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

[tip-bot2 for Chang S. Bae]

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@xxxxxxxxx>
AuthorDate:    Thu, 28 May 2020 16:14:01 -04:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
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@xxxxxxxxxx>
Signed-off-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Reviewed-by: Tony Luck <tony.luck@xxxxxxxxx>
Link: https://lkml.kernel.org/r/20200528201402.1708239-16-sashal@xxxxxxxxxx


---
 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);
 		}
 	}