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.

1 files changed, 10 insertions, 45 deletions

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