x86: don't send SIGBUS for kernel page faults

It's wrong for several reasons, but the most direct one is that the
fault may be for the stack accesses to set up a previous SIGBUS.  When
we have a kernel exception, the kernel exception handler does all the
fixups, not some user-level signal handler.

Even apart from the nested SIGBUS issue, it's also wrong to give out
kernel fault addresses in the signal handler info block, or to send a
SIGBUS when a system call already returns EFAULT.
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

arch/x86/mm/fault.c

@@ -802,8 +802,10 @@ do_sigbus(struct pt_regs *regs, unsigned long error_code, unsigned long address,
 	up_read(&mm->mmap_sem);
 
 	/* Kernel mode? Handle exceptions or die: */
-	if (!(error_code & PF_USER))
+	if (!(error_code & PF_USER)) {
 		no_context(regs, error_code, address);
+		return;
+	}
 
 	/* User-space => ok to do another page fault: */
 	if (is_prefetch(regs, error_code, address))