x86-32: Fix endless loop when processing signals for kernel tasks

The problem occurs on !CONFIG_VM86 kernels [1] when a kernel-mode task
returns from a system call with a pending signal.

A real-life scenario is a child of 'khelper' returning from a failed
kernel_execve() in ____call_usermodehelper() [ kernel/kmod.c ].
kernel_execve() fails due to a pending SIGKILL, which is the result of
"kill -9 -1" (at least, busybox's init does it upon reboot).

The loop is as follows:

* syscall_exit_work:
 - work_pending:            // start_of_the_loop
 - work_notify_sig:
   - do_notify_resume()
     - do_signal()
       - if (!user_mode(regs)) return;
 - resume_userspace         // TIF_SIGPENDING is still set
 - work_pending             // so we call work_pending => goto
                            // start_of_the_loop

More information can be found in another LKML thread:
http://www.serverphorums.com/read.php?12,457826

[1] the problem was also seen on MIPS.

Signed-off-by: Dmitry Adamushko <dmitry.adamushko@gmail.com>
Link: http://lkml.kernel.org/r/1332448765.2299.68.camel@dimm
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Roland McGrath <roland@hack.frob.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>

arch/x86/kernel/entry_32.S

@@ -98,12 +98,6 @@
 #endif
 .endm
 
-#ifdef CONFIG_VM86
-#define resume_userspace_sig	check_userspace
-#else
-#define resume_userspace_sig	resume_userspace
-#endif
-
 /*
  * User gs save/restore
  *
@@ -327,10 +321,19 @@ ret_from_exception:
 	preempt_stop(CLBR_ANY)
 ret_from_intr:
 	GET_THREAD_INFO(%ebp)
-check_userspace:
+resume_userspace_sig:
+#ifdef CONFIG_VM86
 	movl PT_EFLAGS(%esp), %eax	# mix EFLAGS and CS
 	movb PT_CS(%esp), %al
 	andl $(X86_EFLAGS_VM | SEGMENT_RPL_MASK), %eax
+#else
+	/*
+	 * We can be coming here from a syscall done in the kernel space,
+	 * e.g. a failed kernel_execve().
+	 */
+	movl PT_CS(%esp), %eax
+	andl $SEGMENT_RPL_MASK, %eax
+#endif
 	cmpl $USER_RPL, %eax
 	jb resume_kernel		# not returning to v8086 or userspace