ARM: prevent VM_GROWSDOWN mmaps extending below FIRST_USER_ADDRESS

commit 9b61a4d1 upstream.
Reported-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

arch/arm/mm/fault.c

@@ -267,7 +267,9 @@ __do_page_fault(struct mm_struct *mm, unsigned long addr, unsigned int fsr,
 	return fault;
 
 check_stack:
-	if (vma->vm_flags & VM_GROWSDOWN && !expand_stack(vma, addr))
+	/* Don't allow expansion below FIRST_USER_ADDRESS */
+	if (vma->vm_flags & VM_GROWSDOWN &&
+	    addr >= FIRST_USER_ADDRESS && !expand_stack(vma, addr))
 		goto good_area;
 out:
 	return fault;