--- /dev/null
+/*
+ * 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;
+}
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