linux-kernel - Re: [PATCH] pidfd: Stop taking cred_guard

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Message-ID: <87k13sui1p.fsf@x220.int.ebiederm.org>
Date:   Tue, 10 Mar 2020 14:27:30 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Jann Horn <jannh@...gle.com>
Cc:     Christian Brauner <christian.brauner@...ntu.com>,
        Bernd Edlinger <bernd.edlinger@...mail.de>,
        Kees Cook <keescook@...omium.org>,
        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\@vger.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fsdevel\@vger.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-mm\@kvack.org" <linux-mm@...ck.org>,
        "stable\@vger.kernel.org" <stable@...r.kernel.org>,
        "linux-api\@vger.kernel.org" <linux-api@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Sargun Dhillon <sargun@...gun.me>
Subject: Re: [PATCH] pidfd: Stop taking cred_guard_mutex

Jann Horn <jannh@...gle.com> writes:

> On Tue, Mar 10, 2020 at 7:54 PM Eric W. Biederman <ebiederm@...ssion.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.
>
> Please don't. Just use the new exec_update_mutex like everywhere else.
>
>> Cc: Sargun Dhillon <sargun@...gun.me>
>> Cc: Christian Brauner <christian.brauner@...ntu.com>
>> Cc: Arnd Bergmann <arnd@...db.de>
>> Fixes: 8649c322f75c ("pid: Implement pidfd_getfd syscall")
>> Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
>> ---
>>  kernel/pid.c | 6 ------
>>  1 file changed, 6 deletions(-)
>>
>> Christian if you don't have any objections I will take this one through
>> my tree.
>>
>> I tried to figure out why this code path takes the cred_guard_mutex and
>> the archive on lore.kernel.org was not helpful in finding that part of
>> the conversation.
>
> That was my suggestion.
>
>> diff --git a/kernel/pid.c b/kernel/pid.c
>> index 60820e72634c..53646d5616d2 100644
>> --- a/kernel/pid.c
>> +++ b/kernel/pid.c
>> @@ -577,17 +577,11 @@ static struct file *__pidfd_fget(struct task_struct *task, int fd)
>>         struct file *file;
>>         int ret;
>>
>> -       ret = mutex_lock_killable(&task->signal->cred_guard_mutex);
>> -       if (ret)
>> -               return ERR_PTR(ret);
>> -
>>         if (ptrace_may_access(task, PTRACE_MODE_ATTACH_REALCREDS))
>>                 file = fget_task(task, fd);
>>         else
>>                 file = ERR_PTR(-EPERM);
>>
>> -       mutex_unlock(&task->signal->cred_guard_mutex);
>> -
>>         return file ?: ERR_PTR(-EBADF);
>>  }
>
> 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?

Wouldn't it be sufficient to simply test ptrace_may_access after
we get a copy of the file?

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.

Eric