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/*
* sigreturn.c - tests for x86 sigreturn(2) and exit-to-userspace
* 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 series of tests that exercises the sigreturn(2) syscall and
* the IRET / SYSRET paths in the kernel.
*
* For now, this focuses on the effects of unusual CS and SS values,
* and it has a bunch of tests to make sure that ESP/RSP is restored
* properly.
*
* The basic idea behind these tests is to raise(SIGUSR1) to create a
* sigcontext frame, plug in the values to be tested, and then return,
* which implicitly invokes sigreturn(2) and programs the user context
* as desired.
*
* For tests for which we expect sigreturn and the subsequent return to
* user mode to succeed, we return to a short trampoline that generates
* SIGTRAP so that the meat of the tests can be ordinary C code in a
* SIGTRAP handler.
*
* The inner workings of each test is documented below.
*
* Do not run on outdated, unpatched kernels at risk of nasty crashes.
*/
#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>
/* Pull in AR_xyz defines. */
typedef unsigned int u32;
typedef unsigned short u16;
#include "../../../../arch/x86/include/asm/desc_defs.h"
/*
* Copied from asm/ucontext.h, as asm/ucontext.h conflicts badly with the glibc
* headers.
*/
#ifdef __x86_64__
/*
* UC_SIGCONTEXT_SS will be set when delivering 64-bit or x32 signals on
* kernels that save SS in the sigcontext. All kernels that set
* UC_SIGCONTEXT_SS will correctly restore at least the low 32 bits of esp
* regardless of SS (i.e. they implement espfix).
*
* Kernels that set UC_SIGCONTEXT_SS will also set UC_STRICT_RESTORE_SS
* when delivering a signal that came from 64-bit code.
*
* Sigreturn restores SS as follows:
*
* if (saved SS is valid || UC_STRICT_RESTORE_SS is set ||
* saved CS is not 64-bit)
* new SS = saved SS (will fail IRET and signal if invalid)
* else
* new SS = a flat 32-bit data segment
*/
#define UC_SIGCONTEXT_SS 0x2
#define UC_STRICT_RESTORE_SS 0x4
#endif
/*
* In principle, this test can run on Linux emulation layers (e.g.
* Illumos "LX branded zones"). Solaris-based kernels reserve LDT
* entries 0-5 for their own internal purposes, so start our LDT
* allocations above that reservation. (The tests don't pass on LX
* branded zones, but at least this lets them run.)
*/
#define LDT_OFFSET 6
/* An aligned stack accessible through some of our segments. */
static unsigned char stack16[65536] __attribute__((aligned(4096)));
/*
* An aligned int3 instruction used as a trampoline. Some of the tests
* want to fish out their ss values, so this trampoline copies ss to eax
* before the int3.
*/
asm (".pushsection .text\n\t"
".type int3, @function\n\t"
".align 4096\n\t"
"int3:\n\t"
"mov %ss,%ecx\n\t"
"int3\n\t"
".size int3, . - int3\n\t"
".align 4096, 0xcc\n\t"
".popsection");
extern char int3[4096];
/*
* At startup, we prepapre:
*
* - ldt_nonexistent_sel: An LDT entry that doesn't exist (all-zero
* descriptor or out of bounds).
* - code16_sel: A 16-bit LDT code segment pointing to int3.
* - data16_sel: A 16-bit LDT data segment pointing to stack16.
* - npcode32_sel: A 32-bit not-present LDT code segment pointing to int3.
* - npdata32_sel: A 32-bit not-present LDT data segment pointing to stack16.
* - gdt_data16_idx: A 16-bit GDT data segment pointing to stack16.
* - gdt_npdata32_idx: A 32-bit not-present GDT data segment pointing to
* stack16.
*
* For no particularly good reason, xyz_sel is a selector value with the
* RPL and LDT bits filled in, whereas xyz_idx is just an index into the
* descriptor table. These variables will be zero if their respective
* segments could not be allocated.
*/
static unsigned short ldt_nonexistent_sel;
static unsigned short code16_sel, data16_sel, npcode32_sel, npdata32_sel;
static unsigned short gdt_data16_idx, gdt_npdata32_idx;
static unsigned short GDT3(int idx)
{
return (idx << 3) | 3;
}
static unsigned short LDT3(int idx)
{
return (idx << 3) | 7;
}
/* Our sigaltstack scratch space. */
static char altstack_data[SIGSTKSZ];
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");
}
static void add_ldt(const struct user_desc *desc, unsigned short *var,
const char *name)
{
if (syscall(SYS_modify_ldt, 1, desc, sizeof(*desc)) == 0) {
*var = LDT3(desc->entry_number);
} else {
printf("[NOTE]\tFailed to create %s segment\n", name);
*var = 0;
}
}
static void setup_ldt(void)
{
if ((unsigned long)stack16 > (1ULL << 32) - sizeof(stack16))
errx(1, "stack16 is too high\n");
if ((unsigned long)int3 > (1ULL << 32) - sizeof(int3))
errx(1, "int3 is too high\n");
ldt_nonexistent_sel = LDT3(LDT_OFFSET + 2);
const struct user_desc code16_desc = {
.entry_number = LDT_OFFSET + 0,
.base_addr = (unsigned long)int3,
.limit = 4095,
.seg_32bit = 0,
.contents = 2, /* Code, not conforming */
.read_exec_only = 0,
.limit_in_pages = 0,
.seg_not_present = 0,
.useable = 0
};
add_ldt(&code16_desc, &code16_sel, "code16");
const struct user_desc data16_desc = {
.entry_number = LDT_OFFSET + 1,
.base_addr = (unsigned long)stack16,
.limit = 0xffff,
.seg_32bit = 0,
.contents = 0, /* Data, grow-up */
.read_exec_only
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