[PATCH] powerpc: Don't let userspace trigger a kernel WARN_ON

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

In commit 2865d08dd9ea876524652f3900b4b3b9c8b22e77
"powerpc/mm: Move the DSISR_PROTFAULT sanity check",
I completely missed the fact that an attempt at reading
kernel memory *will* trip the warning.

So this partially reverts it. We keep the test in a
helper to keep the code clean, but we move it back to
after the VMA has been found.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
---

diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
index c01d627e687a..20384445ca44 100644
--- a/arch/powerpc/mm/fault.c
+++ b/arch/powerpc/mm/fault.c
@@ -416,9 +416,6 @@ static int __do_page_fault(struct pt_regs *regs, unsigned long address,
 		return SIGBUS;
 	}
 
-	/* Additional sanity check(s) */
-	sanity_check_fault(is_write, error_code);
-
 	/*
 	 * The kernel should never take an execute fault nor should it
 	 * take a page fault to a kernel address.
@@ -511,6 +508,10 @@ static int __do_page_fault(struct pt_regs *regs, unsigned long address,
 		return bad_area(regs, address);
 
 good_area:
+	/* Additional sanity check(s) */
+	sanity_check_fault(is_write, error_code);
+
+	/* Check for VMA access permissions */
 	if (unlikely(access_error(is_write, is_exec, vma)))
 		return bad_access(regs, address);