single_step_syscall.c

  1// SPDX-License-Identifier: GPL-2.0-only
  2/*
  3 * single_step_syscall.c - single-steps various x86 syscalls
  4 * Copyright (c) 2014-2015 Andrew Lutomirski
  5 *
  6 * This is a very simple series of tests that makes system calls with
  7 * the TF flag set.  This exercises some nasty kernel code in the
  8 * SYSENTER case: SYSENTER does not clear TF, so SYSENTER with TF set
  9 * immediately issues #DB from CPL 0.  This requires special handling in
 10 * the kernel.
 11 */
 12
 13#define _GNU_SOURCE
 14
 15#include <sys/time.h>
 16#include <time.h>
 17#include <stdlib.h>
 18#include <sys/syscall.h>
 19#include <unistd.h>
 20#include <stdio.h>
 21#include <string.h>
 22#include <inttypes.h>
 23#include <sys/mman.h>
 24#include <sys/signal.h>
 25#include <sys/ucontext.h>
 26#include <asm/ldt.h>
 27#include <err.h>
 28#include <setjmp.h>
 29#include <stddef.h>
 30#include <stdbool.h>
 31#include <sys/ptrace.h>
 32#include <sys/user.h>
 33
 34#include "helpers.h"
 35
 36static volatile sig_atomic_t sig_traps, sig_eflags;
 37sigjmp_buf jmpbuf;
 38
 39#ifdef __x86_64__
 40# define REG_IP REG_RIP
 41# define WIDTH "q"
 42# define INT80_CLOBBERS "r8", "r9", "r10", "r11"
 43#else
 44# define REG_IP REG_EIP
 45# define WIDTH "l"
 46# define INT80_CLOBBERS
 47#endif
 48
 49static void sigtrap(int sig, siginfo_t *info, void *ctx_void)
 50{
 51	ucontext_t *ctx = (ucontext_t*)ctx_void;
 52
 53	if (get_eflags() & X86_EFLAGS_TF) {
 54		set_eflags(get_eflags() & ~X86_EFLAGS_TF);
 55		printf("[WARN]\tSIGTRAP handler had TF set\n");
 56		_exit(1);
 57	}
 58
 59	sig_traps++;
 60
 61	if (sig_traps == 10000 || sig_traps == 10001) {
 62		printf("[WARN]\tHit %d SIGTRAPs with si_addr 0x%lx, ip 0x%lx\n",
 63		       (int)sig_traps,
 64		       (unsigned long)info->si_addr,
 65		       (unsigned long)ctx->uc_mcontext.gregs[REG_IP]);
 66	}
 67}
 68
 69static char const * const signames[] = {
 70	[SIGSEGV] = "SIGSEGV",
 71	[SIGBUS] = "SIBGUS",
 72	[SIGTRAP] = "SIGTRAP",
 73	[SIGILL] = "SIGILL",
 74};
 75
 76static void print_and_longjmp(int sig, siginfo_t *si, void *ctx_void)
 77{
 78	ucontext_t *ctx = ctx_void;
 79
 80	printf("\tGot %s with RIP=%lx, TF=%ld\n", signames[sig],
 81	       (unsigned long)ctx->uc_mcontext.gregs[REG_IP],
 82	       (unsigned long)ctx->uc_mcontext.gregs[REG_EFL] & X86_EFLAGS_TF);
 83
 84	sig_eflags = (unsigned long)ctx->uc_mcontext.gregs[REG_EFL];
 85	siglongjmp(jmpbuf, 1);
 86}
 87
 88static void check_result(void)
 89{
 90	unsigned long new_eflags = get_eflags();
 91	set_eflags(new_eflags & ~X86_EFLAGS_TF);
 92
 93	if (!sig_traps) {
 94		printf("[FAIL]\tNo SIGTRAP\n");
 95		exit(1);
 96	}
 97
 98	if (!(new_eflags & X86_EFLAGS_TF)) {
 99		printf("[FAIL]\tTF was cleared\n");
100		exit(1);
101	}
102
103	printf("[OK]\tSurvived with TF set and %d traps\n", (int)sig_traps);
104	sig_traps = 0;
105}
106
107static void fast_syscall_no_tf(void)
108{
109	sig_traps = 0;
110	printf("[RUN]\tFast syscall with TF cleared\n");
111	fflush(stdout);  /* Force a syscall */
112	if (get_eflags() & X86_EFLAGS_TF) {
113		printf("[FAIL]\tTF is now set\n");
114		exit(1);
115	}
116	if (sig_traps) {
117		printf("[FAIL]\tGot SIGTRAP\n");
118		exit(1);
119	}
120	printf("[OK]\tNothing unexpected happened\n");
121}
122
123int main()
124{
125#ifdef CAN_BUILD_32
126	int tmp;
127#endif
128
129	sethandler(SIGTRAP, sigtrap, 0);
130
131	printf("[RUN]\tSet TF and check nop\n");
132	set_eflags(get_eflags() | X86_EFLAGS_TF);
133	asm volatile ("nop");
134	check_result();
135
136#ifdef __x86_64__
137	printf("[RUN]\tSet TF and check syscall-less opportunistic sysret\n");
138	set_eflags(get_eflags() | X86_EFLAGS_TF);
139	extern unsigned char post_nop[];
140	asm volatile ("pushf" WIDTH "\n\t"
141		      "pop" WIDTH " %%r11\n\t"
142		      "nop\n\t"
143		      "post_nop:"
144		      : : "c" (post_nop) : "r11");
145	check_result();
146#endif
147#ifdef CAN_BUILD_32
148	printf("[RUN]\tSet TF and check int80\n");
149	set_eflags(get_eflags() | X86_EFLAGS_TF);
150	asm volatile ("int $0x80" : "=a" (tmp) : "a" (SYS_getpid)
151			: INT80_CLOBBERS);
152	check_result();
153#endif
154
155	/*
156	 * This test is particularly interesting if fast syscalls use
157	 * SYSENTER: it triggers a nasty design flaw in SYSENTER.
158	 * Specifically, SYSENTER does not clear TF, so either SYSENTER
159	 * or the next instruction traps at CPL0.  (Of course, Intel
160	 * mostly forgot to document exactly what happens here.)  So we
161	 * get a CPL0 fault with usergs (on 64-bit kernels) and possibly
162	 * no stack.  The only sane way the kernel can possibly handle
163	 * it is to clear TF on return from the #DB handler, but this
164	 * happens way too early to set TF in the saved pt_regs, so the
165	 * kernel has to do something clever to avoid losing track of
166	 * the TF bit.
167	 *
168	 * Needless to say, we've had bugs in this area.
169	 */
170	syscall(SYS_getpid);  /* Force symbol binding without TF set. */
171	printf("[RUN]\tSet TF and check a fast syscall\n");
172	set_eflags(get_eflags() | X86_EFLAGS_TF);
173	syscall(SYS_getpid);
174	check_result();
175
176	/* Now make sure that another fast syscall doesn't set TF again. */
177	fast_syscall_no_tf();
178
179	/*
180	 * And do a forced SYSENTER to make sure that this works even if
181	 * fast syscalls don't use SYSENTER.
182	 *
183	 * Invoking SYSENTER directly breaks all the rules.  Just handle
184	 * the SIGSEGV.
185	 */
186	if (sigsetjmp(jmpbuf, 1) == 0) {
187		unsigned long nr = SYS_getpid;
188		printf("[RUN]\tSet TF and check SYSENTER\n");
189		stack_t stack = {
190			.ss_sp = malloc(sizeof(char) * SIGSTKSZ),
191			.ss_size = SIGSTKSZ,
192		};
193		if (sigaltstack(&stack, NULL) != 0)
194			err(1, "sigaltstack");
195		sethandler(SIGSEGV, print_and_longjmp,
196			   SA_RESETHAND | SA_ONSTACK);
197		sethandler(SIGILL, print_and_longjmp, SA_RESETHAND);
198		set_eflags(get_eflags() | X86_EFLAGS_TF);
199		free(stack.ss_sp);
200		/* Clear EBP first to make sure we segfault cleanly. */
201		asm volatile ("xorl %%ebp, %%ebp; SYSENTER" : "+a" (nr) :: "flags", "rcx"
202#ifdef __x86_64__
203				, "r11"
204#endif
205			);
206
207		/* We're unreachable here.  SYSENTER forgets RIP. */
208	}
209	clearhandler(SIGSEGV);
210	clearhandler(SIGILL);
211	if (!(sig_eflags & X86_EFLAGS_TF)) {
212		printf("[FAIL]\tTF was cleared\n");
213		exit(1);
214	}
215
216	/* Now make sure that another fast syscall doesn't set TF again. */
217	fast_syscall_no_tf();
218
219	return 0;
220}