[PATCH v3 1/7] selftests/x86: Test the FSBASE/GSBASE API and context switching

[Andy Lutomirski]

This catches two distinct bugs in the current code.  I'll fix them.

Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
---
 tools/testing/selftests/x86/Makefile   |   1 +
 tools/testing/selftests/x86/fsgsbase.c | 398 +++++++++++++++++++++++++++++++++
 2 files changed, 399 insertions(+)
 create mode 100644 tools/testing/selftests/x86/fsgsbase.c

diff --git a/tools/testing/selftests/x86/Makefile b/tools/testing/selftests/x86/Makefile
index b47ebd170690..c73425de3cfe 100644
--- a/tools/testing/selftests/x86/Makefile
+++ b/tools/testing/selftests/x86/Makefile
@@ -9,6 +9,7 @@ TARGETS_C_BOTHBITS := single_step_syscall sysret_ss_attrs syscall_nt ptrace_sysc
 TARGETS_C_32BIT_ONLY := entry_from_vm86 syscall_arg_fault test_syscall_vdso unwind_vdso \
 			test_FCMOV test_FCOMI test_FISTTP \
 			vdso_restorer
+TARGETS_C_64BIT_ONLY := fsgsbase
 
 TARGETS_C_32BIT_ALL := $(TARGETS_C_BOTHBITS) $(TARGETS_C_32BIT_ONLY)
 TARGETS_C_64BIT_ALL := $(TARGETS_C_BOTHBITS) $(TARGETS_C_64BIT_ONLY)
diff --git a/tools/testing/selftests/x86/fsgsbase.c b/tools/testing/selftests/x86/fsgsbase.c
new file mode 100644
index 000000000000..b7733055a359
--- /dev/null
+++ b/tools/testing/selftests/x86/fsgsbase.c
@@ -0,0 +1,398 @@
+/*
+ * 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;
+}
-- 
2.5.5