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/*
* sigreturn.c - tests that x86 avoids Intel SYSRET pitfalls
* Copyright (c) 2014-2016 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.
*/
#define _GNU_SOURCE
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>
#include <inttypes.h>
#include <sys/signal.h>
#include <sys/ucontext.h>
#include <sys/syscall.h>
#include <err.h>
#include <stddef.h>
#include <stdbool.h>
#include <setjmp.h>
#include <sys/user.h>
#include <sys/mman.h>
#include <assert.h>
asm (
".pushsection \".text\", \"ax\"\n\t"
".balign 4096\n\t"
"test_page: .globl test_page\n\t"
".fill 4094,1,0xcc\n\t"
"test_syscall_insn:\n\t"
"syscall\n\t"
".ifne . - test_page - 4096\n\t"
".error \"test page is not one page long\"\n\t"
".endif\n\t"
".popsection"
);
extern const char test_page[];
static void const *current_test_page_addr = test_page;
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 void clearhandler(int sig)
{
struct sigaction sa;
memset(&sa, 0, sizeof(sa));
sa.sa_handler = SIG_DFL;
sigemptyset(&sa.sa_mask);
if (sigaction(sig, &sa, 0))
err(1, "sigaction");
}
/* State used by our signal handlers. */
static gregset_t initial_regs;
static volatile unsigned long rip;
static void sigsegv_for_sigreturn_test(int sig, siginfo_t *info, void *ctx_void)
{
ucontext_t *ctx = (ucontext_t*)ctx_void;
if (rip != ctx->uc_mcontext.gregs[REG_RIP]) {
printf("[FAIL]\tRequested RIP=0x%lx but got RIP=0x%lx\n",
rip, (unsigned long)ctx->uc_mcontext.gregs[REG_RIP]);
fflush(stdout);
_exit(1);
}
memcpy(&ctx->uc_mcontext.gregs, &initial_regs, sizeof(gregset_t));
printf("[OK]\tGot SIGSEGV at RIP=0x%lx\n", rip);
}
static void sigusr1(int sig, siginfo_t *info, void *ctx_void)
{
ucontext_t *ctx = (ucontext_t*)ctx_void;
memcpy(&initial_regs, &ctx->uc_mcontext.gregs, sizeof(gregset_t));
/* Set IP and CX to match so that SYSRET can happen. */
ctx->uc_mcontext.gregs[REG_RIP] = rip;
ctx->uc_mcontext.gregs[REG_RCX] = rip;
/* R11 and EFLAGS should already match. */
assert(ctx->uc_mcontext.gregs[REG_EFL] ==
ctx->uc_mcontext.gregs[REG_R11]);
sethandler(SIGSEGV, sigsegv_for_sigreturn_test, SA_RESETHAND);
return;
}
static void test_sigreturn_to(unsigned long ip)
{
rip = ip;
printf("[RUN]\tsigreturn to 0x%lx\n", ip);
raise(SIGUSR1);
}
static jmp_buf jmpbuf;
static void sigsegv_for_fallthrough(int sig, siginfo_t *info, void *ctx_void)
{
ucontext_t *ctx = (ucontext_t*)ctx_void;
if (rip != ctx->uc_mcontext.gregs[REG_RIP]) {
printf("[FAIL]\tExpected SIGSEGV at 0x%lx but got RIP=0x%lx\n",
rip, (unsigned long)ctx->uc_mcontext.gregs[REG_RIP]);
fflush(stdout);
_exit(1);
}
siglongjmp(jmpbuf, 1);
}
static void test_syscall_fallthrough_to(unsigned long ip)
{
void *new_address = (void *)(ip - 4096);
void *ret;
printf("[RUN]\tTrying a SYSCALL that falls through to 0x%lx\n", ip);
ret = mremap((void *)current_test_page_addr, 4096, 4096,
MREMAP_MAYMOVE | MREMAP_FIXED, new_address);
if (ret == MAP_FAILED) {
if (ip <= (1UL << 47) - PAGE_SIZE) {
err(1, "mremap to %p", new_address);
} else {
printf("[OK]\tmremap to %p failed\n", new_address);
return;
}
}
if (ret != new_address)
errx(1, "mremap malfunctioned: asked for %p but got %p\n",
new_address, ret);
current_test_page_addr = new_address;
rip = ip;
if (sigsetjmp(jmpbuf, 1) == 0) {
asm volatile ("call *%[syscall_insn]" :: "a" (SYS_getpid),
[syscall_insn] "rm" (ip - 2));
errx(1, "[FAIL]\tSyscall trampoline returned");
}
printf("[OK]\tWe survived\n");
}
int main()
{
/*
* When the kernel returns from a slow-path syscall, it will
* detect whether SYSRET is appropriate. If it incorrectly
* thinks that SYSRET is appropriate when RIP is noncanonical,
* it'll crash on Intel CPUs.
*/
sethandler(SIGUSR1, sigusr1, 0);
for (int i = 47; i < 64; i++)
test_sigreturn_to(1UL<<i);
clearhandler(SIGUSR1);
sethandler(SIGSEGV, sigsegv_for_fallthrough, 0);
/* One extra test to check that we didn't screw up the mremap logic. */
test_syscall_fallthrough_to((1UL << 47) - 2*PAGE_SIZE);
/* These are the interesting cases. */
for (int i = 47; i < 64; i++) {
test_syscall_fallthrough_to((1UL<<i) - PAGE_SIZE);
test_syscall_fallthrough_to(1UL<<i);
}
return 0;
}
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