Revert "x86/ptrace: Prevent ptrace from clearing the FS/GS selector" and fix the test

This reverts commit 48f5e52e.

The ptrace ABI change was a prerequisite to the proposed design for
FSGSBASE.  Since FSGSBASE support has been reverted, and since I'm not
convinced that the ABI was ever adequately tested, revert the ABI change as
well.

This also modifies the test case so that it tests the preexisting behavior.
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/fca39c478ea7fb15bc76fe8a36bd180810a067f6.1563200250.git.luto@kernel.org

arch/x86/kernel/ptrace.c

@@ -369,12 +369,22 @@ static int putreg(struct task_struct *child,
 	case offsetof(struct user_regs_struct,fs_base):
 		if (value >= TASK_SIZE_MAX)
 			return -EIO;
-		x86_fsbase_write_task(child, value);
+		/*
+		 * When changing the FS base, use do_arch_prctl_64()
+		 * to set the index to zero and to set the base
+		 * as requested.
+		 */
+		if (child->thread.fsbase != value)
+			return do_arch_prctl_64(child, ARCH_SET_FS, value);
 		return 0;
 	case offsetof(struct user_regs_struct,gs_base):
+		/*
+		 * Exactly the same here as the %fs handling above.
+		 */
 		if (value >= TASK_SIZE_MAX)
 			return -EIO;
-		x86_gsbase_write_task(child, value);
+		if (child->thread.gsbase != value)
+			return do_arch_prctl_64(child, ARCH_SET_GS, value);
 		return 0;
 #endif
 	}

tools/testing/selftests/x86/fsgsbase.c

@@ -489,25 +489,11 @@ static void test_ptrace_write_gsbase(void)
 		 * selector value is changed or not by the GSBASE write in
 		 * a ptracer.
 		 */
-		if (gs != *shared_scratch) {
-			nerrs++;
-			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 have no effect.
-			 */
-			if (gs == 0)
-				printf("\tNote: this is expected behavior on older kernels.\n");
-		} else if (have_fsgsbase && (base != 0xFF)) {
-			nerrs++;
-			printf("[FAIL]\tGSBASE changed to %lx\n", base);
+		if (gs == 0 && base == 0xFF) {
+			printf("[OK]\tGS was reset as expected\n");
 		} else {
-			printf("[OK]\tGS remained 0x%hx%s", *shared_scratch, have_fsgsbase ? " and GSBASE changed to 0xFF" : "");
-			printf("\n");
+			nerrs++;
+			printf("[FAIL]\tGS=0x%lx, GSBASE=0x%lx (should be 0, 0xFF)\n", gs, base);
 		}
 	}