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

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

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          | 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