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From:   ebiederm@xmission.com (Eric W. Biederman)
To:     Bernd Edlinger <bernd.edlinger@hotmail.de>
Cc:     Christian Brauner <christian.brauner@ubuntu.com>,
        Kees Cook <keescook@chromium.org>,
        Jann Horn <jannh@google.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>,
        James Morris <jamorris@linux.microsoft.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Shakeel Butt <shakeelb@google.com>,
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Date:   Thu, 05 Mar 2020 23:09:04 -0600
In-Reply-To: <AM6PR03MB5170B05CFDAF21D8A99B7E48E4E20@AM6PR03MB5170.eurprd03.prod.outlook.com>
        (Bernd Edlinger's message of "Thu, 5 Mar 2020 22:56:01 +0000")
Message-ID: <87pndqax3j.fsf@x220.int.ebiederm.org>
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux)
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Subject: Re: [PATCH 1/2] exec: Properly mark the point of no return
Sender: linux-kernel-owner@vger.kernel.org
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Bernd Edlinger <bernd.edlinger@hotmail.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@xmission.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