[PATCH] x86/CET: Update vfork to prevent child return
H.J. Lu
hjl.tools@gmail.com
Wed Oct 14 13:30:57 GMT 2020
On Tue, Oct 13, 2020 at 10:51 AM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> Child of vfork should either call _exit or one of the exec family of
> functions. But normally there is nothing to prevent child of vfork from
> return of the vfork-calling function. Simpilfy x86 vfork when shadow
> stack is in use to introduce mismatched shadow stack in child of vfork
> to trigger SIGSEGV when the child returns from the function in which
> vfork was called.
I will check it in tomorrow if there are no objections.
> ---
> sysdeps/unix/sysv/linux/i386/vfork.S | 55 +++---------
> sysdeps/unix/sysv/linux/x86/Makefile | 5 ++
> sysdeps/unix/sysv/linux/x86/tst-cet-vfork-1.c | 88 +++++++++++++++++++
> sysdeps/unix/sysv/linux/x86_64/vfork.S | 36 +++-----
> 4 files changed, 113 insertions(+), 71 deletions(-)
> create mode 100644 sysdeps/unix/sysv/linux/x86/tst-cet-vfork-1.c
>
> diff --git a/sysdeps/unix/sysv/linux/i386/vfork.S b/sysdeps/unix/sysv/linux/i386/vfork.S
> index ceb41db0bd..91277a639f 100644
> --- a/sysdeps/unix/sysv/linux/i386/vfork.S
> +++ b/sysdeps/unix/sysv/linux/i386/vfork.S
> @@ -21,39 +21,6 @@
> #include <bits/errno.h>
> #include <tcb-offsets.h>
>
> -#if SHSTK_ENABLED
> -/* The shadow stack prevents us from pushing the saved return PC onto
> - the stack and returning normally. Instead we pop the shadow stack
> - and return directly. This is the safest way to return and ensures
> - any stack manipulations done by the vfork'd child doesn't cause the
> - parent to terminate when CET is enabled. */
> -# undef SYSCALL_ERROR_HANDLER
> -# ifdef PIC
> -# define SYSCALL_ERROR_HANDLER \
> -0: \
> - calll .L1; \
> -.L1: \
> - popl %edx; \
> -.L2: \
> - addl $_GLOBAL_OFFSET_TABLE_ + (.L2 - .L1), %edx; \
> - movl __libc_errno@gotntpoff(%edx), %edx; \
> - negl %eax; \
> - movl %eax, %gs:(%edx); \
> - orl $-1, %eax; \
> - jmp 1b;
> -# else
> -# define SYSCALL_ERROR_HANDLER \
> -0: \
> - movl __libc_errno@indntpoff, %edx; \
> - negl %eax; \
> - movl %eax, %gs:(%edx); \
> - orl $-1, %eax; \
> - jmp 1b;
> -# endif
> -# undef SYSCALL_ERROR_LABEL
> -# define SYSCALL_ERROR_LABEL 0f
> -#endif
> -
> /* Clone the calling process, but without copying the whole address space.
> The calling process is suspended until the new process exits or is
> replaced by a call to `execve'. Return -1 for errors, 0 to the new process,
> @@ -70,20 +37,17 @@ ENTRY (__vfork)
> movl $SYS_ify (vfork), %eax
> int $0x80
>
> -#if !SHSTK_ENABLED
> /* Jump to the return PC. Don't jump directly since this
> disturbs the branch target cache. Instead push the return
> address back on the stack. */
> pushl %ecx
> cfi_adjust_cfa_offset (4)
> -#endif
>
> cmpl $-4095, %eax
> /* Branch forward if it failed. */
> jae SYSCALL_ERROR_LABEL
>
> #if SHSTK_ENABLED
> -1:
> /* Check if shadow stack is in use. */
> xorl %edx, %edx
> rdsspd %edx
> @@ -91,18 +55,19 @@ ENTRY (__vfork)
> /* Normal return if shadow stack isn't in use. */
> je L(no_shstk)
>
> - /* Pop return address from shadow stack and jump back to caller
> - directly. */
> - movl $1, %edx
> - incsspd %edx
> + testl %eax, %eax
> + /* In parent, normal return. */
> + jnz L(no_shstk)
> +
> + /* NB: In child, jump back to caller via indirect branch without
> + popping shadow stack which is shared with parent. Keep shadow
> + stack mismatched so that child returns in the vfork-calling
> + function will trigger SIGSEGV. */
> + popl %ecx
> + cfi_adjust_cfa_offset (-4)
> jmp *%ecx
>
> L(no_shstk):
> - /* Jump to the return PC. Don't jump directly since this
> - disturbs the branch target cache. Instead push the return
> - address back on the stack. */
> - pushl %ecx
> - cfi_adjust_cfa_offset (4)
> #endif
>
> ret
> diff --git a/sysdeps/unix/sysv/linux/x86/Makefile b/sysdeps/unix/sysv/linux/x86/Makefile
> index 50fd018fa3..6bfd6bec49 100644
> --- a/sysdeps/unix/sysv/linux/x86/Makefile
> +++ b/sysdeps/unix/sysv/linux/x86/Makefile
> @@ -40,6 +40,11 @@ $(objpfx)tst-cet-property-2.out: $(objpfx)tst-cet-property-2 \
> $(evaluate-test)
> endif
>
> +ifeq ($(subdir),posix)
> +tests += tst-cet-vfork-1
> +CFLAGS-tst-cet-vfork-1.c += -mshstk
> +endif
> +
> ifeq ($(subdir),stdlib)
> tests += tst-cet-setcontext-1
> CFLAGS-tst-cet-setcontext-1.c += -mshstk
> diff --git a/sysdeps/unix/sysv/linux/x86/tst-cet-vfork-1.c b/sysdeps/unix/sysv/linux/x86/tst-cet-vfork-1.c
> new file mode 100644
> index 0000000000..5b9fc8c170
> --- /dev/null
> +++ b/sysdeps/unix/sysv/linux/x86/tst-cet-vfork-1.c
> @@ -0,0 +1,88 @@
> +/* Verify that child of the vfork-calling function can't return when
> + shadow stack is in use.
> + Copyright (C) 2020 Free Software Foundation, Inc.
> + This file is part of the GNU C Library.
> +
> + The GNU C Library is free software; you can redistribute it and/or
> + modify it under the terms of the GNU Lesser General Public
> + License as published by the Free Software Foundation; either
> + version 2.1 of the License, or (at your option) any later version.
> +
> + The GNU C Library is distributed in the hope that it will be useful,
> + but WITHOUT ANY WARRANTY; without even the implied warranty of
> + MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
> + Lesser General Public License for more details.
> +
> + You should have received a copy of the GNU Lesser General Public
> + License along with the GNU C Library; if not, see
> + <https://www.gnu.org/licenses/>. */
> +
> +#include <stdio.h>
> +#include <stdlib.h>
> +#include <unistd.h>
> +#include <errno.h>
> +#include <sys/types.h>
> +#include <sys/wait.h>
> +#include <x86intrin.h>
> +#include <support/test-driver.h>
> +#include <support/xsignal.h>
> +#include <support/xunistd.h>
> +
> +__attribute__ ((noclone, noinline))
> +static void
> +do_test_1 (void)
> +{
> + pid_t p1;
> + int fd[2];
> +
> + if (pipe (fd) == -1)
> + {
> + puts ("pipe failed");
> + _exit (EXIT_FAILURE);
> + }
> +
> + if ((p1 = vfork ()) == 0)
> + {
> + pid_t p = getpid ();
> + TEMP_FAILURE_RETRY (write (fd[1], &p, sizeof (p)));
> + /* Child return should trigger SIGSEGV. */
> + return;
> + }
> + else if (p1 == -1)
> + {
> + puts ("vfork failed");
> + _exit (EXIT_FAILURE);
> + }
> +
> + pid_t p2 = 0;
> + if (TEMP_FAILURE_RETRY (read (fd[0], &p2, sizeof (pid_t)))
> + != sizeof (pid_t))
> + puts ("pipd read failed");
> + else
> + {
> + int r;
> + if (TEMP_FAILURE_RETRY (waitpid (p1, &r, 0)) != p1)
> + puts ("waitpid failed");
> + else if (r != 0)
> + puts ("pip write in child failed");
> + }
> +
> + /* Parent exits immediately so that parent returns without triggering
> + SIGSEGV when shadow stack isn't in use. */
> + _exit (EXIT_FAILURE);
> +}
> +
> +static int
> +do_test (void)
> +{
> + /* NB: This test should trigger SIGSEGV with shadow stack enabled. */
> + if (_get_ssp () == 0)
> + return EXIT_UNSUPPORTED;
> + do_test_1 ();
> + /* Child exits immediately so that child returns without triggering
> + SIGSEGV when shadow stack isn't in use. */
> + _exit (EXIT_FAILURE);
> +}
> +
> +#define EXPECTED_SIGNAL (_get_ssp () == 0 ? 0 : SIGSEGV)
> +#include <support/test-driver.c>
> diff --git a/sysdeps/unix/sysv/linux/x86_64/vfork.S b/sysdeps/unix/sysv/linux/x86_64/vfork.S
> index 776d2fc610..613ff7e846 100644
> --- a/sysdeps/unix/sysv/linux/x86_64/vfork.S
> +++ b/sysdeps/unix/sysv/linux/x86_64/vfork.S
> @@ -20,22 +20,6 @@
> #include <bits/errno.h>
> #include <tcb-offsets.h>
>
> -#if SHSTK_ENABLED
> -/* The shadow stack prevents us from pushing the saved return PC onto
> - the stack and returning normally. Instead we pop the shadow stack
> - and return directly. This is the safest way to return and ensures
> - any stack manipulations done by the vfork'd child doesn't cause the
> - parent to terminate when CET is enabled. */
> -# undef SYSCALL_ERROR_HANDLER
> -# define SYSCALL_ERROR_HANDLER \
> -0: \
> - SYSCALL_SET_ERRNO; \
> - or $-1, %RAX_LP; \
> - jmp 1b;
> -# undef SYSCALL_ERROR_LABEL
> -# define SYSCALL_ERROR_LABEL 0f
> -#endif
> -
> /* Clone the calling process, but without copying the whole address space.
> The calling process is suspended until the new process exits or is
> replaced by a call to `execve'. Return -1 for errors, 0 to the new process,
> @@ -53,17 +37,14 @@ ENTRY (__vfork)
> movl $SYS_ify (vfork), %eax
> syscall
>
> -#if !SHSTK_ENABLED
> /* Push back the return PC. */
> pushq %rdi
> cfi_adjust_cfa_offset(8)
> -#endif
>
> cmpl $-4095, %eax
> jae SYSCALL_ERROR_LABEL /* Branch forward if it failed. */
>
> #if SHSTK_ENABLED
> -1:
> /* Check if shadow stack is in use. */
> xorl %esi, %esi
> rdsspq %rsi
> @@ -71,16 +52,19 @@ ENTRY (__vfork)
> /* Normal return if shadow stack isn't in use. */
> je L(no_shstk)
>
> - /* Pop return address from shadow stack and jump back to caller
> - directly. */
> - movl $1, %esi
> - incsspq %rsi
> + testl %eax, %eax
> + /* In parent, normal return. */
> + jnz L(no_shstk)
> +
> + /* NB: In child, jump back to caller via indirect branch without
> + popping shadow stack which is shared with parent. Keep shadow
> + stack mismatched so that child returns in the vfork-calling
> + function will trigger SIGSEGV. */
> + popq %rdi
> + cfi_adjust_cfa_offset(-8)
> jmp *%rdi
>
> L(no_shstk):
> - /* Push back the return PC. */
> - pushq %rdi
> - cfi_adjust_cfa_offset(8)
> #endif
>
> /* Normal return. */
> --
> 2.26.2
>
--
H.J.
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