1 /* 2 * single_step_syscall.c - single-steps various x86 syscalls 3 * Copyright (c) 2014-2015 Andrew Lutomirski 4 * 5 * This program is free software; you can redistribute it and/or modify 6 * it under the terms and conditions of the GNU General Public License, 7 * version 2, as published by the Free Software Foundation. 8 * 9 * This program is distributed in the hope it will be useful, but 10 * WITHOUT ANY WARRANTY; without even the implied warranty of 11 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU 12 * General Public License for more details. 13 * 14 * This is a very simple series of tests that makes system calls with 15 * the TF flag set. This exercises some nasty kernel code in the 16 * SYSENTER case: SYSENTER does not clear TF, so SYSENTER with TF set 17 * immediately issues #DB from CPL 0. This requires special handling in 18 * the kernel. 19 */ 20 21 #define _GNU_SOURCE 22 23 #include <sys/time.h> 24 #include <time.h> 25 #include <stdlib.h> 26 #include <sys/syscall.h> 27 #include <unistd.h> 28 #include <stdio.h> 29 #include <string.h> 30 #include <inttypes.h> 31 #include <sys/mman.h> 32 #include <sys/signal.h> 33 #include <sys/ucontext.h> 34 #include <asm/ldt.h> 35 #include <err.h> 36 #include <setjmp.h> 37 #include <stddef.h> 38 #include <stdbool.h> 39 #include <sys/ptrace.h> 40 #include <sys/user.h> 41 42 static void sethandler(int sig, void (*handler)(int, siginfo_t *, void *), 43 int flags) 44 { 45 struct sigaction sa; 46 memset(&sa, 0, sizeof(sa)); 47 sa.sa_sigaction = handler; 48 sa.sa_flags = SA_SIGINFO | flags; 49 sigemptyset(&sa.sa_mask); 50 if (sigaction(sig, &sa, 0)) 51 err(1, "sigaction"); 52 } 53 54 static volatile sig_atomic_t sig_traps; 55 56 #ifdef __x86_64__ 57 # define REG_IP REG_RIP 58 # define WIDTH "q" 59 #else 60 # define REG_IP REG_EIP 61 # define WIDTH "l" 62 #endif 63 64 static unsigned long get_eflags(void) 65 { 66 unsigned long eflags; 67 asm volatile ("pushf" WIDTH "\n\tpop" WIDTH " %0" : "=rm" (eflags)); 68 return eflags; 69 } 70 71 static void set_eflags(unsigned long eflags) 72 { 73 asm volatile ("push" WIDTH " %0\n\tpopf" WIDTH 74 : : "rm" (eflags) : "flags"); 75 } 76 77 #define X86_EFLAGS_TF (1UL << 8) 78 79 static void sigtrap(int sig, siginfo_t *info, void *ctx_void) 80 { 81 ucontext_t *ctx = (ucontext_t*)ctx_void; 82 83 if (get_eflags() & X86_EFLAGS_TF) { 84 set_eflags(get_eflags() & ~X86_EFLAGS_TF); 85 printf("[WARN]\tSIGTRAP handler had TF set\n"); 86 _exit(1); 87 } 88 89 sig_traps++; 90 91 if (sig_traps == 10000 || sig_traps == 10001) { 92 printf("[WARN]\tHit %d SIGTRAPs with si_addr 0x%lx, ip 0x%lx\n", 93 (int)sig_traps, 94 (unsigned long)info->si_addr, 95 (unsigned long)ctx->uc_mcontext.gregs[REG_IP]); 96 } 97 } 98 99 static void check_result(void) 100 { 101 unsigned long new_eflags = get_eflags(); 102 set_eflags(new_eflags & ~X86_EFLAGS_TF); 103 104 if (!sig_traps) { 105 printf("[FAIL]\tNo SIGTRAP\n"); 106 exit(1); 107 } 108 109 if (!(new_eflags & X86_EFLAGS_TF)) { 110 printf("[FAIL]\tTF was cleared\n"); 111 exit(1); 112 } 113 114 printf("[OK]\tSurvived with TF set and %d traps\n", (int)sig_traps); 115 sig_traps = 0; 116 } 117 118 int main() 119 { 120 int tmp; 121 122 sethandler(SIGTRAP, sigtrap, 0); 123 124 printf("[RUN]\tSet TF and check nop\n"); 125 set_eflags(get_eflags() | X86_EFLAGS_TF); 126 asm volatile ("nop"); 127 check_result(); 128 129 #ifdef __x86_64__ 130 printf("[RUN]\tSet TF and check syscall-less opportunistic sysret\n"); 131 set_eflags(get_eflags() | X86_EFLAGS_TF); 132 extern unsigned char post_nop[]; 133 asm volatile ("pushf" WIDTH "\n\t" 134 "pop" WIDTH " %%r11\n\t" 135 "nop\n\t" 136 "post_nop:" 137 : : "c" (post_nop) : "r11"); 138 check_result(); 139 #endif 140 141 printf("[RUN]\tSet TF and check int80\n"); 142 set_eflags(get_eflags() | X86_EFLAGS_TF); 143 asm volatile ("int $0x80" : "=a" (tmp) : "a" (SYS_getpid)); 144 check_result(); 145 146 /* 147 * This test is particularly interesting if fast syscalls use 148 * SYSENTER: it triggers a nasty design flaw in SYSENTER. 149 * Specifically, SYSENTER does not clear TF, so either SYSENTER 150 * or the next instruction traps at CPL0. (Of course, Intel 151 * mostly forgot to document exactly what happens here.) So we 152 * get a CPL0 fault with usergs (on 64-bit kernels) and possibly 153 * no stack. The only sane way the kernel can possibly handle 154 * it is to clear TF on return from the #DB handler, but this 155 * happens way too early to set TF in the saved pt_regs, so the 156 * kernel has to do something clever to avoid losing track of 157 * the TF bit. 158 * 159 * Needless to say, we've had bugs in this area. 160 */ 161 syscall(SYS_getpid); /* Force symbol binding without TF set. */ 162 printf("[RUN]\tSet TF and check a fast syscall\n"); 163 set_eflags(get_eflags() | X86_EFLAGS_TF); 164 syscall(SYS_getpid); 165 check_result(); 166 167 /* Now make sure that another fast syscall doesn't set TF again. */ 168 printf("[RUN]\tFast syscall with TF cleared\n"); 169 fflush(stdout); /* Force a syscall */ 170 if (get_eflags() & X86_EFLAGS_TF) { 171 printf("[FAIL]\tTF is now set\n"); 172 exit(1); 173 } 174 if (sig_traps) { 175 printf("[FAIL]\tGot SIGTRAP\n"); 176 exit(1); 177 } 178 printf("[OK]\tNothing unexpected happened\n"); 179 180 return 0; 181 } 182
