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From:   Jann Horn <jannh@google.com>
Date:   Tue, 10 Mar 2020 21:10:59 +0100
Message-ID: <CAG48ez1LjW1xAGe-5tNtstCWxG2bkiHaQUMOcJNjx=z-2Wc2Jw@mail.gmail.com>
Subject: Re: [PATCH] pidfd: Stop taking cred_guard_mutex
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     Christian Brauner <christian.brauner@ubuntu.com>,
        Bernd Edlinger <bernd.edlinger@hotmail.de>,
        Kees Cook <keescook@chromium.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Oleg Nesterov <oleg@redhat.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        Andrei Vagin <avagin@gmail.com>,
        Ingo Molnar <mingo@kernel.org>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Yuyang Du <duyuyang@gmail.com>,
        David Hildenbrand <david@redhat.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        David Howells <dhowells@redhat.com>,
        James Morris <jamorris@linux.microsoft.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Shakeel Butt <shakeelb@google.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Christian Kellner <christian@kellner.me>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Aleksa Sarai <cyphar@cyphar.com>,
        "Dmitry V. Levin" <ldv@altlinux.org>,
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On Tue, Mar 10, 2020 at 9:00 PM Jann Horn <jannh@google.com> wrote:
> On Tue, Mar 10, 2020 at 8:29 PM Eric W. Biederman <ebiederm@xmission.com> wrote:
> > Jann Horn <jannh@google.com> writes:
> > > On Tue, Mar 10, 2020 at 7:54 PM Eric W. Biederman <ebiederm@xmission.com> wrote:
> > >> During exec some file descriptors are closed and the files struct is
> > >> unshared.  But all of that can happen at other times and it has the
> > >> same protections during exec as at ordinary times.  So stop taking the
> > >> cred_guard_mutex as it is useless.
> > >>
> > >> Furthermore he cred_guard_mutex is a bad idea because it is deadlock
> > >> prone, as it is held in serveral while waiting possibly indefinitely
> > >> for userspace to do something.
[...]
> > > If you make this change, then if this races with execution of a setuid
> > > program that afterwards e.g. opens a unix domain socket, an attacker
> > > will be able to steal that socket and inject messages into
> > > communication with things like DBus. procfs currently has the same
> > > race, and that still needs to be fixed, but at least procfs doesn't
> > > let you open things like sockets because they don't have a working
> > > ->open handler, and it enforces the normal permission check for
> > > opening files.
> >
> > It isn't only exec that can change credentials.  Do we need a lock for
> > changing credentials?
[...]
> > If we need a lock around credential change let's design and build that.
> > Having a mismatch between what a lock is designed to do, and what
> > people use it for can only result in other bugs as people get confused.
>
> Hmm... what benefits do we get from making it a separate lock? I guess
> it would allow us to make it a per-task lock instead of a
> signal_struct-wide one? That might be helpful...

But actually, isn't the core purpose of the cred_guard_mutex to guard
against concurrent credential changes anyway? That's what almost
everyone uses it for, and it's in the name...