linux-kernel - Re: [PATCH 1/2] exec: Properly mark the point of no return

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Date:   Thu, 05 Mar 2020 23:09:04 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Bernd Edlinger <bernd.edlinger@...mail.de>
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\@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>
Subject: Re: [PATCH 1/2] exec: Properly mark the point of no return

Bernd Edlinger <bernd.edlinger@...mail.de> writes:

> On 3/5/20 10:15 PM, Eric W. Biederman wrote:
>> 
>> Add a flag binfmt->unrecoverable to mark when execution has gotten to
>> the point where it is impossible to return to userspace with the
>> calling process unchanged.
>> 
>> While techinically this state starts as soon as de_thread starts
>> killing threads, the only return path at that point is if there is a
>> fatal signal pending.  I have choosen instead to set unrecoverable
>> when the killing stops, and there are possibilities of failures other
>> than fatal signals.  In particular it is possible for the allocation
>> of a new sighand structure to fail.
>> 
>> Setting unrecoverable at this point has the benefit that other actions
>> can be taken after the other threads are all dead, and the
>> unrecoverable flag can double as a flag that those actions have been
>> taken.
>> 
>> Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
>> ---
>>  fs/exec.c               | 7 ++++---
>>  include/linux/binfmts.h | 7 ++++++-
>>  2 files changed, 10 insertions(+), 4 deletions(-)
>> 
>> diff --git a/fs/exec.c b/fs/exec.c
>> index db17be51b112..c243f9660d46 100644
>> --- a/fs/exec.c
>> +++ b/fs/exec.c
>> @@ -1061,7 +1061,7 @@ static int exec_mmap(struct mm_struct *mm)
>>   * disturbing other processes.  (Other processes might share the signal
>>   * table via the CLONE_SIGHAND option to clone().)
>>   */
>> -static int de_thread(struct task_struct *tsk)
>> +static int de_thread(struct linux_binprm *bprm, struct task_struct *tsk)
>>  {
>>  	struct signal_struct *sig = tsk->signal;
>>  	struct sighand_struct *oldsighand = tsk->sighand;
>> @@ -1182,6 +1182,7 @@ static int de_thread(struct task_struct *tsk)
>>  		release_task(leader);
>>  	}
>>  
>> +	bprm->unrecoverable = true;
>>  	sig->group_exit_task = NULL;
>>  	sig->notify_count = 0;
>>  
>
> ah, sorry, 
>         if (thread_group_empty(tsk))
>                 goto no_thread_group;
> will skip this:
>
>         sig->group_exit_task = NULL;
>         sig->notify_count = 0;
>
> no_thread_group:
>         /* we have changed execution domain */
>         tsk->exit_signal = SIGCHLD;
>
> so I think the bprm->unrecoverable = true; should be here?

Absolutely.  Thank you very much.

This is why I try and keep things to one clear simple thing per patch so
silly thinkos like that can be caught.

Eric