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

diff --git a/tools/testing/selftests/x86/fsgsbase.c b/tools/testing/selftests/x86/fsgsbase.c
index 15a329d..950a48b 100644 (file)
--- 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);
                }
        }