Re: Q: check_unsafe_exec() races (Was: [PATCH 2/4] fix setuid sometimes doesn't)

Al Viro
2009-03-29T04:11:48Z

On Sun, Mar 29, 2009 at 01:53:43AM +0100, Oleg Nesterov wrote:

> Can't find the patch which introduced check_unsafe_exec(), so
> I am asking here.
>
> How it is supposed to work?
>
> Let's suppose we have two threads T1 and T2. T1 exits, and calls
> exit_fs().
>
> exit_fs:
>
> tsk->fs = NULL;
> // WINDOW
> put_fs_struct(fs);
>
> Now, if T2 does exec() and check_unsafe_exec() happens in the WINDOW
> above, we set LSM_UNSAFE_SHARE.
>
> Or we can race with sub-thread doing clone(CLONE_FS|CLONE_THREAD),
> the new thread is not visible in ->thread_group, buy copy_fs()
> can already increment fs->count.

Frankly, I don't think we really care. Note that having several sub-threads
and doing execve() in one of them will kill the rest, so you really want
to do some kind of synchronization to get something similar to reasonable
behaviour anyway.

> Another question. Why do we check sighand->count? We always unshare
> ->sighand on exec, see de_thread().

Correct. That check can and should go.