linux-kernel - Re: [PATCH 0/2] Infrastructure to allow fixing exec deadlocks

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Date:   Thu, 5 Mar 2020 22:31:24 +0000
From:   Bernd Edlinger <bernd.edlinger@...mail.de>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
CC:     Christian Brauner <christian.brauner@...ntu.com>,
        Kees Cook <keescook@...omium.org>,
        Jann Horn <jannh@...gle.com>, Jonathan Corbet <corbet@....net>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Oleg Nesterov <oleg@...hat.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Andrei Vagin <avagin@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Yuyang Du <duyuyang@...il.com>,
        David Hildenbrand <david@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Anshuman Khandual <anshuman.khandual@....com>,
        David Howells <dhowells@...hat.com>,
        James Morris <jamorris@...ux.microsoft.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Christian Kellner <christian@...lner.me>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Aleksa Sarai <cyphar@...har.com>,
        "Dmitry V. Levin" <ldv@...linux.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>
Subject: Re: [PATCH 0/2] Infrastructure to allow fixing exec deadlocks



On 3/5/20 10:14 PM, Eric W. Biederman wrote:
> 
> Bernd, everyone
> 
> This is how I think the infrastructure change should look that makes way
> for fixing this issue.
> 
> - Correct the point of no return.
> - Add a new mutex to replace cred_guard_mutex
> 
> Then I think it is just going through the existing
> users of cred_guard_mutex and fixing them to use the new one.
> 
> There really aren't that many users of cred_guard_mutex so we should be
> able to get through the easy ones fairly quickly.  And anything that
> isn't easy we can wait until we have a good fix.
> 
> The users of cred_guard_mutex that I saw were:
>     fs/proc/base.c:
>        proc_pid_attr_write
>        do_io_accounting
>        proc_pid_stack
>        proc_pid_syscall
>        proc_pid_personality
>     
>     perf_event_open
>     mm_access
>     kcmp
>     pidfd_fget
>     seccomp_set_mode_filter
> 
> Bernd does this make sense to you?  
> 
> I think we can fix the seccomp/no_new_privs issue with some careful
> refactoring.  We can probably do the same for ptrace but that appears
> to need a little lsm bug fixing.
> 

Yes, for most functions the proposed "exec_update_mutex" is fine,
but we will need a longer-time block for ptrace_attach, seccomp_set_mode_filter
and proc_pid_attr_write need to be blocked for the whole exec duration so
they need a second "mutex", with deadlock-detection as in my previous patch,
if I see that right.

Unfortunately only one of the two test cases can be fixed without the
second mutex, of course the mm_access is what cause the practical problem.

Currently for the unlimited user space delay, I have only the case of
a ptraced sibling thread on my radar, de_thread waits for the parent
to call wait in this case, that can literally take forever.
But I know that also PTRACE_CONT may be needed after a PTRACE_EVENT_EXIT.

Can you explain what else in the user space can go wrong to make an
unlimited delay in the execve?


Bernd.