linux-kernel - Re: possible deadlock in proc_pid

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Message-ID: <20200831074328.GN1362448@hirez.programming.kicks-ass.net>
Date:   Mon, 31 Aug 2020 09:43:28 +0200
From:   peterz@...radead.org
To:     ebiederm@...ssion.com (Eric W. Biederman)
Cc:     syzbot <syzbot+db9cdf3dd1f64252c6ef@...kaller.appspotmail.com>,
        adobriyan@...il.com, akpm@...ux-foundation.org, avagin@...il.com,
        christian@...uner.io, gladkov.alexey@...il.com,
        keescook@...omium.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com,
        walken@...gle.com, Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Miklos Szeredi <miklos@...redi.hu>, jannh@...gle.com
Subject: Re: possible deadlock in proc_pid_syscall (2)

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.