Re: [PATCH] x86/CET: Update vfork to prevent child return

["H.J. Lu via Libc-alpha" <libc-alpha@sourceware.org>]

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On Wed, Sep 16, 2020 at 5:46 PM Florian Weimer <fw@deneb.enyo.de> wrote:
>
> * H. J. Lu via Libc-alpha:
>
> > 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
> > returning to caller of vfork's caller.  With shadow stack enabled, we
> > can introduce mismatched shadow stack in child of vfork.  When the child
> > returns from the function in which vfork was called, mismatched shadow
> > stack will trigger SIGSEGV.
> > ---
> >  sysdeps/unix/sysv/linux/i386/vfork.S          |  6 +++
> >  sysdeps/unix/sysv/linux/x86/Makefile          |  5 ++
> >  sysdeps/unix/sysv/linux/x86/tst-cet-vfork-1.c | 54 +++++++++++++++++++
> >  sysdeps/unix/sysv/linux/x86_64/vfork.S        |  6 +++
> >  4 files changed, 71 insertions(+)
> >  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..e54fdb7e4c 100644
> > --- a/sysdeps/unix/sysv/linux/i386/vfork.S
> > +++ b/sysdeps/unix/sysv/linux/i386/vfork.S
> > @@ -91,6 +91,12 @@ ENTRY (__vfork)
> >       /* Normal return if shadow stack isn't in use.  */
> >       je      L(no_shstk)
> >
> > +     testl   %eax, %eax
> > +     jnz     2f
> > +     /* NB: Jump back to caller directly with mismatched shadow stack
> > +        to prevent child return.  */
> > +     jmp     *%ecx
> > +2:
>
> Doesn't the jmp need a notrack prefix?  Or does GCC generate special
> code for returns_twice functions?

The notrack prefix isn't needed ince GCC has

     /* ECF_RETURNS_TWICE is safe even for -ffreestanding.  */
      if (! strcmp (tname, "setjmp")
          || ! strcmp (tname, "sigsetjmp")
          || ! strcmp (name, "savectx")
          || ! strcmp (name, "vfork")
          || ! strcmp (name, "getcontext"))
        flags |= ECF_RETURNS_TWICE;

> The comment should say that the *function* calling vfork cannot return
> in the subprocess.

Fixed.

> > diff --git a/sysdeps/unix/sysv/linux/x86/Makefile b/sysdeps/unix/sysv/linux/x86/Makefile
> > index 920edd8948..f3fae85c1e 100644
> > --- a/sysdeps/unix/sysv/linux/x86/Makefile
> > +++ b/sysdeps/unix/sysv/linux/x86/Makefile
> > @@ -47,6 +47,11 @@ $(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
>
> Does -mshstk alter the ISA?  Then I think you can't test for the
> presence of support if you build the whole translation unit with
> -mshstk.

The only thing -mshstk does is to enable _get_ssp ().

> > 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..9ca148e857
> > --- /dev/null
> > +++ b/sysdeps/unix/sysv/linux/x86/tst-cet-vfork-1.c
> > @@ -0,0 +1,54 @@
> > +/* Verify that child of vfork can't return with shadow stack.
>
> Likewise: It's the vfork-calling function that must not return.

Fixed.

> > +__attribute__ ((noclone, noinline))
> > +static pid_t
> > +do_test_1 (void)
> > +{
> > +  pid_t pid;
> > +
> > +  pid = vfork ();
> > +  if (pid == 0)
> > +    {
> > +      /* Child return should trigger SIGSEGV.  */
> > +      return 0;
> > +    }
> > +  _exit (EXIT_SUCCESS);
> > +
> > +  return pid;
>
> The return statement immediately above is unreachable.

Fixed.

> > +static int
> > +do_test (void)
> > +{
> > +  /* NB: This test should trigger SIGSEGV with shadow stack enabled.  */
> > +  if (_get_ssp () == 0)
> > +    return EXIT_UNSUPPORTED;
> > +  return do_test_1 () ? EXIT_SUCCESS : EXIT_FAILURE;
> > +}
> > +
> > +#define EXPECTED_SIGNAL (_get_ssp () == 0 ? 0 : SIGSEGV)
> > +#include <support/test-driver.c>
>
> I'm surprised EXPECTED_SIGNAL works here.  I would expect that the
> original test process would have to wait using xwaitpid and check for
> the signal in the subprocess.
>
> I think it would also be good to add a check that the subprocess
> actually returned from vfork without crashing, say using a pipe and a
> write before the return statement.

Fixed.

> > diff --git a/sysdeps/unix/sysv/linux/x86_64/vfork.S b/sysdeps/unix/sysv/linux/x86_64/vfork.S
> > index 776d2fc610..5dd5cb714c 100644
> > --- a/sysdeps/unix/sysv/linux/x86_64/vfork.S
> > +++ b/sysdeps/unix/sysv/linux/x86_64/vfork.S
> > @@ -71,6 +71,12 @@ ENTRY (__vfork)
> >       /* Normal return if shadow stack isn't in use.  */
> >       je      L(no_shstk)
> >
> > +     testl   %eax, %eax
> > +     jnz     2f
> > +     /* NB: Jump back to caller directly with mismatched shadow stack
> > +        to prevent child return.  */
> > +     jmp     *%rdi
> > +2:
> >       /* Pop return address from shadow stack and jump back to caller
> >          directly.  */
> >       movl    $1, %esi
>
> See above.

Here is the updated patch to use indirect branch only for child.

-- 
H.J.

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From 4320a99da3055aed813fe09e240d7ce291955cd6 Mon Sep 17 00:00:00 2001
From: "H.J. Lu" <hjl.tools@gmail.com>
Date: Wed, 16 Sep 2020 16:00:14 -0700
Subject: [PATCH] x86/CET: Update vfork to prevent child return

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.
---
 sysdeps/unix/sysv/linux/i386/vfork.S          | 54 ++----------
 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        | 35 ++------
 4 files changed, 111 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..1bee49c2ca 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 +55,18 @@ 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 directly with mismatched
+	   shadow stack to prevent child from return of the vfork-calling
+	   function.  */
+	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 920edd8948..f3fae85c1e 100644
--- a/sysdeps/unix/sysv/linux/x86/Makefile
+++ b/sysdeps/unix/sysv/linux/x86/Makefile
@@ -47,6 +47,11 @@ $(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..16322031fc 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 +52,18 @@ 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 directly with mismatched
+	   shadow stack to prevent child from return of the vfork-calling
+	   function.  */
+	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