// SPDX-License-Identifier: GPL-2.0-only
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/*
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* sysret_ss_attrs.c - test that syscalls return valid hidden SS attributes
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* Copyright (c) 2015 Andrew Lutomirski
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*
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* On AMD CPUs, SYSRET can return with a valid SS descriptor with with
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* the hidden attributes set to an unusable state. Make sure the kernel
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* doesn't let this happen.
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*/
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#define _GNU_SOURCE
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#include <stdlib.h>
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#include <unistd.h>
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#include <stdio.h>
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#include <string.h>
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#include <sys/mman.h>
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#include <err.h>
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#include <stddef.h>
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#include <stdbool.h>
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#include <pthread.h>
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static void *threadproc(void *ctx)
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{
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/*
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* Do our best to cause sleeps on this CPU to exit the kernel and
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* re-enter with SS = 0.
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*/
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while (true)
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;
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return NULL;
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}
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#ifdef __x86_64__
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extern unsigned long call32_from_64(void *stack, void (*function)(void));
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asm (".pushsection .text\n\t"
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".code32\n\t"
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"test_ss:\n\t"
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"pushl $0\n\t"
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"popl %eax\n\t"
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"ret\n\t"
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".code64");
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extern void test_ss(void);
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#endif
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int main()
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{
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/*
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* Start a busy-looping thread on the same CPU we're on.
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* For simplicity, just stick everything to CPU 0. This will
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* fail in some containers, but that's probably okay.
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*/
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cpu_set_t cpuset;
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CPU_ZERO(&cpuset);
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CPU_SET(0, &cpuset);
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if (sched_setaffinity(0, sizeof(cpuset), &cpuset) != 0)
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printf("[WARN]\tsched_setaffinity failed\n");
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pthread_t thread;
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if (pthread_create(&thread, 0, threadproc, 0) != 0)
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err(1, "pthread_create");
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#ifdef __x86_64__
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unsigned char *stack32 = mmap(NULL, 4096, PROT_READ | PROT_WRITE,
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MAP_32BIT | MAP_ANONYMOUS | MAP_PRIVATE,
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-1, 0);
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if (stack32 == MAP_FAILED)
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err(1, "mmap");
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#endif
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printf("[RUN]\tSyscalls followed by SS validation\n");
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for (int i = 0; i < 1000; i++) {
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/*
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* Go to sleep and return using sysret (if we're 64-bit
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* or we're 32-bit on AMD on a 64-bit kernel). On AMD CPUs,
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* SYSRET doesn't fix up the cached SS descriptor, so the
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* kernel needs some kind of workaround to make sure that we
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* end the system call with a valid stack segment. This
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* can be a confusing failure because the SS *selector*
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* is the same regardless.
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*/
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usleep(2);
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#ifdef __x86_64__
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/*
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* On 32-bit, just doing a syscall through glibc is enough
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* to cause a crash if our cached SS descriptor is invalid.
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* On 64-bit, it's not, so try extra hard.
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*/
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call32_from_64(stack32 + 4088, test_ss);
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#endif
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}
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printf("[OK]\tWe survived\n");
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#ifdef __x86_64__
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munmap(stack32, 4096);
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#endif
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return 0;
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}
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