Staging: android: binder: Fix use-after-free bug

binder_update_page_range could read freed memory if the vma of the
selected process was freed right before the check that the vma
belongs to the mm struct it just locked.

If the vm_mm pointer in that freed vma struct had also been rewritten
with a value that matched the locked mm struct, then the code would
proceed and possibly modify the freed vma.

Signed-off-by: Arve Hjønnevåg <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/staging/android/binder.c

···
 	struct rb_root refs_by_node;
 	int pid;
 	struct vm_area_struct *vma;
+	struct mm_struct *vma_vm_mm;
 	struct task_struct *tsk;
 	struct files_struct *files;
 	struct hlist_node deferred_work_node;
···
 	if (mm) {
 		down_write(&mm->mmap_sem);
 		vma = proc->vma;
-		if (vma && mm != vma->vm_mm) {
+		if (vma && mm != proc->vma_vm_mm) {
 			pr_err("binder: %d: vma mm and task mm mismatch\n",
 				proc->pid);
 			vma = NULL;
···
 		     (vma->vm_end - vma->vm_start) / SZ_1K, vma->vm_flags,
 		     (unsigned long)pgprot_val(vma->vm_page_prot));
 	proc->vma = NULL;
+	proc->vma_vm_mm = NULL;
 	binder_defer_work(proc, BINDER_DEFERRED_PUT_FILES);
 }
 
···
 	barrier();
 	proc->files = get_files_struct(proc->tsk);
 	proc->vma = vma;
+	proc->vma_vm_mm = vma->vm_mm;
 
 	/*printk(KERN_INFO "binder_mmap: %d %lx-%lx maps %p\n",
 		 proc->pid, vma->vm_start, vma->vm_end, proc->buffer);*/