Re: possible deadlock in proc_pid_syscall (2)

[peterz@infradead.org]

On Sun, Aug 30, 2020 at 07:31:39AM -0500, Eric W. Biederman wrote:

> I am thinking that for cases where we want to do significant work it
> might be better to ask the process to pause at someplace safe (probably
> get_signal) and then do all of the work when we know nothing is changing
> in the process.
> 
> I don't really like the idea of checking and then checking again.  We
> might have to do it but it feels like the model is wrong somewhere.
> 
> Given that this is tricky to hit in practice, and given that I am
> already working the general problem of how to sort out the locking I am
> going to work this with the rest of the thorny issues of in exec.  This
> feels like a case where the proper solution is that we simply need
> something better than a mutex.

One possible alternative would be something RCU-like, surround the thing
with get_task_cred() / put_cred() and then have commit_creds() wait for
the usage of the old creds to drop to 0 before continuing.

(Also, get_cred_rcu() is disgusting for casting away const)

But this could be complete garbage, I'm not much familiar with any of
thise code.