x86: don't send SIGBUS for kernel page faults

commit 96054569 upstream.

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: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

arch/x86/mm/fault.c

@@ -801,8 +801,10 @@ 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))