[PATCH] android: binder: fix dangling pointer comparison

[Jann Horn]

On Thu, Jun 16, 2016 at 12:31 AM, Arve Hjønnevåg <arve at android.com> wrote:
> On Wed, Jun 15, 2016 at 3:09 PM, Jann Horn <jannh at google.com> wrote:
>> If /dev/binder is opened and the opener process then e.g. calls execve,
>> proc->vma_vm_mm will still point to the location of the now-freed
>> mm_struct. If the process then calls ioctl(binder_fd, ...), the dangling
>> proc->vma_vm_mm pointer will be compared to current->mm.
>>
>> Let the binder take a reference to the mm_struct to avoid this.
>>
>> In the current code, the BUG_ON() will never trigger; it's just there
>> in case someone changes the conditions under which get_task_mm() can
>> fail. Just WARN_ON() wouldn't work because of the mmput(), and I don't
>> want to complicate the code with a dead error handling code path.
>> (If the BUG_ON() is unacceptable, I'd replace the get_task_mm() with
>> a direct refcount increment or just omit the BUG_ON().)
>>
>> This shouldn't cause additional memory usage in a well-behaved system
>> because you're not supposed to keep binder fds open over fork() or
>> execve().
>>
>> Fixes: a906d6931f3ccaf7de805643190765ddd7378e27
>> Signed-off-by: Jann Horn <jannh at google.com>
>> ---
>>  drivers/android/binder.c | 4 +++-
>>  1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/android/binder.c b/drivers/android/binder.c
>> index 16288e7..4e9131d 100644
>> --- a/drivers/android/binder.c
>> +++ b/drivers/android/binder.c
>> @@ -2962,7 +2962,8 @@ static int binder_open(struct inode *nodp, struct file *filp)
>>                 return -ENOMEM;
>>         get_task_struct(current);
>>         proc->tsk = current;
>> -       proc->vma_vm_mm = current->mm;
>> +       proc->vma_vm_mm = get_task_mm(current);
>> +       BUG_ON(proc->vma_vm_mm == NULL);
>>         INIT_LIST_HEAD(&proc->todo);
>>         init_waitqueue_head(&proc->wait);
>>         proc->default_priority = task_nice(current);
>> @@ -3167,6 +3168,7 @@ static void binder_deferred_release(struct binder_proc *proc)
>>                 vfree(proc->buffer);
>>         }
>>
>> +       mmput(proc->vma_vm_mm);
>>         put_task_struct(proc->tsk);
>>
>>         binder_debug(BINDER_DEBUG_OPEN_CLOSE,
>> --
>> 2.8.0.rc3.226.g39d4020
>>
>
> Does this work? In the past, holding a reference to a task's mm while
> the driver is open would prevent vma close which in turn would prevent
> release of the fd.

Ah, right, mm_struct has two refcounters. I think that should be
atomic_inc(&current->mm->mm_count) / mmdrop() instead.