[PATCH v2] binder: fix proc->files use-after-free

[Todd Kjos <tkjos@android.com>]

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 always use get_files_struct() to obtain struct_files
so that the refcount on the files_struct is used to prevent
a premature free. proc->files is removed since we get it every
time.

Signed-off-by: Todd Kjos <tkjos@google.com>
---
 drivers/android/binder.c | 63 +++++++++++++++++++++++-------------------------
 1 file changed, 30 insertions(+), 33 deletions(-)

diff --git a/drivers/android/binder.c b/drivers/android/binder.c
index fddf76ef5bd6..ea07b35fb533 100644
--- a/drivers/android/binder.c
+++ b/drivers/android/binder.c
@@ -458,9 +458,8 @@ struct binder_ref {
 };
 
 enum binder_deferred_state {
-	BINDER_DEFERRED_PUT_FILES    = 0x01,
-	BINDER_DEFERRED_FLUSH        = 0x02,
-	BINDER_DEFERRED_RELEASE      = 0x04,
+	BINDER_DEFERRED_FLUSH        = 0x01,
+	BINDER_DEFERRED_RELEASE      = 0x02,
 };
 
 /**
@@ -481,8 +480,6 @@ enum binder_deferred_state {
  *                        (invariant after initialized)
  * @tsk                   task_struct for group_leader of process
  *                        (invariant after initialized)
- * @files                 files_struct for process
- *                        (invariant after initialized)
  * @deferred_work_node:   element for binder_deferred_list
  *                        (protected by binder_deferred_lock)
  * @deferred_work:        bitmap of deferred work to perform
@@ -529,7 +526,6 @@ struct binder_proc {
 	struct list_head waiting_threads;
 	int pid;
 	struct task_struct *tsk;
-	struct files_struct *files;
 	struct hlist_node deferred_work_node;
 	int deferred_work;
 	bool is_dead;
@@ -875,22 +871,34 @@ static void binder_free_thread(struct binder_thread *thread);
 static void binder_free_proc(struct binder_proc *proc);
 static void binder_inc_node_tmpref_ilocked(struct binder_node *node);
 
+static struct files_struct *binder_get_files_struct(struct binder_proc *proc)
+{
+	return get_files_struct(proc->tsk);
+}
+
 static int task_get_unused_fd_flags(struct binder_proc *proc, int flags)
 {
-	struct files_struct *files = proc->files;
+	struct files_struct *files;
 	unsigned long rlim_cur;
 	unsigned long irqs;
+	int ret;
 
+	files = binder_get_files_struct(proc);
 	if (files == NULL)
 		return -ESRCH;
 
-	if (!lock_task_sighand(proc->tsk, &irqs))
-		return -EMFILE;
+	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(files, 0, rlim_cur, flags);
+err:
+	put_files_struct(files);
+	return ret;
 }
 
 /*
@@ -899,8 +907,12 @@ 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)
 {
-	if (proc->files)
-		__fd_install(proc->files, fd, file);
+	struct files_struct *files = binder_get_files_struct(proc);
+
+	if (files) {
+		__fd_install(files, fd, file);
+		put_files_struct(files);
+	}
 }
 
 /*
@@ -908,18 +920,20 @@ static void task_fd_install(
  */
 static long task_close_fd(struct binder_proc *proc, unsigned int fd)
 {
+	struct files_struct *files = binder_get_files_struct(proc);
 	int retval;
 
-	if (proc->files == NULL)
+	if (files == NULL)
 		return -ESRCH;
 
-	retval = __close_fd(proc->files, fd);
+	retval = __close_fd(files, fd);
 	/* can't restart close syscall because file table entry was cleared */
 	if (unlikely(retval == -ERESTARTSYS ||
 		     retval == -ERESTARTNOINTR ||
 		     retval == -ERESTARTNOHAND ||
 		     retval == -ERESTART_RESTARTBLOCK))
 		retval = -EINTR;
+	put_files_struct(files);
 
 	return retval;
 }
@@ -4561,7 +4575,6 @@ static void binder_vma_close(struct vm_area_struct *vma)
 		     (vma->vm_end - vma->vm_start) / SZ_1K, vma->vm_flags,
 		     (unsigned long)pgprot_val(vma->vm_page_prot));
 	binder_alloc_vma_close(&proc->alloc);
-	binder_defer_work(proc, BINDER_DEFERRED_PUT_FILES);
 }
 
 static int binder_vm_fault(struct vm_fault *vmf)
@@ -4603,10 +4616,8 @@ static int binder_mmap(struct file *filp, struct vm_area_struct *vma)
 	vma->vm_private_data = proc;
 
 	ret = binder_alloc_mmap_handler(&proc->alloc, vma);
-	if (ret)
-		return ret;
-	proc->files = get_files_struct(current);
-	return 0;
+
+	return ret;
 
 err_bad_arg:
 	pr_err("binder_mmap: %d %lx-%lx %s failed %d\n",
@@ -4778,8 +4789,6 @@ static void binder_deferred_release(struct binder_proc *proc)
 	struct rb_node *n;
 	int threads, nodes, incoming_refs, outgoing_refs, active_transactions;
 
-	BUG_ON(proc->files);
-
 	mutex_lock(&binder_procs_lock);
 	hlist_del(&proc->proc_node);
 	mutex_unlock(&binder_procs_lock);
@@ -4861,8 +4870,6 @@ static void binder_deferred_release(struct binder_proc *proc)
 static void binder_deferred_func(struct work_struct *work)
 {
 	struct binder_proc *proc;
-	struct files_struct *files;
-
 	int defer;
 
 	do {
@@ -4879,21 +4886,11 @@ static void binder_deferred_func(struct work_struct *work)
 		}
 		mutex_unlock(&binder_deferred_lock);
 
-		files = NULL;
-		if (defer & BINDER_DEFERRED_PUT_FILES) {
-			files = proc->files;
-			if (files)
-				proc->files = NULL;
-		}
-
 		if (defer & BINDER_DEFERRED_FLUSH)
 			binder_deferred_flush(proc);
 
 		if (defer & BINDER_DEFERRED_RELEASE)
 			binder_deferred_release(proc); /* frees proc */
-
-		if (files)
-			put_files_struct(files);
 	} while (proc);
 }
 static DECLARE_WORK(binder_deferred_work, binder_deferred_func);
-- 
2.15.0.448.gf294e3d99a-goog