x86, selftests: Add single_step_syscall test

This is a very simple test that makes system calls with TF set.

This test currently fails when running the 32-bit build on a
64-bit kernel on an Intel CPU.  This bug will be fixed by the
next commit.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Denys Vlasenko <vda.linux@googlemail.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Shuah Khan <shuah.kh@samsung.com>
Link: http://lkml.kernel.org/r/20e68021155f6ab5c60590dcad81d37c68ea2c4f.1429139075.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

tools/testing/selftests/x86/Makefile

@@ -1,6 +1,6 @@
 .PHONY: all all_32 all_64 check_build32 clean run_tests
 
-TARGETS_C_BOTHBITS := sigreturn
+TARGETS_C_BOTHBITS := sigreturn single_step_syscall
 
 BINARIES_32 := $(TARGETS_C_BOTHBITS:%=%_32)
 BINARIES_64 := $(TARGETS_C_BOTHBITS:%=%_64)

tools/testing/selftests/x86/run_x86_tests.sh

@@ -3,9 +3,11 @@
 # This is deliberately minimal.  IMO kselftests should provide a standard
 # script here.
 ./sigreturn_32 || exit 1
+./single_step_syscall_32 || exit 1
 
 if [[ "$uname -p" -eq "x86_64" ]]; then
     ./sigreturn_64 || exit 1
+    ./single_step_syscall_64 || exit 1
 fi
 
 exit 0

tools/testing/selftests/x86/single_step_syscall.c

@@ -0,0 +1,181 @@
+/*
+ * single_step_syscall.c - single-steps various x86 syscalls
+ * Copyright (c) 2014-2015 Andrew Lutomirski
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms and conditions of the GNU General Public License,
+ * version 2, as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * This is a very simple series of tests that makes system calls with
+ * the TF flag set.  This exercises some nasty kernel code in the
+ * SYSENTER case: SYSENTER does not clear TF, so SYSENTER with TF set
+ * immediately issues #DB from CPL 0.  This requires special handling in
+ * the kernel.
+ */
+
+#define _GNU_SOURCE
+
+#include <sys/time.h>
+#include <time.h>
+#include <stdlib.h>
+#include <sys/syscall.h>
+#include <unistd.h>
+#include <stdio.h>
+#include <string.h>
+#include <inttypes.h>
+#include <sys/mman.h>
+#include <sys/signal.h>
+#include <sys/ucontext.h>
+#include <asm/ldt.h>
+#include <err.h>
+#include <setjmp.h>
+#include <stddef.h>
+#include <stdbool.h>
+#include <sys/ptrace.h>
+#include <sys/user.h>
+
+static void sethandler(int sig, void (*handler)(int, siginfo_t *, void *),
+		       int flags)
+{
+	struct sigaction sa;
+	memset(&sa, 0, sizeof(sa));
+	sa.sa_sigaction = handler;
+	sa.sa_flags = SA_SIGINFO | flags;
+	sigemptyset(&sa.sa_mask);
+	if (sigaction(sig, &sa, 0))
+		err(1, "sigaction");
+}
+
+static volatile sig_atomic_t sig_traps;
+
+#ifdef __x86_64__
+# define REG_IP REG_RIP
+# define WIDTH "q"
+#else
+# define REG_IP REG_EIP
+# define WIDTH "l"
+#endif
+
+static unsigned long get_eflags(void)
+{
+	unsigned long eflags;
+	asm volatile ("pushf" WIDTH "\n\tpop" WIDTH " %0" : "=rm" (eflags));
+	return eflags;
+}
+
+static void set_eflags(unsigned long eflags)
+{
+	asm volatile ("push" WIDTH " %0\n\tpopf" WIDTH
+		      : : "rm" (eflags) : "flags");
+}
+
+#define X86_EFLAGS_TF (1UL << 8)
+
+static void sigtrap(int sig, siginfo_t *info, void *ctx_void)
+{
+	ucontext_t *ctx = (ucontext_t*)ctx_void;
+
+	if (get_eflags() & X86_EFLAGS_TF) {
+		set_eflags(get_eflags() & ~X86_EFLAGS_TF);
+		printf("[WARN]\tSIGTRAP handler had TF set\n");
+		_exit(1);
+	}
+
+	sig_traps++;
+
+	if (sig_traps == 10000 || sig_traps == 10001) {
+		printf("[WARN]\tHit %d SIGTRAPs with si_addr 0x%lx, ip 0x%lx\n",
+		       (int)sig_traps,
+		       (unsigned long)info->si_addr,
+		       (unsigned long)ctx->uc_mcontext.gregs[REG_IP]);
+	}
+}
+
+static void check_result(void)
+{
+	unsigned long new_eflags = get_eflags();
+	set_eflags(new_eflags & ~X86_EFLAGS_TF);
+
+	if (!sig_traps) {
+		printf("[FAIL]\tNo SIGTRAP\n");
+		exit(1);
+	}
+
+	if (!(new_eflags & X86_EFLAGS_TF)) {
+		printf("[FAIL]\tTF was cleared\n");
+		exit(1);
+	}
+
+	printf("[OK]\tSurvived with TF set and %d traps\n", (int)sig_traps);
+	sig_traps = 0;
+}
+
+int main()
+{
+	int tmp;
+
+	sethandler(SIGTRAP, sigtrap, 0);
+
+	printf("[RUN]\tSet TF and check nop\n");
+	set_eflags(get_eflags() | X86_EFLAGS_TF);
+	asm volatile ("nop");
+	check_result();
+
+#ifdef __x86_64__
+	printf("[RUN]\tSet TF and check syscall-less opportunistic sysret\n");
+	set_eflags(get_eflags() | X86_EFLAGS_TF);
+	extern unsigned char post_nop[];
+	asm volatile ("pushf" WIDTH "\n\t"
+		      "pop" WIDTH " %%r11\n\t"
+		      "nop\n\t"
+		      "post_nop:"
+		      : : "c" (post_nop) : "r11");
+	check_result();
+#endif
+
+	printf("[RUN]\tSet TF and check int80\n");
+	set_eflags(get_eflags() | X86_EFLAGS_TF);
+	asm volatile ("int $0x80" : "=a" (tmp) : "a" (SYS_getpid));
+	check_result();
+
+	/*
+	 * This test is particularly interesting if fast syscalls use
+	 * SYSENTER: it triggers a nasty design flaw in SYSENTER.
+	 * Specifically, SYSENTER does not clear TF, so either SYSENTER
+	 * or the next instruction traps at CPL0.  (Of course, Intel
+	 * mostly forgot to document exactly what happens here.)  So we
+	 * get a CPL0 fault with usergs (on 64-bit kernels) and possibly
+	 * no stack.  The only sane way the kernel can possibly handle
+	 * it is to clear TF on return from the #DB handler, but this
+	 * happens way too early to set TF in the saved pt_regs, so the
+	 * kernel has to do something clever to avoid losing track of
+	 * the TF bit.
+	 *
+	 * Needless to say, we've had bugs in this area.
+	 */
+	syscall(SYS_getpid);  /* Force symbol binding without TF set. */
+	printf("[RUN]\tSet TF and check a fast syscall\n");
+	set_eflags(get_eflags() | X86_EFLAGS_TF);
+	syscall(SYS_getpid);
+	check_result();
+
+	/* Now make sure that another fast syscall doesn't set TF again. */
+	printf("[RUN]\tFast syscall with TF cleared\n");
+	fflush(stdout);  /* Force a syscall */
+	if (get_eflags() & X86_EFLAGS_TF) {
+		printf("[FAIL]\tTF is now set\n");
+		exit(1);
+	}
+	if (sig_traps) {
+		printf("[FAIL]\tGot SIGTRAP\n");
+		exit(1);
+	}
+	printf("[OK]\tNothing unexpected happened\n");
+
+	return 0;
+}