Re: [PATCH v2] android: binder: Sanity check at binder ioctl
From: David Rientjes
Date: Tue Jan 19 2016 - 17:41:37 EST
On Tue, 19 Jan 2016, Chen Feng wrote:
> When a process fork a child process, we should not allow the
> child process use the binder which opened by parent process.
> But if the binder-object creater is a thread of one process who exit,
> the other thread can also use this binder-object normally.
> We can distinguish this by the member proc->tsk->mm.
> If the thread exit the tsk->mm will be NULL.
Why does exit_mm(), the point where tsk->mm == NULL, signify the point at
which the binder can now be used by other threads?
> proc->tsk->mm != current->mm && proc->tsk->mm
> So only allow the shared mm_struct to use the same binder-object and
> check the existence of mm_struct.
> V2: Fix compile error for error commit
> Signed-off-by: Chen Feng <puck.chen@xxxxxxxxxxxxx>
> Signed-off-by: Wei Dong <weidong2@xxxxxxxxxxxxx>
> Signed-off-by: Junmin Zhao <zhaojunmin@xxxxxxxxxx>
> Reviewed-by: Zhuangluan Su <suzhuangluan@xxxxxxxxxxxxx>
> drivers/android/binder.c | 2 ++
> 1 file changed, 2 insertions(+)
> diff --git a/drivers/android/binder.c b/drivers/android/binder.c
> index a39e85f..279063c 100644
> --- a/drivers/android/binder.c
> +++ b/drivers/android/binder.c
> @@ -2736,6 +2736,8 @@ static long binder_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
> /*pr_info("binder_ioctl: %d:%d %x %lx\n",
> proc->pid, current->pid, cmd, arg);*/
> + if (unlikely(proc->tsk->mm != current->mm && proc->tsk->mm))
> + return -EINVAL;
> trace_binder_ioctl(cmd, arg);
I would imagine that you would want to do READ_ONCE(proc->tsk->mm) so you
are guaranteed to be testing the same value.