staging: binder: add vm_fault handler

An issue was observed when a userspace task exits.
The page which hits error here is the zero page.
In binder mmap, the whole of vma is not mapped.
On a task crash, when debuggerd reads the binder regions,
the unmapped areas fall to do_anonymous_page in handle_pte_fault,
due to the absence of a vm_fault handler. This results in
zero page being mapped. Later in zap_pte_range, vm_normal_page
returns zero page in the case of VM_MIXEDMAP and it results in the
error.

BUG: Bad page map in process mediaserver  pte:9dff379f pmd:9bfbd831
page:c0ed8e60 count:1 mapcount:-1 mapping:  (null) index:0x0
page flags: 0x404(referenced|reserved)
addr:40c3f000 vm_flags:10220051 anon_vma:  (null) mapping:d9fe0764 index:fd
vma->vm_ops->fault:   (null)
vma->vm_file->f_op->mmap: binder_mmap+0x0/0x274
CPU: 0 PID: 1463 Comm: mediaserver Tainted: G        W    3.10.17+ #1
[<c001549c>] (unwind_backtrace+0x0/0x11c) from [<c001200c>] (show_stack+0x10/0x14)
[<c001200c>] (show_stack+0x10/0x14) from [<c0103d78>] (print_bad_pte+0x158/0x190)
[<c0103d78>] (print_bad_pte+0x158/0x190) from [<c01055f0>] (unmap_single_vma+0x2e4/0x598)
[<c01055f0>] (unmap_single_vma+0x2e4/0x598) from [<c010618c>] (unmap_vmas+0x34/0x50)
[<c010618c>] (unmap_vmas+0x34/0x50) from [<c010a9e4>] (exit_mmap+0xc8/0x1e8)
[<c010a9e4>] (exit_mmap+0xc8/0x1e8) from [<c00520f0>] (mmput+0x54/0xd0)
[<c00520f0>] (mmput+0x54/0xd0) from [<c005972c>] (do_exit+0x360/0x990)
[<c005972c>] (do_exit+0x360/0x990) from [<c0059ef0>] (do_group_exit+0x84/0xc0)
[<c0059ef0>] (do_group_exit+0x84/0xc0) from [<c0066de0>] (get_signal_to_deliver+0x4d4/0x548)
[<c0066de0>] (get_signal_to_deliver+0x4d4/0x548) from [<c0011500>] (do_signal+0xa8/0x3b8)

Add a vm_fault handler which returns VM_FAULT_SIGBUS, and prevents the
wrong fallback to do_anonymous_page.
Signed-off-by: Vinayak Menon <vinayakm.list@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/android/binder.c

@@ -2768,9 +2768,15 @@ static void binder_vma_close(struct vm_area_struct *vma)
 	binder_defer_work(proc, BINDER_DEFERRED_PUT_FILES);
 }
 
+static int binder_vm_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
+{
+	return VM_FAULT_SIGBUS;
+}
+
 static struct vm_operations_struct binder_vm_ops = {
 	.open = binder_vma_open,
 	.close = binder_vma_close,
+	.fault = binder_vm_fault,
 };
 
 static int binder_mmap(struct file *filp, struct vm_area_struct *vma)