Date:   Tue, 20 Dec 2016 02:30:33 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     x86@...nel.org, Borislav Petkov <bp@...en8.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH] selftests/x86: Add a selftest for SYSRET to noncanonical
 addresses

On Mon, Dec 19, 2016 at 11:12:42AM -0800, Andy Lutomirski wrote:
> SYSRET to a noncanonical address will blow up on Intel CPUs.  Linux
> needs to prevent this from happening in two major cases, and the
> criteria will become more complicated when support for larger virtual
> address spaces is added.
> 
> A fast-path SYSCALL will fallthrough to the following instruction
> using SYSRET without any particular checking.  To prevent fallthrough
> to a noncanonical address, Linux prevents the highest canonical page
> from being mapped.  This test case checks a variety of possible maximum
> addresses to make sure that either we can't map code there or that
> SYSCALL fallthrough works.
> 
> A slow-path system call can return anywhere.  Linux needs to make sure
> that, if the return address is non-canonical, it won't use SYSRET.
> This test cases causes sigreturn() to return to a variety of addresses
> (with RCX == RIP) and makes sure that nothing explodes.
> 
> Some of this code comes from Kirill Shutemov.
> 
> Cc: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
> Signed-off-by: Andy Lutomirski <luto@...nel.org>

Thanks a lot.

That's what I've got with 5-level paging:

[RUN]   sigreturn to 0x800000000000
[OK]    Got SIGSEGV at RIP=0x800000000000
[RUN]   sigreturn to 0x1000000000000
[OK]    Got SIGSEGV at RIP=0x1000000000000
[RUN]   sigreturn to 0x2000000000000
[OK]    Got SIGSEGV at RIP=0x2000000000000
[RUN]   sigreturn to 0x4000000000000
[OK]    Got SIGSEGV at RIP=0x4000000000000
[RUN]   sigreturn to 0x8000000000000
[OK]    Got SIGSEGV at RIP=0x8000000000000
[RUN]   sigreturn to 0x10000000000000
[OK]    Got SIGSEGV at RIP=0x10000000000000
[RUN]   sigreturn to 0x20000000000000
[OK]    Got SIGSEGV at RIP=0x20000000000000
[RUN]   sigreturn to 0x40000000000000
[OK]    Got SIGSEGV at RIP=0x40000000000000
[RUN]   sigreturn to 0x80000000000000
[OK]    Got SIGSEGV at RIP=0x80000000000000
[RUN]   sigreturn to 0x100000000000000
[OK]    Got SIGSEGV at RIP=0x100000000000000
[RUN]   sigreturn to 0x200000000000000
[OK]    Got SIGSEGV at RIP=0x200000000000000
[RUN]   sigreturn to 0x400000000000000
[OK]    Got SIGSEGV at RIP=0x400000000000000
[RUN]   sigreturn to 0x800000000000000
[OK]    Got SIGSEGV at RIP=0x800000000000000
[RUN]   sigreturn to 0x1000000000000000
[OK]    Got SIGSEGV at RIP=0x1000000000000000
[RUN]   sigreturn to 0x2000000000000000
[OK]    Got SIGSEGV at RIP=0x2000000000000000
[RUN]   sigreturn to 0x4000000000000000
[OK]    Got SIGSEGV at RIP=0x4000000000000000
[RUN]   sigreturn to 0x8000000000000000
[OK]    Got SIGSEGV at RIP=0x8000000000000000
[RUN]   Trying a SYSCALL that falls through to 0x7ffffffff000
[OK]    We survived
[RUN]   Trying a SYSCALL that falls through to 0x7fffffffe000
[OK]    We survived
[RUN]   Trying a SYSCALL that falls through to 0x800000000000
[OK]    We survived
[RUN]   Trying a SYSCALL that falls through to 0x1000000000000
[OK]    We survived
[RUN]   Trying a SYSCALL that falls through to 0x2000000000000
[OK]    We survived
[RUN]   Trying a SYSCALL that falls through to 0x4000000000000
[OK]    We survived
[RUN]   Trying a SYSCALL that falls through to 0x8000000000000
[OK]    We survived
[RUN]   Trying a SYSCALL that falls through to 0x10000000000000
[OK]    We survived
[RUN]   Trying a SYSCALL that falls through to 0x20000000000000
[OK]    We survived
[RUN]   Trying a SYSCALL that falls through to 0x40000000000000
[OK]    We survived
[RUN]   Trying a SYSCALL that falls through to 0x80000000000000
[OK]    We survived
[RUN]   Trying a SYSCALL that falls through to 0x100000000000000
[OK]    mremap to 0x0xfffffffffff000 failed
[RUN]   Trying a SYSCALL that falls through to 0x200000000000000
[OK]    mremap to 0x0x1fffffffffff000 failed
[RUN]   Trying a SYSCALL that falls through to 0x400000000000000
[OK]    mremap to 0x0x3fffffffffff000 failed
[RUN]   Trying a SYSCALL that falls through to 0x800000000000000
[OK]    mremap to 0x0x7fffffffffff000 failed
[RUN]   Trying a SYSCALL that falls through to 0x1000000000000000
[OK]    mremap to 0x0xffffffffffff000 failed
[RUN]   Trying a SYSCALL that falls through to 0x2000000000000000
[OK]    mremap to 0x0x1ffffffffffff000 failed
[RUN]   Trying a SYSCALL that falls through to 0x4000000000000000
[OK]    mremap to 0x0x3ffffffffffff000 failed
[RUN]   Trying a SYSCALL that falls through to 0x8000000000000000
[OK]    mremap to 0x0x7ffffffffffff000 failed

> ---
>  tools/testing/selftests/x86/Makefile     |   2 +-
>  tools/testing/selftests/x86/sysret_rip.c | 196 +++++++++++++++++++++++++++++++
>  2 files changed, 197 insertions(+), 1 deletion(-)
>  create mode 100644 tools/testing/selftests/x86/sysret_rip.c
> 
> diff --git a/tools/testing/selftests/x86/Makefile b/tools/testing/selftests/x86/Makefile
> index 8c1cb423cfe6..25d4067c11e4 100644
> --- a/tools/testing/selftests/x86/Makefile
> +++ b/tools/testing/selftests/x86/Makefile
> @@ -10,7 +10,7 @@ TARGETS_C_BOTHBITS := single_step_syscall sysret_ss_attrs syscall_nt ptrace_sysc
>  TARGETS_C_32BIT_ONLY := entry_from_vm86 syscall_arg_fault test_syscall_vdso unwind_vdso \
>  			test_FCMOV test_FCOMI test_FISTTP \
>  			vdso_restorer
> -TARGETS_C_64BIT_ONLY := fsgsbase
> +TARGETS_C_64BIT_ONLY := fsgsbase sysret_rip
>  
>  TARGETS_C_32BIT_ALL := $(TARGETS_C_BOTHBITS) $(TARGETS_C_32BIT_ONLY)
>  TARGETS_C_64BIT_ALL := $(TARGETS_C_BOTHBITS) $(TARGETS_C_64BIT_ONLY)
> diff --git a/tools/testing/selftests/x86/sysret_rip.c b/tools/testing/selftests/x86/sysret_rip.c
> new file mode 100644
> index 000000000000..3aa7d27c2d42
> --- /dev/null
> +++ b/tools/testing/selftests/x86/sysret_rip.c
> @@ -0,0 +1,196 @@
> +/*
> + * sigreturn.c - tests that x86 avoids Intel SYSRET pitfalls
> + * Copyright (c) 2014-2016 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.
> + */
> +
> +#define _GNU_SOURCE
> +
> +#include <stdlib.h>
> +#include <unistd.h>
> +#include <stdio.h>
> +#include <string.h>
> +#include <inttypes.h>
> +#include <sys/signal.h>
> +#include <sys/ucontext.h>
> +#include <sys/syscall.h>
> +#include <err.h>
> +#include <stddef.h>
> +#include <stdbool.h>
> +#include <setjmp.h>
> +#include <sys/user.h>
> +#include <sys/mman.h>
> +#include <assert.h>
> +
> +
> +asm (
> +	".pushsection \".text\", \"ax\"\n\t"
> +	".balign 4096\n\t"
> +	"test_page: .globl test_page\n\t"
> +	".fill 4094,1,0xcc\n\t"
> +	"test_syscall_insn:\n\t"
> +	"syscall\n\t"
> +	".ifne . - test_page - 4096\n\t"
> +	".error \"test page is not one page long\"\n\t"
> +	".endif\n\t"
> +	".popsection"
> +    );
> +
> +extern const char test_page[];
> +static void const *current_test_page_addr = test_page;
> +
> +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");
> +}
> +
> +/* State used by our signal handlers. */
> +static gregset_t initial_regs;
> +
> +static volatile unsigned long rip;
> +
> +static void sigsegv_for_sigreturn_test(int sig, siginfo_t *info, void *ctx_void)
> +{
> +	ucontext_t *ctx = (ucontext_t*)ctx_void;
> +
> +	if (rip != ctx->uc_mcontext.gregs[REG_RIP]) {
> +		printf("[FAIL]\tRequested RIP=0x%lx but got RIP=0x%lx\n",
> +		       rip, (unsigned long)ctx->uc_mcontext.gregs[REG_RIP]);
> +		fflush(stdout);
> +		_exit(1);
> +	}
> +
> +	memcpy(&ctx->uc_mcontext.gregs, &initial_regs, sizeof(gregset_t));
> +
> +	printf("[OK]\tGot SIGSEGV at RIP=0x%lx\n", rip);
> +}
> +
> +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));
> +
> +	/* Set IP and CX to match so that SYSRET can happen. */
> +	ctx->uc_mcontext.gregs[REG_RIP] = rip;
> +	ctx->uc_mcontext.gregs[REG_RCX] = rip;
> +
> +	/* R11 and EFLAGS should already match. */
> +	assert(ctx->uc_mcontext.gregs[REG_EFL] ==
> +	       ctx->uc_mcontext.gregs[REG_R11]);
> +
> +	sethandler(SIGSEGV, sigsegv_for_sigreturn_test, SA_RESETHAND);
> +
> +	return;
> +}
> +
> +static void test_sigreturn_to(unsigned long ip)
> +{
> +	rip = ip;
> +	printf("[RUN]\tsigreturn to 0x%lx\n", ip);
> +	raise(SIGUSR1);
> +}
> +
> +static jmp_buf jmpbuf;
> +
> +static void sigsegv_for_fallthrough(int sig, siginfo_t *info, void *ctx_void)
> +{
> +	ucontext_t *ctx = (ucontext_t*)ctx_void;
> +
> +	if (rip != ctx->uc_mcontext.gregs[REG_RIP]) {
> +		printf("[FAIL]\tExpected SIGSEGV at 0x%lx but got RIP=0x%lx\n",
> +		       rip, (unsigned long)ctx->uc_mcontext.gregs[REG_RIP]);
> +		fflush(stdout);
> +		_exit(1);
> +	}
> +
> +	siglongjmp(jmpbuf, 1);
> +}
> +
> +static void test_syscall_fallthrough_to(unsigned long ip)
> +{
> +	void *new_address = (void *)(ip - 4096);
> +	void *ret;
> +
> +	printf("[RUN]\tTrying a SYSCALL that falls through to 0x%lx\n", ip);
> +
> +	ret = mremap((void *)current_test_page_addr, 4096, 4096,
> +		     MREMAP_MAYMOVE | MREMAP_FIXED, new_address);
> +	if (ret == MAP_FAILED) {
> +		if (ip <= (1UL << 47) - PAGE_SIZE) {
> +			err(1, "mremap to 0x%p", new_address);

"0x" is redundant for %p.

> +		} else {
> +			printf("[OK]\tmremap to 0x%p failed\n", new_address);

Ditto.

> +			return;
> +		}
> +	}
> +
> +	if (ret != new_address)
> +		errx(1, "mremap malfunctioned: asked for %p but got %p\n",
> +		     new_address, ret);
> +
> +	current_test_page_addr = new_address;
> +	rip = ip;
> +
> +	if (sigsetjmp(jmpbuf, 1) == 0) {
> +		asm volatile ("call *%[syscall_insn]" :: "a" (SYS_getpid),
> +			      [syscall_insn] "rm" (ip - 2));
> +		errx(1, "[FAIL]\tSyscall trampoline returned");
> +	}
> +
> +	printf("[OK]\tWe survived\n");
> +}
> +
> +int main()
> +{
> +	/*
> +	 * When the kernel returns from a slow-path syscall, it will
> +	 * detect whether SYSRET is appropriate.  If it incorrectly
> +	 * thinks that SYSRET is appropriate when RIP is noncanonical,
> +	 * it'll crash on Intel CPUs.
> +	 */
> +	sethandler(SIGUSR1, sigusr1, 0);
> +	for (int i = 47; i < 64; i++)
> +		test_sigreturn_to((1UL<<i));

Redundant parenthesis?

> +
> +	clearhandler(SIGUSR1);
> +
> +	sethandler(SIGSEGV, sigsegv_for_fallthrough, 0);
> +
> +	/* This should execute on all kernels. */
> +	test_syscall_fallthrough_to((1UL << 47) - PAGE_SIZE);
> +
> +	/* Make sure that we didn't screw up the mremap logic. */
> +	test_syscall_fallthrough_to((1UL << 47) - 2*PAGE_SIZE);
> +
> +	/* These are the interesting cases. */
> +	for (int i = 47; i < 64; i++)
> +		test_syscall_fallthrough_to((1UL<<i));

Ditto.

Also, "(1UL << i) - PAGE_SIZE" is interesting too. TASK_SIZE for 5-level
paging would be (1UL << 56) - PAGE_SIZE. I would be better to catch both
corner cases.

> +
> +	return 0;
> +}
> -- 
> 2.9.3
> 

-- 
 Kirill A. Shutemov

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