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 <[email protected]>

---
v2: declare binder_get_files_struct as static
v3: rework to protect proc->files with a mutex instead of using get_files_struct

Also needed in 4.14

 drivers/android/binder.c | 44 +++++++++++++++++++++++++++++++-------------
 1 file changed, 31 insertions(+), 13 deletions(-)

drivers/android/binder.c +31 -14

index a73596a4f804..7c027ee61375 100644
@@ -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;
 }
 

@@ -4605,7 +4618,9 @@ 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:

@@ -4629,6 +4644,7 @@ 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,

@@ -4881,9 +4897,11 @@ 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)
--

2.15.0.417.g466bffb3ac-goog

Greg- when this is in, we'll want it in 4.14 as well.