linux-kernel - Re: possible deadlock in proc_pid

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Date:   Wed, 2 Sep 2020 11:57:30 +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:
> peterz@...radead.org writes:

> > Could we check privs twice instead?
> >
> > Something like the completely untested below..
> 
> That might work.
> 
> 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.

Another possible aproach might be to grab a copy of the cred pointer and
have the final install check that. It means we need to allow
perf_install_in_context() to fail though. That might be a little more
work.

> I had not realized before this how much setting up tracing in
> perf_even_open looks like attaching a debugger in ptrace_attach.

Same problem; once you've attached a perf event you can observe much of
what the task does.