// SPDX-License-Identifier: GPL-2.0-only
/*
 * sysret_ss_attrs.c - test that syscalls return valid hidden SS attributes
 * Copyright (c) 2015 Andrew Lutomirski
 *
 * On AMD CPUs, SYSRET can return with a valid SS descriptor with with
 * the hidden attributes set to an unusable state.  Make sure the kernel
 * doesn't let this happen.
 */

#define _GNU_SOURCE

#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>
#include <sys/mman.h>
#include <err.h>
#include <stddef.h>
#include <stdbool.h>
#include <pthread.h>

static void *threadproc(void *ctx)
{
        /*
         * Do our best to cause sleeps on this CPU to exit the kernel and
         * re-enter with SS = 0.
         */
        while (true)
                ;

        return NULL;
}

#ifdef __x86_64__
extern unsigned long call32_from_64(void *stack, void (*function)(void));

asm (".pushsection .text\n\t"
     ".code32\n\t"
     "test_ss:\n\t"
     "pushl $0\n\t"
     "popl %eax\n\t"
     "ret\n\t"
     ".code64");
extern void test_ss(void);
#endif

int main()
{
        /*
         * Start a busy-looping thread on the same CPU we're on.
         * For simplicity, just stick everything to CPU 0.  This will
         * fail in some containers, but that's probably okay.
         */
        cpu_set_t cpuset;
        CPU_ZERO(&cpuset);
        CPU_SET(0, &cpuset);
        if (sched_setaffinity(0, sizeof(cpuset), &cpuset) != 0)
                printf("[WARN]\tsched_setaffinity failed\n");

        pthread_t thread;
        if (pthread_create(&thread, 0, threadproc, 0) != 0)
                err(1, "pthread_create");

#ifdef __x86_64__
        unsigned char *stack32 = mmap(NULL, 4096, PROT_READ | PROT_WRITE,
                                      MAP_32BIT | MAP_ANONYMOUS | MAP_PRIVATE,
                                      -1, 0);
        if (stack32 == MAP_FAILED)
                err(1, "mmap");
#endif

        printf("[RUN]\tSyscalls followed by SS validation\n");

        for (int i = 0; i < 1000; i++) {
                /*
                 * Go to sleep and return using sysret (if we're 64-bit
                 * or we're 32-bit on AMD on a 64-bit kernel).  On AMD CPUs,
                 * SYSRET doesn't fix up the cached SS descriptor, so the
                 * kernel needs some kind of workaround to make sure that we
                 * end the system call with a valid stack segment.  This
                 * can be a confusing failure because the SS *selector*
                 * is the same regardless.
                 */
                usleep(2);

#ifdef __x86_64__
                /*
                 * On 32-bit, just doing a syscall through glibc is enough
                 * to cause a crash if our cached SS descriptor is invalid.
                 * On 64-bit, it's not, so try extra hard.
                 */
                call32_from_64(stack32 + 4088, test_ss);
#endif
        }

        printf("[OK]\tWe survived\n");

#ifdef __x86_64__
        munmap(stack32, 4096);
#endif

        return 0;
}