UPSTREAM: 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>
(cherry picked from commit 7f3dc0088b98533f17128058fac73cd8b2752ef1)

Change-Id: I40982bb0b4615bda5459538c20eb2a913964042c

diff --git a/drivers/android/binder.c b/drivers/android/binder.c
index dc7a4f5..3d49fd1 100644 (file)
--- a/drivers/android/binder.c
+++ b/drivers/android/binder.c
@@ -501,7 +501,8 @@ struct binder_priority {
  * @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
@@ -547,6 +548,7 @@ 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;
@@ -943,20 +945,26 @@ 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;
 }
 
 /*
@@ -965,8 +973,10 @@ 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);
 }
 
 /*
@@ -976,9 +986,11 @@ 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 ||
@@ -986,7 +998,8 @@ 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;
 }
 
@@ -4894,7 +4907,9 @@ 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:
@@ -4918,6 +4933,7 @@ 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);
        if (binder_supported_policy(current->policy)) {
                proc->default_priority.sched_policy = current->policy;
@@ -5177,9 +5193,11 @@ 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)