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From:   Jann Horn <jannh@google.com>
Date:   Mon, 2 Mar 2020 18:37:27 +0100
Message-ID: <CAG48ez1jj_J3PtENWvu8piFGsik6RvuyD38ie48TYr2k1Rbf3A@mail.gmail.com>
Subject: Re: [PATCHv2] exec: Fix a deadlock in ptrace
To:     Bernd Edlinger <bernd.edlinger@hotmail.de>
Cc:     "Eric W. Biederman" <ebiederm@xmission.com>,
        James Morris <jamorris@linux.microsoft.com>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        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>,
        Kees Cook <keescook@chromium.org>,
        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 Mon, Mar 2, 2020 at 6:01 PM Bernd Edlinger <bernd.edlinger@hotmail.de> wrote:
> On 3/2/20 5:43 PM, Jann Horn wrote:
> > On Mon, Mar 2, 2020 at 5:19 PM Eric W. Biederman <ebiederm@xmission.com> wrote:
> >>
> >> Bernd Edlinger <bernd.edlinger@hotmail.de> writes:
> >>
> >>> On 3/2/20 4:57 PM, Eric W. Biederman wrote:
> >>>> Bernd Edlinger <bernd.edlinger@hotmail.de> writes:
> >>>>
> >>>>>
> >>>>> I tried this with s/EACCESS/EACCES/.
> >>>>>
> >>>>> The test case in this patch is not fixed, but strace does not freeze,
> >>>>> at least with my setup where it did freeze repeatable.
> >>>>
> >>>> Thanks, That is what I was aiming at.
> >>>>
> >>>> So we have one method we can pursue to fix this in practice.
> >>>>
> >>>>> That is
> >>>>> obviously because it bypasses the cred_guard_mutex.  But all other
> >>>>> process that access this file still freeze, and cannot be
> >>>>> interrupted except with kill -9.
> >>>>>
> >>>>> However that smells like a denial of service, that this
> >>>>> simple test case which can be executed by guest, creates a /proc/$pid/mem
> >>>>> that freezes any process, even root, when it looks at it.
> >>>>> I mean: "ln -s README /proc/$pid/mem" would be a nice bomb.
> >>>>
> >>>> Yes.  Your the test case in your patch a variant of the original
> >>>> problem.
> >>>>
> >>>>
> >>>> I have been staring at this trying to understand the fundamentals of the
> >>>> original deeper problem.
> >>>>
> >>>> The current scope of cred_guard_mutex in exec is because being ptraced
> >>>> causes suid exec to act differently.  So we need to know early if we are
> >>>> ptraced.
> >>>>
> >>>
> >>> It has a second use, that it prevents two threads entering execve,
> >>> which would probably result in disaster.
> >>
> >> Exec can fail with an error code up until de_thread.  de_thread causes
> >> exec to fail with the error code -EAGAIN for the second thread to get
> >> into de_thread.
> >>
> >> So no.  The cred_guard_mutex is not needed for that case at all.
> >>
> >>>> If that case did not exist we could reduce the scope of the
> >>>> cred_guard_mutex in exec to where your patch puts the cred_change_mutex.
> >>>>
> >>>> I am starting to think reworking how we deal with ptrace and exec is the
> >>>> way to solve this problem.
> >>
> >>
> >> I am 99% convinced that the fix is to move cred_guard_mutex down.
> >
> > "move cred_guard_mutex down" as in "take it once we've already set up
> > the new process, past the point of no return"?
> >
> >> Then right after we take cred_guard_mutex do:
> >>         if (ptraced) {
> >>                 use_original_creds();
> >>         }
> >>
> >> And call it a day.
> >>
> >> The details suck but I am 99% certain that would solve everyones
> >> problems, and not be too bad to audit either.
> >
> > Ah, hmm, that sounds like it'll work fine at least when no LSMs are involved.
> >
> > SELinux normally doesn't do the execution-degrading thing, it just
> > blocks the execution completely - see their selinux_bprm_set_creds()
> > hook. So I think they'd still need to set some state on the task that
> > says "we're currently in the middle of an execution where the target
> > task will run in context X", and then check against that in the
> > ptrace_may_access hook. Or I suppose they could just kill the task
> > near the end of execve, although that'd be kinda ugly.
> >
>
> We have current->in_execve for that, right?
> I think when the cred_guard_mutex is taken only in the critical section,
> then PTRACE_ATTACH could take the guard_mutex, and look at current->in_execve,
> and just return -EAGAIN in that case, right, everybody happy :)

It's probably going to mean that things like strace will just randomly
fail to attach to processes if they happen to be in the middle of
execve... but I guess that works?