[POWERPC] Make alignment exception always check exception table

The alignment exception used to only check the exception table for
-EFAULT, not for other errors. That opens an oops window if we can
coerce the kernel into getting an alignment exception for other reasons
in what would normally be a user-protected accessor, which can be done
via some of the futex ops. This fixes it by always checking the
exception tables.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/kernel/traps.c

@@ -843,7 +843,7 @@ void __kprobes program_check_exception(struct pt_regs *regs)
 
 void alignment_exception(struct pt_regs *regs)
 {
-	int fixed = 0;
+	int sig, code, fixed = 0;
 
 	/* we don't implement logging of alignment exceptions */
 	if (!(current->thread.align_ctl & PR_UNALIGN_SIGBUS))
@@ -857,14 +857,16 @@ void alignment_exception(struct pt_regs *regs)
 
 	/* Operand address was bad */
 	if (fixed == -EFAULT) {
-		if (user_mode(regs))
-			_exception(SIGSEGV, regs, SEGV_ACCERR, regs->dar);
-		else
-			/* Search exception table */
-			bad_page_fault(regs, regs->dar, SIGSEGV);
-		return;
+		sig = SIGSEGV;
+		code = SEGV_ACCERR;
+	} else {
+		sig = SIGBUS;
+		code = BUS_ADRALN;
 	}
-	_exception(SIGBUS, regs, BUS_ADRALN, regs->dar);
+	if (user_mode(regs))
+		_exception(sig, regs, code, regs->dar);
+	else
+		bad_page_fault(regs, regs->dar, sig);
 }
 
 void StackOverflow(struct pt_regs *regs)

arch/ppc/kernel/traps.c

@@ -708,7 +708,7 @@ void single_step_exception(struct pt_regs *regs)
 
 void alignment_exception(struct pt_regs *regs)
 {
-	int fixed;
+	int sig, code, fixed = 0;
 
 	fixed = fix_alignment(regs);
 	if (fixed == 1) {
@@ -717,14 +717,16 @@ void alignment_exception(struct pt_regs *regs)
 		return;
 	}
 	if (fixed == -EFAULT) {
-		/* fixed == -EFAULT means the operand address was bad */
-		if (user_mode(regs))
-			_exception(SIGSEGV, regs, SEGV_ACCERR, regs->dar);
-		else
-			bad_page_fault(regs, regs->dar, SIGSEGV);
-		return;
+		sig = SIGSEGV;
+		code = SEGV_ACCERR;
+	} else {
+		sig = SIGBUS;
+		code = BUS_ADRALN;
 	}
-	_exception(SIGBUS, regs, BUS_ADRALN, regs->dar);
+	if (user_mode(regs))
+		_exception(sig, regs, code, regs->dar);
+	else
+		bad_page_fault(regs, regs->dar, sig);
 }
 
 void StackOverflow(struct pt_regs *regs)