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SubjectRe: [PATCH v3] binder: fix proc->files use-after-free
Greg- when this is in, we'll want it in 4.14 as well.

On Mon, Nov 27, 2017 at 9:32 AM, Todd Kjos <tkjos@android.com> wrote:
> 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>
> ---
> 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(-)
>
> diff --git a/drivers/android/binder.c b/drivers/android/binder.c
> index a73596a4f804..7c027ee61375 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;
> }
>
> @@ -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
>

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