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

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>
Cc: stable <stable@vger.kernel.org> # 4.14
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 31 insertions, 13 deletions

diff --git a/drivers/android/binder.c b/drivers/android/binder.c
index bccec9de0533..a7ecfde66b7b 100644
--- a/drivers/android/binder.c
+++ b/drivers/android/binder.c
@@ -482,7 +482,8 @@ enum binder_deferred_state {
  * @tsk                   task_struct for group_leader of process
  *                        (invariant after initialized)
  * @files                 files_struct for process
- *                        (invariant after initialized)
+ *                        (protected by @files_lock)
+ * @files_lock            mutex to protect @files
  * @deferred_work_node:   element for binder_deferred_list
  *                        (protected by binder_deferred_lock)
  * @deferred_work:        bitmap of deferred work to perform
@@ -530,6 +531,7 @@ struct binder_proc {
         int pid;
         struct task_struct *tsk;
         struct files_struct *files;
+        struct mutex files_lock;
         struct hlist_node deferred_work_node;
         int deferred_work;
         bool is_dead;
@@ -877,20 +879,26 @@ static void binder_inc_node_tmpref_ilocked(struct binder_node *node);
 
 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;
 }
 
 /*
@@ -899,8 +907,10 @@ 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);
 }
 
 /*
@@ -910,9 +920,11 @@ 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 ||
@@ -920,7 +932,8 @@ 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;
 }
 
@@ -4627,7 +4640,9 @@ static int binder_mmap(struct file *filp, struct vm_area_struct *vma)
         ret = binder_alloc_mmap_handler(&proc->alloc, vma);
         if (ret)
                 return ret;
+        mutex_lock(&proc->files_lock);
         proc->files = get_files_struct(current);
+        mutex_unlock(&proc->files_lock);
         return 0;
 
 err_bad_arg:
@@ -4651,6 +4666,7 @@ static int binder_open(struct inode *nodp, struct file *filp)
         spin_lock_init(&proc->outer_lock);
         get_task_struct(current->group_leader);
         proc->tsk = current->group_leader;
+        mutex_init(&proc->files_lock);
         INIT_LIST_HEAD(&proc->todo);
         proc->default_priority = task_nice(current);
         binder_dev = container_of(filp->private_data, struct binder_device,
@@ -4903,9 +4919,11 @@ 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)