 2020 15:47:06 +02:00

selftests/x86/fsgsbase: Test GS selector on ptracer-induced GS base write

The test validates that the selector is not changed when a ptracer writes
the ptracee's GS base.

Originally-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Link: https://lkml.kernel.org/r/20200528201402.1708239-16-sashal@kernel.org


---
tools/testing/selftests/x86/fsgsbase.c | 21 +++++++++++++++------
1 file changed, 15 insertions(+), 6 deletions(-)

diff --git a/tools/testing/selftests/x86/fsgsbase.c b/tools/testing/selftests/x86/fsgsbase.c
index 15a329d..950a48b 100644
--- a/tools/testing/selftests/x86/fsgsbase.c
+++ b/tools/testing/selftests/x86/fsgsbase.c
@@ -465,7 +465,7 @@ static void test_ptrace_write_gsbase(void)
wait(&status);

if (WSTOPSIG(status) == SIGTRAP) {
- unsigned long gs, base;
+ unsigned long gs;
unsigned long gs_offset = USER_REGS_OFFSET(gs);
unsigned long base_offset = USER_REGS_OFFSET(gs_base);

@@ -481,7 +481,6 @@ static void test_ptrace_write_gsbase(void)
err(1, "PTRACE_POKEUSER");

gs = ptrace(PTRACE_PEEKUSER, child, gs_offset, NULL);
- base = ptrace(PTRACE_PEEKUSER, child, base_offset, NULL);

/*
* In a non-FSGSBASE system, the nonzero selector will load
@@ -489,11 +488,21 @@ static void test_ptrace_write_gsbase(void)
* selector value is changed or not by the GSBASE write in
* a ptracer.
*/
- if (gs == 0 && base == 0xFF) {
- printf("[OK]\tGS was reset as expected\n");
- } else {
+ if (gs != *shared_scratch) {
nerrs++;
- printf("[FAIL]\tGS=0x%lx, GSBASE=0x%lx (should be 0, 0xFF)\n", gs, base);
+ printf("[FAIL]\tGS changed to %lx\n", gs);
+
+ /*
+ * On older kernels, poking a nonzero value into the
+ * base would zero the selector. On newer kernels,
+ * this behavior has changed -- poking the base
+ * changes only the base and, if FSGSBASE is not
+ * available, this may not effect.
+ */
+ if (gs == 0)
+ printf("\tNote: this is expected behavior on older kernels.\n");
+ } else {
+ printf("[OK]\tGS remained 0x%hx\n", *shared_scratch);
}
}

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