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

Al Viro
2009-03-29T06:03:50Z

On Sun, Mar 29, 2009 at 06:55:13AM +0100, Al Viro wrote:
> On Sun, Mar 29, 2009 at 06:52:06AM +0200, Oleg Nesterov wrote:
>
> > Let's suppose that check_unsafe_exec() does not set LSM_UNSAFE_SHARE and
> > drops ->siglock. After that, another sub-thread does clone(CLONE_FS) without
> > CLONE_THREAD.
>
> Lovely. And yes, AFAICS that's a hole.
>
> > Unless we killed other threads, I can't see how we can check ->fs is not
> > shared with another process, we can fool ->bprm_set_creds() anyway.
>
> We can't do that, until we are past the point of no return. Charming...
> In principle, we can mark these threads as "-EAGAIN on such clone()" and
> clean that on exec failure.

... or just do that to fs_struct. After finding that there's no outside
users. Commenst?