binder: fix proc->files use-after-free

commit 7f3dc008 upstream.

proc->files cleanup is initiated by binder_vma_close. Therefore
a reference on the binder_proc is not enough to prevent the
files_struct from being released while the binder_proc still has
a reference. This can lead to an attempt to dereference the
stale pointer obtained from proc->files prior to proc->files
cleanup. This has been seen once in task_get_unused_fd_flags()
when __alloc_fd() is called with a stale "files".

The fix is to protect proc->files with a mutex to prevent cleanup
while in use.
Signed-off-by: Todd Kjos <tkjos@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/android/binder.c

@@ -302,6 +302,7 @@ struct binder_proc {
 	struct mm_struct *vma_vm_mm;
 	struct task_struct *tsk;
 	struct files_struct *files;
+	struct mutex files_lock;
 	struct hlist_node deferred_work_node;
 	int deferred_work;
 	void *buffer;
@@ -375,20 +376,26 @@ binder_defer_work(struct binder_proc *proc, enum binder_deferred_state defer);
 
 static int task_get_unused_fd_flags(struct binder_proc *proc, int flags)
 {
-	struct files_struct *files = proc->files;
 	unsigned long rlim_cur;
 	unsigned long irqs;
+	int ret;
 
-	if (files == NULL)
-		return -ESRCH;
-
-	if (!lock_task_sighand(proc->tsk, &irqs))
-		return -EMFILE;
-
+	mutex_lock(&proc->files_lock);
+	if (proc->files == NULL) {
+		ret = -ESRCH;
+		goto err;
+	}
+	if (!lock_task_sighand(proc->tsk, &irqs)) {
+		ret = -EMFILE;
+		goto err;
+	}
 	rlim_cur = task_rlimit(proc->tsk, RLIMIT_NOFILE);
 	unlock_task_sighand(proc->tsk, &irqs);
 
-	return __alloc_fd(files, 0, rlim_cur, flags);
+	ret = __alloc_fd(proc->files, 0, rlim_cur, flags);
+err:
+	mutex_unlock(&proc->files_lock);
+	return ret;
 }
 
 /*
@@ -397,8 +404,10 @@ static int task_get_unused_fd_flags(struct binder_proc *proc, int flags)
 static void task_fd_install(
 	struct binder_proc *proc, unsigned int fd, struct file *file)
 {
+	mutex_lock(&proc->files_lock);
 	if (proc->files)
 		__fd_install(proc->files, fd, file);
+	mutex_unlock(&proc->files_lock);
 }
 
 /*
@@ -408,9 +417,11 @@ static long task_close_fd(struct binder_proc *proc, unsigned int fd)
 {
 	int retval;
 
-	if (proc->files == NULL)
-		return -ESRCH;
-
+	mutex_lock(&proc->files_lock);
+	if (proc->files == NULL) {
+		retval = -ESRCH;
+		goto err;
+	}
 	retval = __close_fd(proc->files, fd);
 	/* can't restart close syscall because file table entry was cleared */
 	if (unlikely(retval == -ERESTARTSYS ||
@@ -418,7 +429,8 @@ static long task_close_fd(struct binder_proc *proc, unsigned int fd)
 		     retval == -ERESTARTNOHAND ||
 		     retval == -ERESTART_RESTARTBLOCK))
 		retval = -EINTR;
-
+err:
+	mutex_unlock(&proc->files_lock);
 	return retval;
 }
 
@@ -2946,7 +2958,9 @@ static int binder_mmap(struct file *filp, struct vm_area_struct *vma)
 	binder_insert_free_buffer(proc, buffer);
 	proc->free_async_space = proc->buffer_size / 2;
 	barrier();
+	mutex_lock(&proc->files_lock);
 	proc->files = get_files_struct(current);
+	mutex_unlock(&proc->files_lock);
 	proc->vma = vma;
 	proc->vma_vm_mm = vma->vm_mm;
 
@@ -2982,6 +2996,7 @@ static int binder_open(struct inode *nodp, struct file *filp)
 		return -ENOMEM;
 	get_task_struct(current->group_leader);
 	proc->tsk = current->group_leader;
+	mutex_init(&proc->files_lock);
 	INIT_LIST_HEAD(&proc->todo);
 	init_waitqueue_head(&proc->wait);
 	proc->default_priority = task_nice(current);
@@ -3220,9 +3235,11 @@ static void binder_deferred_func(struct work_struct *work)
 
 		files = NULL;
 		if (defer & BINDER_DEFERRED_PUT_FILES) {
+			mutex_lock(&proc->files_lock);
 			files = proc->files;
 			if (files)
 				proc->files = NULL;
+			mutex_unlock(&proc->files_lock);
 		}
 
 		if (defer & BINDER_DEFERRED_FLUSH)