x86, selftests: Add a test for the "sysret_ss_attrs" bug

.PHONY: all all_32 all_64 check_build32 clean run_tests

TARGETS_C_BOTHBITS := sigreturn single_step_syscall

TARGETS_C_BOTHBITS := sigreturn single_step_syscall sysret_ss_attrs

BINARIES_32 := $(TARGETS_C_BOTHBITS:%=%_32)

BINARIES_64 := $(TARGETS_C_BOTHBITS:%=%_64)

	  echo "  yum install glibc-devel.*i686";			\

	  exit 1;							\

	fi

# Some tests have additional dependencies.

sysret_ss_attrs_64: thunks.S

# script here.

./sigreturn_32 || exit 1

./single_step_syscall_32 || exit 1

./sysret_ss_attrs_32 || exit 1

if [[ "$uname -p" -eq "x86_64" ]]; then

    ./sigreturn_64 || exit 1

    ./single_step_syscall_64 || exit 1

    ./sysret_ss_attrs_64 || exit 1

fi

exit 0

/*

 * sysret_ss_attrs.c - test that syscalls return valid hidden SS attributes

 * Copyright (c) 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.

 *

 * 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;

}

/*

 * thunks.S - assembly helpers for mixed-bitness code

 * Copyright (c) 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.

 *

 * These are little helpers that make it easier to switch bitness on

 * the fly.

 */

	.text

	.global call32_from_64

	.type call32_from_64, @function

call32_from_64:

	// rdi: stack to use

	// esi: function to call

	// Save registers

	pushq %rbx

	pushq %rbp

	pushq %r12

	pushq %r13

	pushq %r14

	pushq %r15

	pushfq

	// Switch stacks

	mov %rsp,(%rdi)

	mov %rdi,%rsp

	// Switch to compatibility mode

	pushq $0x23  /* USER32_CS */

	pushq $1f

	lretq

1:

	.code32

	// Call the function

	call *%esi

	// Switch back to long mode

	jmp $0x33,$1f

	.code64

1:

	// Restore the stack

	mov (%rsp),%rsp

	// Restore registers

	popfq

	popq %r15

	popq %r14

	popq %r13

	popq %r12

	popq %rbp

	popq %rbx

	ret

.size call32_from_64, .-call32_from_64