243 lines
6.1 KiB
C
243 lines
6.1 KiB
C
// SPDX-License-Identifier: GPL-2.0-only
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/*
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* single_step_syscall.c - single-steps various x86 syscalls
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* Copyright (c) 2014-2015 Andrew Lutomirski
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*
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* This is a very simple series of tests that makes system calls with
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* the TF flag set. This exercises some nasty kernel code in the
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* SYSENTER case: SYSENTER does not clear TF, so SYSENTER with TF set
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* immediately issues #DB from CPL 0. This requires special handling in
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* the kernel.
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*/
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#define _GNU_SOURCE
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#include <sys/time.h>
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#include <time.h>
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#include <stdlib.h>
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#include <sys/syscall.h>
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#include <unistd.h>
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#include <stdio.h>
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#include <string.h>
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#include <inttypes.h>
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#include <sys/mman.h>
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#include <sys/signal.h>
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#include <sys/ucontext.h>
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#include <asm/ldt.h>
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#include <err.h>
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#include <setjmp.h>
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#include <stddef.h>
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#include <stdbool.h>
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#include <sys/ptrace.h>
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#include <sys/user.h>
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#include "helpers.h"
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static void sethandler(int sig, void (*handler)(int, siginfo_t *, void *),
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int flags)
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{
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struct sigaction sa;
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memset(&sa, 0, sizeof(sa));
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sa.sa_sigaction = handler;
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sa.sa_flags = SA_SIGINFO | flags;
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sigemptyset(&sa.sa_mask);
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if (sigaction(sig, &sa, 0))
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err(1, "sigaction");
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}
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static void clearhandler(int sig)
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{
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struct sigaction sa;
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memset(&sa, 0, sizeof(sa));
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sa.sa_handler = SIG_DFL;
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sigemptyset(&sa.sa_mask);
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if (sigaction(sig, &sa, 0))
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err(1, "sigaction");
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}
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static volatile sig_atomic_t sig_traps, sig_eflags;
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sigjmp_buf jmpbuf;
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#ifdef __x86_64__
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# define REG_IP REG_RIP
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# define WIDTH "q"
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# define INT80_CLOBBERS "r8", "r9", "r10", "r11"
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#else
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# define REG_IP REG_EIP
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# define WIDTH "l"
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# define INT80_CLOBBERS
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#endif
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static void sigtrap(int sig, siginfo_t *info, void *ctx_void)
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{
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ucontext_t *ctx = (ucontext_t*)ctx_void;
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if (get_eflags() & X86_EFLAGS_TF) {
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set_eflags(get_eflags() & ~X86_EFLAGS_TF);
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printf("[WARN]\tSIGTRAP handler had TF set\n");
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_exit(1);
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}
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sig_traps++;
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if (sig_traps == 10000 || sig_traps == 10001) {
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printf("[WARN]\tHit %d SIGTRAPs with si_addr 0x%lx, ip 0x%lx\n",
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(int)sig_traps,
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(unsigned long)info->si_addr,
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(unsigned long)ctx->uc_mcontext.gregs[REG_IP]);
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}
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}
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static char const * const signames[] = {
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[SIGSEGV] = "SIGSEGV",
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[SIGBUS] = "SIBGUS",
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[SIGTRAP] = "SIGTRAP",
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[SIGILL] = "SIGILL",
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};
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static void print_and_longjmp(int sig, siginfo_t *si, void *ctx_void)
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{
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ucontext_t *ctx = ctx_void;
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printf("\tGot %s with RIP=%lx, TF=%ld\n", signames[sig],
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(unsigned long)ctx->uc_mcontext.gregs[REG_IP],
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(unsigned long)ctx->uc_mcontext.gregs[REG_EFL] & X86_EFLAGS_TF);
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sig_eflags = (unsigned long)ctx->uc_mcontext.gregs[REG_EFL];
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siglongjmp(jmpbuf, 1);
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}
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static void check_result(void)
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{
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unsigned long new_eflags = get_eflags();
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set_eflags(new_eflags & ~X86_EFLAGS_TF);
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if (!sig_traps) {
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printf("[FAIL]\tNo SIGTRAP\n");
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exit(1);
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}
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if (!(new_eflags & X86_EFLAGS_TF)) {
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printf("[FAIL]\tTF was cleared\n");
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exit(1);
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}
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printf("[OK]\tSurvived with TF set and %d traps\n", (int)sig_traps);
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sig_traps = 0;
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}
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static void fast_syscall_no_tf(void)
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{
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sig_traps = 0;
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printf("[RUN]\tFast syscall with TF cleared\n");
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fflush(stdout); /* Force a syscall */
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if (get_eflags() & X86_EFLAGS_TF) {
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printf("[FAIL]\tTF is now set\n");
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exit(1);
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}
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if (sig_traps) {
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printf("[FAIL]\tGot SIGTRAP\n");
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exit(1);
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}
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printf("[OK]\tNothing unexpected happened\n");
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}
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int main()
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{
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#ifdef CAN_BUILD_32
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int tmp;
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#endif
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sethandler(SIGTRAP, sigtrap, 0);
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printf("[RUN]\tSet TF and check nop\n");
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set_eflags(get_eflags() | X86_EFLAGS_TF);
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asm volatile ("nop");
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check_result();
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#ifdef __x86_64__
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printf("[RUN]\tSet TF and check syscall-less opportunistic sysret\n");
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set_eflags(get_eflags() | X86_EFLAGS_TF);
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extern unsigned char post_nop[];
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asm volatile ("pushf" WIDTH "\n\t"
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"pop" WIDTH " %%r11\n\t"
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"nop\n\t"
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"post_nop:"
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: : "c" (post_nop) : "r11");
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check_result();
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#endif
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#ifdef CAN_BUILD_32
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printf("[RUN]\tSet TF and check int80\n");
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set_eflags(get_eflags() | X86_EFLAGS_TF);
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asm volatile ("int $0x80" : "=a" (tmp) : "a" (SYS_getpid)
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: INT80_CLOBBERS);
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check_result();
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#endif
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/*
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* This test is particularly interesting if fast syscalls use
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* SYSENTER: it triggers a nasty design flaw in SYSENTER.
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* Specifically, SYSENTER does not clear TF, so either SYSENTER
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* or the next instruction traps at CPL0. (Of course, Intel
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* mostly forgot to document exactly what happens here.) So we
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* get a CPL0 fault with usergs (on 64-bit kernels) and possibly
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* no stack. The only sane way the kernel can possibly handle
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* it is to clear TF on return from the #DB handler, but this
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* happens way too early to set TF in the saved pt_regs, so the
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* kernel has to do something clever to avoid losing track of
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* the TF bit.
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*
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* Needless to say, we've had bugs in this area.
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*/
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syscall(SYS_getpid); /* Force symbol binding without TF set. */
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printf("[RUN]\tSet TF and check a fast syscall\n");
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set_eflags(get_eflags() | X86_EFLAGS_TF);
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syscall(SYS_getpid);
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check_result();
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/* Now make sure that another fast syscall doesn't set TF again. */
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fast_syscall_no_tf();
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/*
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* And do a forced SYSENTER to make sure that this works even if
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* fast syscalls don't use SYSENTER.
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*
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* Invoking SYSENTER directly breaks all the rules. Just handle
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* the SIGSEGV.
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*/
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if (sigsetjmp(jmpbuf, 1) == 0) {
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unsigned long nr = SYS_getpid;
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printf("[RUN]\tSet TF and check SYSENTER\n");
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stack_t stack = {
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.ss_sp = malloc(sizeof(char) * SIGSTKSZ),
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.ss_size = SIGSTKSZ,
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};
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if (sigaltstack(&stack, NULL) != 0)
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err(1, "sigaltstack");
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sethandler(SIGSEGV, print_and_longjmp,
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SA_RESETHAND | SA_ONSTACK);
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sethandler(SIGILL, print_and_longjmp, SA_RESETHAND);
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set_eflags(get_eflags() | X86_EFLAGS_TF);
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free(stack.ss_sp);
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/* Clear EBP first to make sure we segfault cleanly. */
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asm volatile ("xorl %%ebp, %%ebp; SYSENTER" : "+a" (nr) :: "flags", "rcx"
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#ifdef __x86_64__
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, "r11"
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#endif
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);
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/* We're unreachable here. SYSENTER forgets RIP. */
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}
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clearhandler(SIGSEGV);
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clearhandler(SIGILL);
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if (!(sig_eflags & X86_EFLAGS_TF)) {
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printf("[FAIL]\tTF was cleared\n");
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exit(1);
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}
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/* Now make sure that another fast syscall doesn't set TF again. */
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fast_syscall_no_tf();
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return 0;
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}
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