x86, selftests: Add sigreturn selftest

This is my sigreturn test, added mostly unchanged from its old
home. It exercises the sigreturn(2) syscall, specifically
focusing on its interactions with various IRET corner cases.

It tests for correct behavior in several areas that were
historically dangerously buggy. For example, it exercises espfix
on kernels of both bitnesses under various conditions, and it
contains testcases for several now-fixed bugs in IRET error
handling.

If you run it on older kernels without the fixes, your system will
crash. It probably won't eat your data in the process.

There is no released kernel on which the sigreturn_64 test will
pass, but it passes on tip:x86/asm.

I plan to switch to lib.mk for Linux 4.2.

I'm not using the ksft_ helpers at all yet.  I can do that later.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Acked-by: Shuah Khan <shuahkh@osg.samsung.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Denys Vlasenko <vda.linux@googlemail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Shuah Khan <shuah.kh@samsung.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/89d10b76b92c7202d8123654dc8d36701c017b3d.1428386971.git.luto@kernel.org
[ Fixed empty format string GCC build warning in trivial_32bit_program.c ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>

6 files changed, 760 insertions, 0 deletions

diff --git a/tools/testing/selftests/Makefile b/tools/testing/selftests/Makefile
index 4e511221a0c1..2ad56d451469 100644
--- a/tools/testing/selftests/Makefile
+++ b/tools/testing/selftests/Makefile
@@ -17,6 +17,7 @@ TARGETS += sysctl
 TARGETS += timers
 TARGETS += user
 TARGETS += vm
+TARGETS += x86
 #Please keep the TARGETS list alphabetically sorted
 
 TARGETS_HOTPLUG = cpu-hotplug
diff --git a/tools/testing/selftests/x86/.gitignore b/tools/testing/selftests/x86/.gitignore
new file mode 100644
index 000000000000..15034fef9698
--- /dev/null
+++ b/tools/testing/selftests/x86/.gitignore
@@ -0,0 +1,2 @@
+*_32
+*_64
diff --git a/tools/testing/selftests/x86/Makefile b/tools/testing/selftests/x86/Makefile
new file mode 100644
index 000000000000..f0a7918178dd
--- /dev/null
+++ b/tools/testing/selftests/x86/Makefile
@@ -0,0 +1,48 @@
+.PHONY: all all_32 all_64 check_build32 clean run_tests
+
+TARGETS_C_BOTHBITS := sigreturn
+
+BINARIES_32 := $(TARGETS_C_BOTHBITS:%=%_32)
+BINARIES_64 := $(TARGETS_C_BOTHBITS:%=%_64)
+
+CFLAGS := -O2 -g -std=gnu99 -pthread -Wall
+
+UNAME_P := $(shell uname -p)
+
+# Always build 32-bit tests
+all: all_32
+
+# If we're on a 64-bit host, build 64-bit tests as well
+ifeq ($(shell uname -p),x86_64)
+all: all_64
+endif
+
+all_32: check_build32 $(BINARIES_32)
+
+all_64: $(BINARIES_64)
+
+clean:
+        $(RM) $(BINARIES_32) $(BINARIES_64)
+
+run_tests:
+        ./run_x86_tests.sh
+
+$(TARGETS_C_BOTHBITS:%=%_32): %_32: %.c
+        $(CC) -m32 -o $@ $(CFLAGS) $(EXTRA_CFLAGS) $^ -lrt -ldl
+
+$(TARGETS_C_BOTHBITS:%=%_64): %_64: %.c
+        $(CC) -m64 -o $@ $(CFLAGS) $(EXTRA_CFLAGS) $^ -lrt -ldl
+
+check_build32:
+        @if ! $(CC) -m32 -o /dev/null trivial_32bit_program.c; then     \
+          echo "Warning: you seem to have a broken 32-bit build" 2>&1;  \
+          echo "environment.  If you are using a Debian-like";          \
+          echo " distribution, try:";                                   \
+          echo "";                                                      \
+          echo "  apt-get install gcc-multilib libc6-i386 libc6-dev-i386"; \
+          echo "";                                                      \
+          echo "If you are using a Fedora-like distribution, try:";     \
+          echo "";                                                      \
+          echo "  yum install glibc-devel.*i686";                       \
+          exit 1;                                                       \
+        fi
diff --git a/tools/testing/selftests/x86/run_x86_tests.sh b/tools/testing/selftests/x86/run_x86_tests.sh
new file mode 100644
index 000000000000..3d3ec65f3e7c
--- /dev/null
+++ b/tools/testing/selftests/x86/run_x86_tests.sh
@@ -0,0 +1,11 @@
+#!/bin/bash
+
+# This is deliberately minimal.  IMO kselftests should provide a standard
+# script here.
+./sigreturn_32 || exit 1
+
+if [[ "$uname -p" -eq "x86_64" ]]; then
+    ./sigreturn_64 || exit 1
+fi
+
+exit 0
diff --git a/tools/testing/selftests/x86/sigreturn.c b/tools/testing/selftests/x86/sigreturn.c
new file mode 100644
index 000000000000..b5aa1bab7416
--- /dev/null
+++ b/tools/testing/selftests/x86/sigreturn.c
@@ -0,0 +1,684 @@
+/*
+ * 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>
+
+/*
+ * 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,%eax\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  = 0,
+                .limit_in_pages  = 0,
+                .seg_not_present = 0,
+                .useable         = 0
+        };
+        add_ldt(&data16_desc, &data16_sel, "data16");
+
+        const struct user_desc npcode32_desc = {
+                .entry_number    = LDT_OFFSET + 3,
+                .base_addr       = (unsigned long)int3,
+                .limit           = 4095,
+                .seg_32bit       = 1,
+                .contents        = 2, /* Code, not conforming */
+                .read_exec_only  = 0,
+                .limit_in_pages  = 0,
+                .seg_not_present = 1,
+                .useable         = 0
+        };
+        add_ldt(&npcode32_desc, &npcode32_sel, "npcode32");
+
+        const struct user_desc npdata32_desc = {
+                .entry_number    = LDT_OFFSET + 4,
+                .base_addr       = (unsigned long)stack16,
+                .limit           = 0xffff,
+                .seg_32bit       = 1,
+                .contents        = 0, /* Data, grow-up */
+                .read_exec_only  = 0,
+                .limit_in_pages  = 0,
+                .seg_not_present = 1,
+                .useable         = 0
+        };
+        add_ldt(&npdata32_desc, &npdata32_sel, "npdata32");
+
+        struct user_desc gdt_data16_desc = {
+                .entry_number    = -1,
+                .base_addr       = (unsigned long)stack16,
+                .limit           = 0xffff,
+                .seg_32bit       = 0,
+                .contents        = 0, /* Data, grow-up */
+                .read_exec_only  = 0,
+                .limit_in_pages  = 0,
+                .seg_not_present = 0,
+                .useable         = 0
+        };
+
+        if (syscall(SYS_set_thread_area, &gdt_data16_desc) == 0) {
+                /*
+                 * This probably indicates vulnerability to CVE-2014-8133.
+                 * Merely getting here isn't definitive, though, and we'll
+                 * diagnose the problem for real later on.
+                 */
+                printf("[WARN]\tset_thread_area allocated data16 at index %d\n",
+                       gdt_data16_desc.entry_number);
+                gdt_data16_idx = gdt_data16_desc.entry_number;
+        } else {
+                printf("[OK]\tset_thread_area refused 16-bit data\n");
+        }
+
+        struct user_desc gdt_npdata32_desc = {
+                .entry_number    = -1,
+                .base_addr       = (unsigned long)stack16,
+                .limit           = 0xffff,
+                .seg_32bit       = 1,
+                .contents        = 0, /* Data, grow-up */
+                .read_exec_only  = 0,
+                .limit_in_pages  = 0,
+                .seg_not_present = 1,
+                .useable         = 0
+        };
+
+        if (syscall(SYS_set_thread_area, &gdt_npdata32_desc) == 0) {
+                /*
+                 * As a hardening measure, newer kernels don't allow this.
+                 */
+                printf("[WARN]\tset_thread_area allocated npdata32 at index %d\n",
+                       gdt_npdata32_desc.entry_number);
+                gdt_npdata32_idx = gdt_npdata32_desc.entry_number;
+        } else {
+                printf("[OK]\tset_thread_area refused 16-bit data\n");
+        }
+}
+
+/* State used by our signal handlers. */
+static gregset_t initial_regs, requested_regs, resulting_regs;
+
+/* Instructions for the SIGUSR1 handler. */
+static volatile unsigned short sig_cs, sig_ss;
+static volatile sig_atomic_t sig_trapped, sig_err, sig_trapno;
+
+/* Abstractions for some 32-bit vs 64-bit differences. */
+#ifdef __x86_64__
+# define REG_IP REG_RIP
+# define REG_SP REG_RSP
+# define REG_AX REG_RAX
+
+struct selectors {
+        unsigned short cs, gs, fs, ss;
+};
+
+static unsigned short *ssptr(ucontext_t *ctx)
+{
+        struct selectors *sels = (void *)&ctx->uc_mcontext.gregs[REG_CSGSFS];
+        return &sels->ss;
+}
+
+static unsigned short *csptr(ucontext_t *ctx)
+{
+        struct selectors *sels = (void *)&ctx->uc_mcontext.gregs[REG_CSGSFS];
+        return &sels->cs;
+}
+#else
+# define REG_IP REG_EIP
+# define REG_SP REG_ESP
+# define REG_AX REG_EAX
+
+static greg_t *ssptr(ucontext_t *ctx)
+{
+        return &ctx->uc_mcontext.gregs[REG_SS];
+}
+
+static greg_t *csptr(ucontext_t *ctx)
+{
+        return &ctx->uc_mcontext.gregs[REG_CS];
+}
+#endif
+
+/* Number of errors in the current test case. */
+static volatile sig_atomic_t nerrs;
+
+/*
+ * SIGUSR1 handler.  Sets CS and SS as requested and points IP to the
+ * int3 trampoline.  Sets SP to a large known value so that we can see
+ * whether the value round-trips back to user mode correctly.
+ */
+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));
+
+        *csptr(ctx) = sig_cs;
+        *ssptr(ctx) = sig_ss;
+
+        ctx->uc_mcontext.gregs[REG_IP] =
+                sig_cs == code16_sel ? 0 : (unsigned long)&int3;
+        ctx->uc_mcontext.gregs[REG_SP] = (unsigned long)0x8badf00d5aadc0deULL;
+        ctx->uc_mcontext.gregs[REG_AX] = 0;
+
+        memcpy(&requested_regs, &ctx->uc_mcontext.gregs, sizeof(gregset_t));
+        requested_regs[REG_AX] = *ssptr(ctx);   /* The asm code does this. */
+
+        return;
+}
+
+/*
+ * Called after a successful sigreturn.  Restores our state so that
+ * the original raise(SIGUSR1) returns.
+ */
+static void sigtrap(int sig, siginfo_t *info, void *ctx_void)
+{
+        ucontext_t *ctx = (ucontext_t*)ctx_void;
+
+        sig_err = ctx->uc_mcontext.gregs[REG_ERR];
+        sig_trapno = ctx->uc_mcontext.gregs[REG_TRAPNO];
+
+        unsigned short ss;
+        asm ("mov %%ss,%0" : "=r" (ss));
+
+        greg_t asm_ss = ctx->uc_mcontext.gregs[REG_AX];
+        if (asm_ss != sig_ss && sig == SIGTRAP) {
+                /* Sanity check failure. */
+                printf("[FAIL]\tSIGTRAP: ss = %hx, frame ss = %hx, ax = %llx\n",
+                       ss, *ssptr(ctx), (unsigned long long)asm_ss);
+                nerrs++;
+        }
+
+        memcpy(&resulting_regs, &ctx->uc_mcontext.gregs, sizeof(gregset_t));
+        memcpy(&ctx->uc_mcontext.gregs, &initial_regs, sizeof(gregset_t));
+
+        sig_trapped = sig;
+}
+
+/*
+ * Checks a given selector for its code bitness or returns -1 if it's not
+ * a usable code segment selector.
+ */
+int cs_bitness(unsigned short cs)
+{
+        uint32_t valid = 0, ar;
+        asm ("lar %[cs], %[ar]\n\t"
+             "jnz 1f\n\t"
+             "mov $1, %[valid]\n\t"
+             "1:"
+             : [ar] "=r" (ar), [valid] "+rm" (valid)
+             : [cs] "r" (cs));
+
+        if (!valid)
+                return -1;
+
+        bool db = (ar & (1 << 22));
+        bool l = (ar & (1 << 21));
+
+        if (!(ar & (1<<11)))
+            return -1;  /* Not code. */
+
+        if (l && !db)
+                return 64;
+        else if (!l && db)
+                return 32;
+        else if (!l && !db)
+                return 16;
+        else
+                return -1;      /* Unknown bitness. */
+}
+
+/* Finds a usable code segment of the requested bitness. */
+int find_cs(int bitness)
+{
+        unsigned short my_cs;
+
+        asm ("mov %%cs,%0" :  "=r" (my_cs));
+
+        if (cs_bitness(my_cs) == bitness)
+                return my_cs;
+        if (cs_bitness(my_cs + (2 << 3)) == bitness)
+                return my_cs + (2 << 3);
+        if (my_cs > (2<<3) && cs_bitness(my_cs - (2 << 3)) == bitness)
+            return my_cs - (2 << 3);
+        if (cs_bitness(code16_sel) == bitness)
+                return code16_sel;
+
+        printf("[WARN]\tCould not find %d-bit CS\n", bitness);
+        return -1;
+}
+
+static int test_valid_sigreturn(int cs_bits, bool use_16bit_ss, int force_ss)
+{
+        int cs = find_cs(cs_bits);
+        if (cs == -1) {
+                printf("[SKIP]\tCode segment unavailable for %d-bit CS, %d-bit SS\n",
+                       cs_bits, use_16bit_ss ? 16 : 32);
+                return 0;
+        }
+
+        if (force_ss != -1) {
+                sig_ss = force_ss;
+        } else {
+                if (use_16bit_ss) {
+                        if (!data16_sel) {
+                                printf("[SKIP]\tData segment unavailable for %d-bit CS, 16-bit SS\n",
+                                       cs_bits);
+                                return 0;
+                        }
+                        sig_ss = data16_sel;
+                } else {
+                        asm volatile ("mov %%ss,%0" : "=r" (sig_ss));
+                }
+        }
+
+        sig_cs = cs;
+
+        printf("[RUN]\tValid sigreturn: %d-bit CS (%hx), %d-bit SS (%hx%s)\n",
+               cs_bits, sig_cs, use_16bit_ss ? 16 : 32, sig_ss,
+               (sig_ss & 4) ? "" : ", GDT");
+
+        raise(SIGUSR1);
+
+        nerrs = 0;
+
+        /*
+         * Check that each register had an acceptable value when the
+         * int3 trampoline was invoked.
+         */
+        for (int i = 0; i < NGREG; i++) {
+                greg_t req = requested_regs[i], res = resulting_regs[i];
+                if (i == REG_TRAPNO || i == REG_IP)
+                        continue;       /* don't care */
+                if (i == REG_SP) {
+                        printf("\tSP: %llx -> %llx\n", (unsigned long long)req,
+                               (unsigned long long)res);
+
+                        /*
+                         * In many circumstances, the high 32 bits of rsp
+                         * are zeroed.  For example, we could be a real
+                         * 32-bit program, or we could hit any of a number
+                         * of poorly-documented IRET or segmented ESP
+                         * oddities.  If this happens, it's okay.
+                         */
+                        if (res == (req & 0xFFFFFFFF))
+                                continue;  /* OK; not expected to work */
+                }
+
+                bool ignore_reg = false;
+#if __i386__
+                if (i == REG_UESP)
+                        ignore_reg = true;
+#else
+                if (i == REG_CSGSFS) {
+                        struct selectors *req_sels =
+                                (void *)&requested_regs[REG_CSGSFS];
+                        struct selectors *res_sels =
+                                (void *)&resulting_regs[REG_CSGSFS];
+                        if (req_sels->cs != res_sels->cs) {
+                                printf("[FAIL]\tCS mismatch: requested 0x%hx; got 0x%hx\n",
+                                       req_sels->cs, res_sels->cs);
+                                nerrs++;
+                        }
+
+                        if (req_sels->ss != res_sels->ss) {
+                                printf("[FAIL]\tSS mismatch: requested 0x%hx; got 0x%hx\n",
+                                       req_sels->ss, res_sels->ss);
+                                nerrs++;
+                        }
+
+                        continue;
+                }
+#endif
+
+                /* Sanity check on the kernel */
+                if (i == REG_AX && requested_regs[i] != resulting_regs[i]) {
+                        printf("[FAIL]\tAX (saved SP) mismatch: requested 0x%llx; got 0x%llx\n",
+                               (unsigned long long)requested_regs[i],
+                               (unsigned long long)resulting_regs[i]);
+                        nerrs++;
+                        continue;
+                }
+
+                if (requested_regs[i] != resulting_regs[i] && !ignore_reg) {
+                        /*
+                         * SP is particularly interesting here.  The
+                         * usual cause of failures is that we hit the
+                         * nasty IRET case of returning to a 16-bit SS,
+                         * in which case bits 16:31 of the *kernel*
+                         * stack pointer persist in ESP.
+                         */
+                        printf("[FAIL]\tReg %d mismatch: requested 0x%llx; got 0x%llx\n",
+                               i, (unsigned long long)requested_regs[i],
+                               (unsigned long long)resulting_regs[i]);
+                        nerrs++;
+                }
+        }
+
+        if (nerrs == 0)
+                printf("[OK]\tall registers okay\n");
+
+        return nerrs;
+}
+
+static int test_bad_iret(int cs_bits, unsigned short ss, int force_cs)
+{
+        int cs = force_cs == -1 ? find_cs(cs_bits) : force_cs;
+        if (cs == -1)
+                return 0;
+
+        sig_cs = cs;
+        sig_ss = ss;
+
+        printf("[RUN]\t%d-bit CS (%hx), bogus SS (%hx)\n",
+               cs_bits, sig_cs, sig_ss);
+
+        sig_trapped = 0;
+        raise(SIGUSR1);
+        if (sig_trapped) {
+                char errdesc[32] = "";
+                if (sig_err) {
+                        const char *src = (sig_err & 1) ? " EXT" : "";
+                        const char *table;
+                        if ((sig_err & 0x6) == 0x0)
+                                table = "GDT";
+                        else if ((sig_err & 0x6) == 0x4)
+                                table = "LDT";
+                        else if ((sig_err & 0x6) == 0x2)
+                                table = "IDT";
+                        else
+                                table = "???";
+
+                        sprintf(errdesc, "%s%s index %d, ",
+                                table, src, sig_err >> 3);
+                }
+
+                char trapname[32];
+                if (sig_trapno == 13)
+                        strcpy(trapname, "GP");
+                else if (sig_trapno == 11)
+                        strcpy(trapname, "NP");
+                else if (sig_trapno == 12)
+                        strcpy(trapname, "SS");
+                else if (sig_trapno == 32)
+                        strcpy(trapname, "IRET");  /* X86_TRAP_IRET */
+                else
+                        sprintf(trapname, "%d", sig_trapno);
+
+                printf("[OK]\tGot #%s(0x%lx) (i.e. %s%s)\n",
+                       trapname, (unsigned long)sig_err,
+                       errdesc, strsignal(sig_trapped));
+                return 0;
+        } else {
+                printf("[FAIL]\tDid not get SIGSEGV\n");
+                return 1;
+        }
+}
+
+int main()
+{
+        int total_nerrs = 0;
+        unsigned short my_cs, my_ss;
+
+        asm volatile ("mov %%cs,%0" : "=r" (my_cs));
+        asm volatile ("mov %%ss,%0" : "=r" (my_ss));
+        setup_ldt();
+
+        stack_t stack = {
+                .ss_sp = altstack_data,
+                .ss_size = SIGSTKSZ,
+        };
+        if (sigaltstack(&stack, NULL) != 0)
+                err(1, "sigaltstack");
+
+        sethandler(SIGUSR1, sigusr1, 0);
+        sethandler(SIGTRAP, sigtrap, SA_ONSTACK);
+
+        /* Easy cases: return to a 32-bit SS in each possible CS bitness. */
+        total_nerrs += test_valid_sigreturn(64, false, -1);
+        total_nerrs += test_valid_sigreturn(32, false, -1);
+        total_nerrs += test_valid_sigreturn(16, false, -1);
+
+        /*
+         * Test easy espfix cases: return to a 16-bit LDT SS in each possible
+         * CS bitness.  NB: with a long mode CS, the SS bitness is irrelevant.
+         *
+         * This catches the original missing-espfix-on-64-bit-kernels issue
+         * as well as CVE-2014-8134.
+         */
+        total_nerrs += test_valid_sigreturn(64, true, -1);
+        total_nerrs += test_valid_sigreturn(32, true, -1);
+        total_nerrs += test_valid_sigreturn(16, true, -1);
+
+        if (gdt_data16_idx) {
+                /*
+                 * For performance reasons, Linux skips espfix if SS points
+                 * to the GDT.  If we were able to allocate a 16-bit SS in
+                 * the GDT, see if it leaks parts of the kernel stack pointer.
+                 *
+                 * This tests for CVE-2014-8133.
+                 */
+                total_nerrs += test_valid_sigreturn(64, true,
+                                                    GDT3(gdt_data16_idx));
+                total_nerrs += test_valid_sigreturn(32, true,
+                                                    GDT3(gdt_data16_idx));
+                total_nerrs += test_valid_sigreturn(16, true,
+                                                    GDT3(gdt_data16_idx));
+        }
+
+        /*
+         * We're done testing valid sigreturn cases.  Now we test states
+         * for which sigreturn itself will succeed but the subsequent
+         * entry to user mode will fail.
+         *
+         * Depending on the failure mode and the kernel bitness, these
+         * entry failures can generate SIGSEGV, SIGBUS, or SIGILL.
+         */
+        clearhandler(SIGTRAP);
+        sethandler(SIGSEGV, sigtrap, SA_ONSTACK);
+        sethandler(SIGBUS, sigtrap, SA_ONSTACK);
+        sethandler(SIGILL, sigtrap, SA_ONSTACK);  /* 32-bit kernels do this */
+
+        /* Easy failures: invalid SS, resulting in #GP(0) */
+        test_bad_iret(64, ldt_nonexistent_sel, -1);
+        test_bad_iret(32, ldt_nonexistent_sel, -1);
+        test_bad_iret(16, ldt_nonexistent_sel, -1);
+
+        /* These fail because SS isn't a data segment, resulting in #GP(SS) */
+        test_bad_iret(64, my_cs, -1);
+        test_bad_iret(32, my_cs, -1);
+        test_bad_iret(16, my_cs, -1);
+
+        /* Try to return to a not-present code segment, triggering #NP(SS). */
+        test_bad_iret(32, my_ss, npcode32_sel);
+
+        /*
+         * Try to return to a not-present but otherwise valid data segment.
+         * This will cause IRET to fail with #SS on the espfix stack.  This
+         * exercises CVE-2014-9322.
+         *
+         * Note that, if espfix is enabled, 64-bit Linux will lose track
+         * of the actual cause of failure and report #GP(0) instead.
+         * This would be very difficult for Linux to avoid, because
+         * espfix64 causes IRET failures to be promoted to #DF, so the
+         * original exception frame is never pushed onto the stack.
+         */
+        test_bad_iret(32, npdata32_sel, -1);
+
+        /*
+         * Try to return to a not-present but otherwise valid data
+         * segment without invoking espfix.  Newer kernels don't allow
+         * this to happen in the first place.  On older kernels, though,
+         * this can trigger CVE-2014-9322.
+         */
+        if (gdt_npdata32_idx)
+                test_bad_iret(32, GDT3(gdt_npdata32_idx), -1);
+
+        return total_nerrs ? 1 : 0;
+}
diff --git a/tools/testing/selftests/x86/trivial_32bit_program.c b/tools/testing/selftests/x86/trivial_32bit_program.c
new file mode 100644
index 000000000000..2e231beb0a39
--- /dev/null
+++ b/tools/testing/selftests/x86/trivial_32bit_program.c
@@ -0,0 +1,14 @@
+/*
+ * Trivial program to check that we have a valid 32-bit build environment.
+ * Copyright (c) 2015 Andy Lutomirski
+ * GPL v2
+ */
+
+#include <stdio.h>
+
+int main()
+{
+        printf("\n");
+
+        return 0;
+}