// SPDX-License-Identifier: GPL-2.0-only
/*
* fsgsbase_restore.c, test ptrace vs fsgsbase
* Copyright (c) 2020 Andy Lutomirski
*
* This test case simulates a tracer redirecting tracee execution to
* a function and then restoring tracee state using PTRACE_GETREGS and
* PTRACE_SETREGS. This is similar to what gdb does when doing
* 'p func()'. The catch is that this test has the called function
* modify a segment register. This makes sure that ptrace correctly
* restores segment state when using PTRACE_SETREGS.
*
* This is not part of fsgsbase.c, because that test is 64-bit only.
*/
#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <string.h>
#include <sys/syscall.h>
#include <unistd.h>
#include <err.h>
#include <sys/user.h>
#include <asm/prctl.h>
#include <sys/prctl.h>
#include <asm/ldt.h>
#include <sys/mman.h>
#include <stddef.h>
#include <sys/ptrace.h>
#include <sys/wait.h>
#include <stdint.h>
#define EXPECTED_VALUE 0x1337f00d
#ifdef __x86_64__
# define SEG "%gs"
#else
# define SEG "%fs"
#endif
static unsigned int dereference_seg_base(void)
{
int ret;
asm volatile ("mov %" SEG ":(0), %0" : "=rm" (ret));
return ret;
}
static void init_seg(void)
{
unsigned int *target = mmap(
NULL, sizeof(unsigned int),
PROT_READ | PROT_WRITE,
MAP_PRIVATE | MAP_ANONYMOUS | MAP_32BIT, -1, 0);
if (target == MAP_FAILED)
err(1, "mmap");
*target = EXPECTED_VALUE;
printf("\tsegment base address = 0x%lx\n", (unsigned long)target);
struct user_desc desc = {
.entry_number = 0,
.base_addr = (unsigned int)(uintptr_t)target,
.limit = sizeof(unsigned int) - 1,
.seg_32bit = 1,
.contents = 0, /* Data, grow-up */
.read_exec_only = 0,
.limit_in_pages = 0,
.seg_not_present = 0,
.useable = 0
};
if (syscall(SYS_modify_ldt, 1, &desc, sizeof(desc)) == 0) {
printf("\tusing LDT slot 0\n");
asm volatile ("mov %0, %" SEG :: "rm" ((unsigned short)0x7));
} else {
/* No modify_ldt for us (configured out, perhaps) */
struct user_desc *low_desc = mmap(
NULL, sizeof(desc),
PROT_READ | PROT_WRITE,
MAP_PRIVATE | MAP_ANONYMOUS | MAP_32BIT, -1, 0);
memcpy(low_desc, &desc, sizeof(desc));
low_desc->entry_number = -1;
/* 32-bit set_thread_area */
long ret;
asm volatile ("int $0x80"
: "=a" (ret), "+m" (*low_desc)
: "a" (243), "b" (low_desc)
#ifdef __x86_64__
: "r8", "r9", "r10", "r11"
#endif
);
memcpy(&desc, low_desc, sizeof(desc));
munmap(low_desc, sizeof(desc));
if (ret != 0) {
printf("[NOTE]\tcould not create a segment -- can't test anything\n");
exit(0);
}
printf("\tusing GDT slot %d\n", desc.entry_number);
unsigned short sel = (unsigned short)((desc.entry_number << 3) | 0x3);
asm volatile ("mov %0, %" SEG :: "rm" (sel));
}
}
static void tracee_zap_segment(void)
{
/*
* The tracer will redirect execution here. This is meant to
* work like gdb's 'p func()' feature. The tricky bit is that
* we modify a segment register in order to make sure that ptrace
* can correctly restore segment registers.
*/
printf("\tTracee: in tracee_zap_segment()\n");
/*
* Write a nonzero selector with base zero to the segment register.
* Using a null selector would defeat the test on AMD pre-Zen2
* CPUs, as such CPUs don't clear the base when loading a null
* selector.
*/
unsigned short sel;
asm volatile ("mov %%ss, %0\n\t"
"mov %0, %" SEG
: "=rm" (sel));
pid_t pid = getpid(), tid = syscall(SYS_gettid);
printf("\tTracee is going back to sleep\n");
syscall(SYS_tgkill, pid, tid, SIGSTOP);
/* Should not get here. */
while (true) {
printf("[FAIL]\tTracee hit unreachable code\n");
pause();
}
}
int main()
{
printf("\tSetting up a segment\n");
init_seg();
unsigned int val = dereference_seg_base();
if (val != EXPECTED_VALUE) {
printf("[FAIL]\tseg[0] == %x; should be %x\n", val, EXPECTED_VALUE);
return 1;
}
printf("[OK]\tThe segment points to the right place.\n");
pid_t chld = fork();
if (chld < 0)
err(1, "fork");
if (chld == 0) {
prctl(PR_SET_PDEATHSIG, SIGKILL, 0, 0, 0, 0);
if (ptrace(PTRACE_TRACEME, 0, 0, 0) != 0)
err(1, "PTRACE_TRACEME");
pid_t pid = getpid(), tid = syscall(SYS_gettid);
printf("\tTracee will take a nap until signaled\n");
syscall(SYS_tgkill, pid, tid, SIGSTOP);
printf("\tTracee was resumed. Will re-check segment.\n");
val = dereference_seg_base();
if (val != EXPECTED_VALUE) {
printf("[FAIL]\tseg[0] == %x; should be %x\n", val, EXPECTED_VALUE);
exit(1);
}
printf("[OK]\tThe segment points to the right place.\n");
exit(0);
}
int status;
/* Wait for SIGSTOP. */
if (waitpid(chld, &status, 0) != chld || !WIFSTOPPED(status))
err(1, "waitpid");
struct user_regs_struct regs;
if (ptrace(PTRACE_GETREGS, chld, NULL, ®s) != 0)
err(1, "PTRACE_GETREGS");
#ifdef __x86_64__
printf("\tChild GS=0x%lx, GSBASE=0x%lx\n", (unsigned long)regs.gs, (unsigned long)regs.gs_base);
#else
printf("\tChild FS=0x%lx\n", (unsigned long)regs.xfs);
#endif
struct user_regs_struct regs2 = regs;
#ifdef __x86_64__
regs2.rip = (unsigned long)tracee_zap_segment;
regs2.rsp -= 128; /* Don't clobber the redzone. */
#else
regs2.eip = (unsigned long)tracee_zap_segment;
#endif
printf("\tTracer: redirecting tracee to tracee_zap_segment()\n");
if (ptrace(PTRACE_SETREGS, chld, NULL, ®s2) != 0)
err(1, "PTRACE_GETREGS");
if (ptrace(PTRACE_CONT, chld, NULL, NULL) != 0)
err(1, "PTRACE_GETREGS");
/* Wait for SIGSTOP. */
if (waitpid(chld, &status, 0) != chld || !WIFSTOPPED(status))
err(1, "waitpid");
printf("\tTracer: restoring tracee state\n");
if (ptrace(PTRACE_SETREGS, chld, NULL, ®s) != 0)
err(1, "PTRACE_GETREGS");
if (ptrace(PTRACE_DETACH, chld, NULL, NULL) != 0)
err(1, "PTRACE_GETREGS");
/* Wait for SIGSTOP. */
if (waitpid(chld, &status, 0) != chld)
err(1, "waitpid");
if (WIFSIGNALED(status)) {
printf("[FAIL]\tTracee crashed\n");
return 1;
}
if (!WIFEXITED(status)) {
printf("[FAIL]\tTracee stopped for an unexpected reason: %d\n", status);
return 1;
}
int exitcode = WEXITSTATUS(status);
if (exitcode != 0) {
printf("[FAIL]\tTracee reported failure\n");
return 1;
}
printf("[OK]\tAll is well.\n");
return 0;
}