--- /dev/null
+/*
+ * fsgsbase.c, an fsgsbase test
+ * Copyright (c) 2014-2016 Andy Lutomirski
+ * GPL v2
+ */
+
+#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 <signal.h>
+#include <limits.h>
+#include <sys/ucontext.h>
+#include <sched.h>
+#include <linux/futex.h>
+#include <pthread.h>
+#include <asm/ldt.h>
+#include <sys/mman.h>
+
+#ifndef __x86_64__
+# error This test is 64-bit only
+#endif
+
+static volatile sig_atomic_t want_segv;
+static volatile unsigned long segv_addr;
+
+static int nerrs;
+
+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 sigsegv(int sig, siginfo_t *si, void *ctx_void)
+{
+ ucontext_t *ctx = (ucontext_t*)ctx_void;
+
+ if (!want_segv) {
+ clearhandler(SIGSEGV);
+ return; /* Crash cleanly. */
+ }
+
+ want_segv = false;
+ segv_addr = (unsigned long)si->si_addr;
+
+ ctx->uc_mcontext.gregs[REG_RIP] += 4; /* Skip the faulting mov */
+
+}
+
+enum which_base { FS, GS };
+
+static unsigned long read_base(enum which_base which)
+{
+ unsigned long offset;
+ /*
+ * Unless we have FSGSBASE, there's no direct way to do this from
+ * user mode. We can get at it indirectly using signals, though.
+ */
+
+ want_segv = true;
+
+ offset = 0;
+ if (which == FS) {
+ /* Use a constant-length instruction here. */
+ asm volatile ("mov %%fs:(%%rcx), %%rax" : : "c" (offset) : "rax");
+ } else {
+ asm volatile ("mov %%gs:(%%rcx), %%rax" : : "c" (offset) : "rax");
+ }
+ if (!want_segv)
+ return segv_addr + offset;
+
+ /*
+ * If that didn't segfault, try the other end of the address space.
+ * Unless we get really unlucky and run into the vsyscall page, this
+ * is guaranteed to segfault.
+ */
+
+ offset = (ULONG_MAX >> 1) + 1;
+ if (which == FS) {
+ asm volatile ("mov %%fs:(%%rcx), %%rax"
+ : : "c" (offset) : "rax");
+ } else {
+ asm volatile ("mov %%gs:(%%rcx), %%rax"
+ : : "c" (offset) : "rax");
+ }
+ if (!want_segv)
+ return segv_addr + offset;
+
+ abort();
+}
+
+static void check_gs_value(unsigned long value)
+{
+ unsigned long base;
+ unsigned short sel;
+
+ printf("[RUN]\tARCH_SET_GS to 0x%lx\n", value);
+ if (syscall(SYS_arch_prctl, ARCH_SET_GS, value) != 0)
+ err(1, "ARCH_SET_GS");
+
+ asm volatile ("mov %%gs, %0" : "=rm" (sel));
+ base = read_base(GS);
+ if (base == value) {
+ printf("[OK]\tGSBASE was set as expected (selector 0x%hx)\n",
+ sel);
+ } else {
+ nerrs++;
+ printf("[FAIL]\tGSBASE was not as expected: got 0x%lx (selector 0x%hx)\n",
+ base, sel);
+ }
+
+ if (syscall(SYS_arch_prctl, ARCH_GET_GS, &base) != 0)
+ err(1, "ARCH_GET_GS");
+ if (base == value) {
+ printf("[OK]\tARCH_GET_GS worked as expected (selector 0x%hx)\n",
+ sel);
+ } else {
+ nerrs++;
+ printf("[FAIL]\tARCH_GET_GS was not as expected: got 0x%lx (selector 0x%hx)\n",
+ base, sel);
+ }
+}
+
+static void mov_0_gs(unsigned long initial_base, bool schedule)
+{
+ unsigned long base, arch_base;
+
+ printf("[RUN]\tARCH_SET_GS to 0x%lx then mov 0 to %%gs%s\n", initial_base, schedule ? " and schedule " : "");
+ if (syscall(SYS_arch_prctl, ARCH_SET_GS, initial_base) != 0)
+ err(1, "ARCH_SET_GS");
+
+ if (schedule)
+ usleep(10);
+
+ asm volatile ("mov %0, %%gs" : : "rm" (0));
+ base = read_base(GS);
+ if (syscall(SYS_arch_prctl, ARCH_GET_GS, &arch_base) != 0)
+ err(1, "ARCH_GET_GS");
+ if (base == arch_base) {
+ printf("[OK]\tGSBASE is 0x%lx\n", base);
+ } else {
+ nerrs++;
+ printf("[FAIL]\tGSBASE changed to 0x%lx but kernel reports 0x%lx\n", base, arch_base);
+ }
+}
+
+static volatile unsigned long remote_base;
+static volatile bool remote_hard_zero;
+static volatile unsigned int ftx;
+
+/*
+ * ARCH_SET_FS/GS(0) may or may not program a selector of zero. HARD_ZERO
+ * means to force the selector to zero to improve test coverage.
+ */
+#define HARD_ZERO 0xa1fa5f343cb85fa4
+
+static void do_remote_base()
+{
+ unsigned long to_set = remote_base;
+ bool hard_zero = false;
+ if (to_set == HARD_ZERO) {
+ to_set = 0;
+ hard_zero = true;
+ }
+
+ if (syscall(SYS_arch_prctl, ARCH_SET_GS, to_set) != 0)
+ err(1, "ARCH_SET_GS");
+
+ if (hard_zero)
+ asm volatile ("mov %0, %%gs" : : "rm" ((unsigned short)0));
+
+ unsigned short sel;
+ asm volatile ("mov %%gs, %0" : "=rm" (sel));
+ printf("\tother thread: ARCH_SET_GS(0x%lx)%s -- sel is 0x%hx\n",
+ to_set, hard_zero ? " and clear gs" : "", sel);
+}
+
+void do_unexpected_base(void)
+{
+ /*
+ * The goal here is to try to arrange for GS == 0, GSBASE !=
+ * 0, and for the the kernel the think that GSBASE == 0.
+ *
+ * To make the test as reliable as possible, this uses
+ * explicit descriptorss. (This is not the only way. This
+ * could use ARCH_SET_GS with a low, nonzero base, but the
+ * relevant side effect of ARCH_SET_GS could change.)
+ */
+
+ /* Step 1: tell the kernel that we have GSBASE == 0. */
+ if (syscall(SYS_arch_prctl, ARCH_SET_GS, 0) != 0)
+ err(1, "ARCH_SET_GS");
+
+ /* Step 2: change GSBASE without telling the kernel. */
+ struct user_desc desc = {
+ .entry_number = 0,
+ .base_addr = 0xBAADF00D,
+ .limit = 0xfffff,
+ .seg_32bit = 1,
+ .contents = 0, /* Data, grow-up */
+ .read_exec_only = 0,
+ .limit_in_pages = 1,
+ .seg_not_present = 0,
+ .useable = 0
+ };
+ if (syscall(SYS_modify_ldt, 1, &desc, sizeof(desc)) == 0) {
+ printf("\tother thread: using LDT slot 0\n");
+ asm volatile ("mov %0, %%gs" : : "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) : "a" (243), "b" (low_desc)
+ : "flags");
+ memcpy(&desc, low_desc, sizeof(desc));
+ munmap(low_desc, sizeof(desc));
+
+ if (ret != 0) {
+ printf("[NOTE]\tcould not create a segment -- test won't do anything\n");
+ return;
+ }
+ printf("\tother thread: using GDT slot %d\n", desc.entry_number);
+ asm volatile ("mov %0, %%gs" : : "rm" ((unsigned short)((desc.entry_number << 3) | 0x3)));
+ }
+
+ /*
+ * Step 3: set the selector back to zero. On AMD chips, this will
+ * preserve GSBASE.
+ */
+
+ asm volatile ("mov %0, %%gs" : : "rm" ((unsigned short)0));
+}
+
+static void *threadproc(void *ctx)
+{
+ while (1) {
+ while (ftx == 0)
+ syscall(SYS_futex, &ftx, FUTEX_WAIT, 0, NULL, NULL, 0);
+ if (ftx == 3)
+ return NULL;
+
+ if (ftx == 1)
+ do_remote_base();
+ else if (ftx == 2)
+ do_unexpected_base();
+ else
+ errx(1, "helper thread got bad command");
+
+ ftx = 0;
+ syscall(SYS_futex, &ftx, FUTEX_WAKE, 0, NULL, NULL, 0);
+ }
+}
+
+static void set_gs_and_switch_to(unsigned long local, unsigned long remote)
+{
+ unsigned long base;
+
+ bool hard_zero = false;
+ if (local == HARD_ZERO) {
+ hard_zero = true;
+ local = 0;
+ }
+
+ printf("[RUN]\tARCH_SET_GS(0x%lx)%s, then schedule to 0x%lx\n",
+ local, hard_zero ? " and clear gs" : "", remote);
+ if (syscall(SYS_arch_prctl, ARCH_SET_GS, local) != 0)
+ err(1, "ARCH_SET_GS");
+ if (hard_zero)
+ asm volatile ("mov %0, %%gs" : : "rm" ((unsigned short)0));
+
+ if (read_base(GS) != local) {
+ nerrs++;
+ printf("[FAIL]\tGSBASE wasn't set as expected\n");
+ }
+
+ remote_base = remote;
+ ftx = 1;
+ syscall(SYS_futex, &ftx, FUTEX_WAKE, 0, NULL, NULL, 0);
+ while (ftx != 0)
+ syscall(SYS_futex, &ftx, FUTEX_WAIT, 1, NULL, NULL, 0);
+
+ base = read_base(GS);
+ if (base == local) {
+ printf("[OK]\tGSBASE remained 0x%lx\n", local);
+ } else {
+ nerrs++;
+ printf("[FAIL]\tGSBASE changed to 0x%lx\n", base);
+ }
+}
+
+static void test_unexpected_base(void)
+{
+ unsigned long base;
+
+ printf("[RUN]\tARCH_SET_GS(0), clear gs, then manipulate GSBASE in a different thread\n");
+ if (syscall(SYS_arch_prctl, ARCH_SET_GS, 0) != 0)
+ err(1, "ARCH_SET_GS");
+ asm volatile ("mov %0, %%gs" : : "rm" ((unsigned short)0));
+
+ ftx = 2;
+ syscall(SYS_futex, &ftx, FUTEX_WAKE, 0, NULL, NULL, 0);
+ while (ftx != 0)
+ syscall(SYS_futex, &ftx, FUTEX_WAIT, 1, NULL, NULL, 0);
+
+ base = read_base(GS);
+ if (base == 0) {
+ printf("[OK]\tGSBASE remained 0\n");
+ } else {
+ nerrs++;
+ printf("[FAIL]\tGSBASE changed to 0x%lx\n", base);
+ }
+}
+
+int main()
+{
+ pthread_t thread;
+
+ sethandler(SIGSEGV, sigsegv, 0);
+
+ check_gs_value(0);
+ check_gs_value(1);
+ check_gs_value(0x200000000);
+ check_gs_value(0);
+ check_gs_value(0x200000000);
+ check_gs_value(1);
+
+ for (int sched = 0; sched < 2; sched++) {
+ mov_0_gs(0, !!sched);
+ mov_0_gs(1, !!sched);
+ mov_0_gs(0x200000000, !!sched);
+ }
+
+ /* Set up for multithreading. */
+
+ cpu_set_t cpuset;
+ CPU_ZERO(&cpuset);
+ CPU_SET(0, &cpuset);
+ if (sched_setaffinity(0, sizeof(cpuset), &cpuset) != 0)
+ err(1, "sched_setaffinity to CPU 0"); /* should never fail */
+
+ if (pthread_create(&thread, 0, threadproc, 0) != 0)
+ err(1, "pthread_create");
+
+ static unsigned long bases_with_hard_zero[] = {
+ 0, HARD_ZERO, 1, 0x200000000,
+ };
+
+ for (int local = 0; local < 4; local++) {
+ for (int remote = 0; remote < 4; remote++) {
+ set_gs_and_switch_to(bases_with_hard_zero[local],
+ bases_with_hard_zero[remote]);
+ }
+ }
+
+ test_unexpected_base();
+
+ ftx = 3; /* Kill the thread. */
+ syscall(SYS_futex, &ftx, FUTEX_WAKE, 0, NULL, NULL, 0);
+
+ if (pthread_join(thread, NULL) != 0)
+ err(1, "pthread_join");
+
+ return nerrs == 0 ? 0 : 1;
+}
projects / cascardo / linux.git / commitdiff