sigreturn.c

#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>

struct selectors {
	unsigned short cs, gs, fs, ss;
};

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 unsigned char stack16[65536] __attribute__((aligned(4096)));

asm (".pushsection .text\n\t"
     ".type int3, @function\n\t"
     ".align 4096\n\t"
     "int3:\n\t"
     "mov %ss,%ax\n\t"
     "int3\n\t"
     ".size int3, . - int3\n\t"
     ".align 4096, 0xcc\n\t"
     ".popsection");
extern char int3[4096];

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");

	// Borrowed from a test case by hpa
	const struct user_desc code16_desc = {
		.entry_number    = 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
	};
	const struct user_desc 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
	};
	const struct user_desc npcode16_desc = {
		.entry_number    = 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
	};
	const struct user_desc npdata16_desc = {
		.entry_number    = 4,
		.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 = 1,
		.useable         = 0
	};

	if (syscall(SYS_modify_ldt, 1, &code16_desc, sizeof code16_desc) != 0)
		err(1, "modify_ldt");
	if (syscall(SYS_modify_ldt, 1, &data16_desc, sizeof data16_desc) != 0)
		err(1, "modify_ldt");
	if (syscall(SYS_modify_ldt, 1, &npcode16_desc, sizeof npcode16_desc) != 0)
		err(1, "modify_ldt");
	if (syscall(SYS_modify_ldt, 1, &npdata16_desc, sizeof npdata16_desc) != 0)
		err(1, "modify_ldt");
}

static gregset_t initial_regs, requested_regs, resulting_regs;

static volatile unsigned short sig_cs, sig_ss;
static volatile bool sig_trapped;

#ifdef __x86_64__
# define REG_IP REG_RIP
# define REG_SP REG_RSP
# define REG_AX REG_RAX

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

static int nerrs;

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 == 0x7 || sig_cs == 0x1f) ? 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;
}

static void sigtrap(int sig, siginfo_t *info, void *ctx_void)
{
	ucontext_t *ctx = (ucontext_t*)ctx_void;

	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) {
		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 = true;
}

static char altstack_data[SIGSTKSZ];

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. */
}

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(0x7) == bitness)
		return 0x7;

	printf("[WARN]\tCould not find %d-bit CS\n", bitness);
	return -1;
}

static int do_test(int cs_bits, bool use_16bit_ss)
{
	int cs = find_cs(cs_bits);
	if (cs == -1)
		return 0;

	if (use_16bit_ss)
		sig_ss = (1 << 3) | 7;	/* LDT selector 1, RPL = 3 */
	else
		asm volatile ("mov %%ss,%0" : "=r" (sig_ss));

	sig_cs = cs;

	printf("[RUN]\t%d-bit CS (%hx), %d-bit SS (%hx)\n",
	       cs_bits, sig_cs, use_16bit_ss ? 16 : 32, sig_ss);

	raise(SIGUSR1);

	nerrs = 0;

	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);
			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) {
			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 = false;
	raise(SIGUSR1);
	if (sig_trapped) {
		printf("[OK]\tGot SIGSEGV\n");
		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);

	total_nerrs += do_test(64, false);
	total_nerrs += do_test(32, false);
	total_nerrs += do_test(16, false);
	total_nerrs += do_test(64, true);
	total_nerrs += do_test(32, true);
	total_nerrs += do_test(16, true);

	clearhandler(SIGTRAP);
	sethandler(SIGSEGV, sigtrap, SA_ONSTACK);

	test_bad_iret(64, (2 << 3) | 7, -1);
	test_bad_iret(32, (2 << 3) | 7, -1);
	test_bad_iret(16, (2 << 3) | 7, -1);

	test_bad_iret(64, my_cs, -1);
	test_bad_iret(32, my_cs, -1);
	test_bad_iret(16, my_cs, -1);

	/* IRET will fail with #NP */
	test_bad_iret(32, my_ss, (3 << 3) | 7);

	/* IRET will fail with #SS */
	test_bad_iret(32, (4 << 3) | 7, -1);

	return total_nerrs ? 1 : 0;
}