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" <[email protected]>
> Signed-off-by: Andy Lutomirski <[email protected]>
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
